Could ImGUI Be the Future of GUIs? | Hacker News Reader

Could ImGUI Be the Future of GUIs?

games.greggman.com

224 pointstokyodude124 comments

Really probably not. I love them and they're very handy in certain situations (debugging tools, quick UIs) but once you need a lot of customization they become extremely cumbersome. Also really kind of makes it impossible for designers or less technical people to do anything. Also, while I think separating view logic is slightly overrated, it is useful, and it's very hard to do that with IMGUI. Also it doesn't thread well. Also... Look there's like a million downsides.

Also saying there's no memory allocation is really misleading. There's PLENTY of memory allocation, per frame, you're just not explicitly doing it yourself. It's actually much worse than an RMGUI in this regard, because at least with an RMGUI you get the allocations over with once. With an IMGUI you're allocating things all the time. They're probably much smaller allocations, but lots of small allocations does not make for good performance.

One final note, the Unity 3D example always gets used. If you've ever written a plugin for unity or a custom editor, you're very familiar with the fact that it's editor gui system is extremely limiting and kind of sucks. I mean, it's an example, but once you're past the basics it's kind of a bad example.

On the allocation point, the efficient way to handle this is to use a per-frame memory pool. Because nothing in the IMGUI can persist between frames, you can allocate a single arena of memory for any UI elements that need to store bounding boxes or callbacks or whatever. Each frame just reset your next_ptr to the start of the arena. Technically you are allocating memory, but in practice your allocations are free.

Adding a separate complex ad hoc memory allocation scheme is not what I would call free. Granted, computationally-wise it may be relatively cheap but it does add multiple forms of complexity to a problem that doesn't exist in rmguis.

Arenas are the simplest allocation scheme ever.

  // Start of frame
  void *arena    = malloc(LOTS_OF_MEMORY);
  void *next_ptr = arena;

  // Allocate something
  object   = *next_ptr;               // return that value
  next_ptr = object_size + alignment; // crash if out of memory
 
  // End of frame
  free(arena);

By the way, such "separate complex ad hoc" memory allocation schemes are the reason why manual memory management is faster than garbage collection. If you did everything with malloc(), it would be slower (unless the GC language allocates much more than C, which they often do).

No need to malloc each frame. Malloc at startup and memset each frame (don't even need to do that, tbh).

Nope, just set the pointer back to where it started from. You do need to be super careful doing this though, as anything that relies on RAII (in c++ land) will be busted. You could manually call the destructor on the object in that case, but kind of defeats the purpose of the "no allocation" goals

C++ includes it's "placement new" feature specifically to cater to the memory pool needs. There's no allocation, and only constructor and destructor calls.

It’s a good idea to have a mode that does malloc every frame, as that makes stale pointers much easier to find.

A memory arena that is reset every render loop is the simplest memory allocation possible.

Where do you store pointers to textures? Or do you upload textured to the GPU every frame? What about scroll positions an other non application state?

Specifically Unity would have been much better served using something like Electron. Unity's UI looks bad, feels bad, and is a huge pain in the ass if you're authoring extensions.

Whoa there.

Electron is a catastrophic of hog of resources and performance. Electron for mobile game UI would be a disaster!

This is referring to Electron for the (PC-based) editor UI, not the deployed game UI.

(Plenty of criticisms of Electron still hold, of course.)

Ah, that's fair.

Unity launched in 2005. Electron released in 2013. Still, saying Unity would have been "better served" by using Electron is more than a little unreasonable!

Using a primarily JavaScript environment for primarily C# content sounds like an absolute nightmare to me.

Weirdly, when Unity was first starting out it had this "UnityScript" thing, which was javascript-but-not-quite. Luckily it's mostly been phased out (I don't think newer versions even support it)

2019.1 has a new retained mode editor UI system based on CSS.

Unity 2019.1 has a new retained mode GUI system for editor UI.

FWIW, Unity are moving away from ImGUI (to a classic retained mode UI system) https://blogs.unity3d.com/2019/04/23/whats-new-with-uielemen...

Also to clarify, UIElements (Unity's new retained mode UI framework) is available now in 2019.1 for Editor UI. Runtime UI will also be using UIElements in the future.

In the context of creating debugging UIs for games and graphics applications, Dear imGUI is a godsend. Programmers love it because there is literally only the code to worry about. It's very easy to get it up and running, and all the code that handles the UI drawing and interaction is in one place so it's easy to reason about.

It works very well in the context where you already have fast graphics and an update loop, and you're already expecting to redraw the whole screen every frame. It does not really suit more complex, text-heavy UIs where you're rendering thousands of glyphs with proper kerning and ligatures and anti-aliasing, etc, and want the result of that hard work to be retained in the framebuffer unless it absolutely needs to change.

> want the result of that hard work to be retained in the framebuffer unless it absolutely needs to change

I think immediate mode GUI libraries can get around this issue by still caching and reusing between frames. Conrod does this by still having the state in the background although you are programming to an immediate mode API:

https://docs.rs/conrod/latest/conrod/guide/chapter_1/index.h...

That's a bit antithetical, given the other side of the debate being Retained Mode GUIs.

That said it's a good middle ground. Use whatever API you prefer over a well optimized implementation.

The author does a good job explaining some benefits of an immediate mode renderer but vastly misses the disadvantages.

The immediate mode renderer is great for toy programs. Similar to how you could reproduce 'look here is how simple it is to write hello world and compute the millionth digit of PI' in a new esoteric language...

Occlusion, hit-testing, state changes, scrolling/animations even in the simplest forms will fall over. Infact, that's why we have every major browser move their touch and animation systems into a layer based compositor system (into a separate thread / process).

The author also grossly misses their own example of 'how a spreadsheet with many rows and columns will update faster using ImgUI' and how Instagram styled apps will fare better ImgUi.

A retained mode renderer will use virtual scrollers, efficient culling of nodes for both display and hit-testing (scale to billions of virtual cells) and more importantly help a team of people coordinate their work and get things done.

We are no longer in the 90s.

In fact, with Javascript JIT engines (whose teams deeply understand the use of libraries like React) relentlessly attacking the overhead of DOM nodes and their initialization, and with users who actually expect the design flexibility provided by CSS... a system like React is actually an ideal layer of abstraction for modern UI implementation on practically any platform.

Sure, if you're on an embedded platform, somewhere a JIT can't run, or if you're doing something with real-time rendering requirements (and honestly modern React Suspense should even make that feasible), you may want to use something lower-level. But most people won't need to do this.

The DOM being slow is really more an artifact of browsers/HTML and the history behind doing document based layout. React is a nice solution to that specific problem; but this article suggests immediate mode gui's are the future, which, I don't think that's really the case. And I'm not sure I'd really call React an IMGUI anyway. It's similar, but it's also pretty different.

> A retained mode renderer will use virtual scrollers, efficient culling of nodes for both display and hit-testing (scale to billions of virtual cells)

A good IMGUI will do this too (e.g. Dear IMGUI already does)

The problem with ImGUI is that your gui tree is function call tree and you can't do much with that unless you are in lisp.

Function call tree has some advantages. Functions are wonderfully composable and flexible.

React is such a revolution because it translates ImGUI usage into whatever insane retained mode API is at the bottom of the stack, through intermediate representation of virtual dom.

Presto. You have flexibility and composability of ImGUI without disadvantages of keeping your gui tree only in your function calls, and you can use it without ability to directly control how the GUI is drawn in the engine.

This is really reads like someone trying to sell you something. I've done work on frameworks for both immediate mode and retained mode GUIs. They both absolutely allocate memory behind the scene. There absolutely is state being marshaled around. Caching commonly used state is important. Performance can be bad and great in both. You're really just subscribing to different sets of opinions

Any moderately complex “immediate mode” GUI system is going to do something equivalent to constructing a “retained mode” GUI on the fly I'm guessing.

It will. In fact, even a simple one will require this. But it's not much of a problem - 99% of the time, the UI is the same from one frame to the next, and it isn't much work to detect this. So even if every change requires a complete rebuild of everything, it's not much of a problem.

(I've written systems like this for Cocoa and Win32, and it never turned out to be necessary to do anything other than just regenerate the entire UI any time anything changed. The update runs at 30Hz or 60Hz, and when anything is changing, the UI gets regenerated a lot! - but so what? Most of the time, the UI doesn't get regenerated at all. Then something happens, and the code spends 2 seconds getting absolutely hammered continuously, malloc malloc malloc malloc malloc, god help us all... and then, once again, nothing. The operator puts their fingers to one side and stares at the result with their eyes. Repeat.)

I think the author is over exaggerating the problem of object creation and destruction in traditional gui frameworks. List/collection views are designed to reuse objects as you scroll. Secondly I think the author is also downplaying the fact that retained GUIs can also cache object rendering. Just as the gpu doesn't have to draw the whole screen when only the cursor is blinking, it also doesn't have to redraw widgets unless their size changes.

Immediate vs retained is a simple case of budgeting against cpu usage or memory usage, and it should be considered in that light. (immediate uses more processing, retained uses more memory)

Thinking about this decision just as a performance one disregards the fact that the code is substantially different. It does seems likely that one way is more intuitive / easier to work with than the other, I wouldn't know which though.

It depends on the probkem younare trying to solve with your GUI, I suppose. Sometimes, it is better to just recreate the whole GUI to adapt to a model change (e.g. the user moved half of the tree nodes somewhere else), sometimes it is easier/faster/... to just update the existing GUI (e.g. update the text of a label inside a complex dialog widget).

Retained GUIs vary wildly in implementation.

Many of authors most significant criticisms on retained GUIs are implementation considerations. GUI frameworks exist that solve his key criticisms of complexity and are pleasant to work with.

Criticisms that target core architecture of retained GUI I don't consider to be valuable design goals, at least in settings where I work on GUIs. e.g. memory usage.

Alot of things are glossed over that remain challenges in both, e.g. layout management.

HTML is an interesting example. First iteration of HTML was essentially immediate mode if you think about a single client/server interaction as a GUI update cycle. Server sends draws to client browser and client browser sends back to server user selections. There is no retained state on gui side. Now with programmatic access to DOM, ability to attach events to DOM elements from client side it is now a retained GUI. Seems to be where things evolve to naturally.

The GUI framework I use nearly daily is retained and very pleasant to work with in terms of ease of belting out screens & readability/maintainability of code. The simplicity comes with compromises though as there are limits on GUI idioms that can be expressed. Occasionally run into those boundaries and resulting GUI does look a little plain and unexciting, but for something that is substantially about data entry its fine.

Recently I started playing with https://github.com/ajnsit/concur-documentation/blob/master/R... which has been the most refreshing UI paradigm i've used in a while. and it reminds me a lot of this ImGUI approach, but behind the scenes it uses coroutines instead.

The idea is that a button is a UI element that _blocks_ until an event is fired. You can then compose elements in time like:

    button "hey" >> label "clicked"

which is a program that displays a button, you click it, the button goes away and the text "clicked appears"

Or you can compose programs in space:

    (button "hey" <> label "not clicked")

this is a program that displays both a button, and a label at the same time.

Now, by combining both space and time, we can create a program that changes the label when clicking as follows:

    program = (button "toggle" <> label "off") >> (button "toggle" <> label "on") >> program

This is an application that toggles a label on and off when you press a button. (Note that the definition is recursive)

There's a difference bt poor impl and poor design.

As the author points out, HTML has a poor design (eg. if you want to have a 1000x1000 cell table, you have to have 10^6 actual cells - that's a lot of tds or whatever to parse).

Modern OO GUI frameworks don't do this - they say something like:

cellRenderer.draw(target, row,col,position,size)

No creation of objects required. Of course since it's so easy to create OO programs a lot of code isn't great... and then others copy that code and so it goes.

Seems like we keep re-creating software b/c we haven't taken the time to look at what exists and only then decide on what to keep and what to change. "This is too complex - I'll re-write it!". 10 years later: "We added all the features that the existing software had and now the new one... is just as slow... but we did sell a lot of conference tickets and books so... totally worth it."

When I was 20 I also thought I knew better so I get it.

I feel that the future lies in combining retained and immediate interfaces, preferably with the granularity to allow deeply nested retained interfaces that are fast for complex ui (or for a realtime system with memory to spare), whilst still allowing one to go the other direction, such that a simple ui can be written cleanly and logically for low-memory systems (such as embedded or boot guis). It would need a very well designed api for this, but I feel the benefits are worth the effort (and I'll probably look into this next time I have the freedom to choose ui apis).

A balance may be letting people define it either way, so that manually written ui still can have auto-layout yet intuitive code (following control flow primitives), whilst allowing generated retained uis to be manually editable -- perhaps even allowing one to then embed one within the other, a boon for scripted interfaces that perhaps have people of various levels of experience producing ui elements, such as a musician with little experience being able to add a simple visualiser in an immediate manner to a deeply retained daw gui.

Of course, there's a lot here that is implementation, and some criticism either way can be optimised out. Immediate mode can still cache its rendering, we've had optimised blitting since the early days, and is only usually a problem with complex ui. Retained would get fewer cache misses if we weren't allocating madly across the heap and took a more disciplined approach allocating to contiguous memory -- which is almost entirely a language/api problem (in my experience) that can also happen with immediate but we typically don't see since it's often done in a more procedural style that is allocating to some pool.

Other api elements, such as handling lists etc aren't really a differentiation between retained and immediate, those can be made in either.

For me, I often find that the ability to write out prototype ui code in an immediate style in very quick and satisfying (exactly what I want in prototyping), however once I start to expand upon a ui, I find it best to over time refactor towards a retained style, since by then I will typically have some templates for what ui elements look like, and so I just have to pass a string and function pointer to fill in the template.

Can't see why we can't have nice things and let both coexist...

Conrod is an immediate mode GUI library for rust[1]. I've been using it for an image processing app[2] and have enjoyed the way the code turns out. Everything is much more straightforward as you don't have to reason about callbacks interacting with a loop that's not yours to control and the performance seems good.

[1] https://github.com/pistondevelopers/conrod

[2] https://github.com/pedrocr/chimper

Immediate mode uis are fine for debug displays or for displaying data that's changes every single frame, but for anything else, in a shipping product they are just a waste of resources. The primary wasters are excess memory allocation, string generation, and the sheer amount of redundant function calls. Anything you do to address those problems result in converting your ui to a retained mode UI. For those advocating react like approaches to solving this.. similar problems are involved. Diffing state is wasting cycles unless it's done so optimally and carefully that it becomes a technical feat and ends up being as complex as just doing something retained. Source: game developer for 25 years on console, desktop, and mobile.

Probably not. With technologies like React that make retained-mode UIs look more like immediate-mode ones, there is less need for full blown immediate-mode UIs. React achieves the programmability of an immediate-mode UI without sacrificing the performance of a retained-mode UI (at least, that’s the goal).

> A few problems with this GUI style are:

> You have to write lots of code to manage the the creation and destruction of GUI objects. [..]

> The creation and destruction problem leads to slow unresponsive UIs [..]

> You have to marshal your data into and out of the widgets. [..]

My biggest pain point with the retained mode GUIs I worked with was none of the issues mentioned above. It was always the centralized GUI thread and the consequential synchronization complications. I don't know if this is an inherent problem of retained mode GUI frameworks and if there are some that don't force all widgets into a single thread. If not, this alone is a reason to for me to find immediate mode interesting.

Immediate mode, if anything, makes this harder. (You absolutely have to run the GUI on one thread, for instance). Really though you can get around that on either of them by spawning a worker thread/coroutine/etc. on button clicks and so on.

The real problems start, when the worker thread needs to update the GUI, for example to advance a progress bar. With the frameworks I know I have to build a communication channel between the threads and tell the GUI thread to show and update the progress bar.

My hope was that with an immediate mode framework I could just show a progress bar on top of the existing GUI right from the worker thread. I don't know enough about immediate mode to say if this is really possible. It would simplify a lot of my code though.

I recall reading this article in your comments on the last GUI-specific link posted here... where you just kept disagreeing with comments that took the time to point out how this stuff is largely off base.

We moved away from WM_PAINT for a reason.

Here’s a downside that wasn’t mentioned:

    if (ImGUI::Button("Click Me")) {
        IWasClickedSoDoSomething();
    }

This forces Button to be stateless, which limits the possible quality of implementation. If you mouse-down on a button and the button changes before you mouse-up, it shouldn’t register as a click. Similarly, if you mouse-down on a button, drag to the button, and mouse-up, it shouldn’t be a click. Implementing this in a fully immediate-mode GUI with this interface is either impossible or requires gross hacks.

A naive implementation would have the problem you describe. However, a smarter library implementation avoids this (e.g., by generating an id for the button, saving the id on mouse-down, and checking the id on mouse-up). The user of such a library won't have to worry about it.

And how, exactly, is the id created? Some hash of the button’s text and coordinates?

I would call that a dirty hack, no to mention being implicitly stateful.

But a stateful GUI is not a sin: the program as a whole has state, application code needs to process GUI inputs to update application-level state (e.g. currently set game options) while tracking mechanical details like whether a button is "half clicked" is suitable for automation at the GUI library level.

Regarding IDs, they can (and must) be provided by client code, as client code is responsible for managing widget identity (i.e. whether the button that might have a mouse up event this frame is "the same" that received a mouse down event some frames ago). In C or C++, string names could usually be autogenerated with trivial macros relying on __LINE__ and __FILE__.

That's not how it would go, you don't need state to handle different events, in the case you describe the button is not clicked, of course the button would not trigger on anything else than a mouseup that would be incredibly dumb. It will go something like this: - The mouse is on top of the button, you could render your button differently with that information. - The mousdown event trigger on top of the button again you can do something to make the button render differently. - If the mouseup event occur the button trigger. Instead of keeping a state on your button you render your uis elements conditionned on the cursor position and state of the mouse (up or down)

Can we please just stop moving around in circles in the tech industry? Nobody seems to learn anything from past methods, tech and everything.

LOL. My favorite part of it was the last line:

"More research into ImGUI style UIs could lead to huge gains in productivity."

Don't tell this cat that the research on this stuff goes back >40 years and that the introductory chapter of any book on computer graphics would have talked about all of this. Not like it would help him - he hasn't read anything about it, didn't even do a cursory Google search. Sheesh. A low, low bar.

Do you have any recommended links?

I do!

One of my favorite is "Don't Fidget with Widgets, Draw!" (https://www.hpl.hp.com/techreports/Compaq-DEC/WRL-91-6.pdf) It's modern enough to be understandable, and while it's referencing Ezd (a Scheme drawing system) it greatly influenced Tk (which is still used in all kinds of heavy-hitting EDA software).

That one's only been around for 28 years though (well, the paper was published in 1991, so code was before that...) but let's go further:

The drawing system(s) that greatly influenced Ezd came largely from Xerox PARC, such as ALTO: https://www.computerhistory.org/atchm/xerox-alto-source-code...

There's code in there for a vector drawing program (in 1980!), as well as interacting widgets. Let's go back further...

Finally, we have to mention the Mother of All Demos, in 1968: https://en.wikipedia.org/wiki/The_Mother_of_All_Demos

Which if you haven't watched MOAD before, give it a spin. It will still blow your mind, but the interactive graphical drawing mechanisms will be recognizable.

Also, this "paint the screen every time" method is how a tremendous number of people who cut their teeth on DOS did things on the screen. DOS release date: 1981. So you don't have to have been an academic (which I am NOT) to have tried these techniques while solving practical problems.

As far as books: Computer Graphics Principles and Practice in C (later editions use C#/C++/etc. - the ideas are the same)

The 1995/96 one talks about retained mode graphics directly. That book has been standard in "intro computer graphics" courses for as long as it's been out. So at least 20+ years it's been the "start here" book.

So yea, this stuff has been around for a while...

I don't know. I think every circle we get a bit of new insights. Last time we passed ImGUI we didn't invent react. This time we did.

IMGUI must sound appealing to people who have never done UI work. Just try to implement a responsive UI in an IMGUI. Layout code is not fun.

I got fed up of ImGUI when I wanted to route events to my application instead of the GUI. Its good "out the box" ut when you need to get down and dirty to customise, it can be awkward.

Nowadays I do a hybrid approach, so I have use NanoGUI and create my own "live data" "retained mode" controls. Now I have either the best of both worlds or the worst of both worlds. I think the best:

Pros: - I don't have to bother with data binding. As the control is passed a pointer to the actual memory for he value, it can "go get it" when rendering, rather than my application setting its state. - I still have classes to represent elements and state, so its conceptually simple to build controls on controls. I found this difficult with Imgui.

Cons: - renders 100% full speed, but I am working on way to speed up and slow down render loop depending on user activity, so that when sitting idle, the cpu is not burning.

This is the exact issue I'm running into right now with dear ImGui. There is no event propagation. You can either transfer control over to ImGui, or you retain control. In Qt it would be possible for the focus widget to not accept events, so they would bubble up the hierarchy. Considering how the hierarchy is tied to the callstack, this seems difficult to achieve with an ImGui.

NanoGUI + live data custom controls = happy days

You seem to be missing the point of IM GUIs. They are just intended to render the UI in a stateless way. If you try to use them in another way you'll find them awkward, of course.

I may have misunderstood you, tho :)

I have implemented a bunch of UIs for games. Immediate mode sounds good, but each time the thing that has bit me has been layout.

Sometimes you need to traverse the hierarchy to figure out where things will be placed. Before traversing it for render. If your hierarchy is implicit in a call graph, you have to either duplicate your calls, or implement some kind of backend retained system so you can defer rendering until after layout.

Beyond the absolute simplest of toy UIs, immediate mode doesn't work in my opinion.

Every time a developer decries some abstractions and tools as unnecessary complicated and too "enterprise", it probably means he haven't encountered a problem that this solution was created to address.

As a Unity developer, I love immediate mode GUI for debugging. But I would never in my right mind attempt to use it for actual in-game GUI. Project I'm working on right now is not incredibly complicated, it's just a typical mobile match3 game. But a typical screen here has: (1) background that has to be scaled over all screen, eveloping it around while keeping aspect ratio, (2) frame that has to be scaled to screen without keeping aspect ratio, (3) a "window" background that has to be scaled somewhat to screen (with margin), being enveloped by it, (4) interactive elements, that have to be scaled down from the "safe area" (so that there are no button under the iPhone bevel), (5) match3 game field that has to be scaled according to physical sizes of the screen, (6) pixel-perfect elements that have to be chosen according to pixel size of the screen (1x, 2x and 3x options) and scaled appropriately.

So, no, immediate GUI is definitely not the solution here.

From dear ImGui's mission statement:

> Designed for developers and content-creators, not the typical end-user! Some of the weaknesses includes:

> - Doesn't look fancy, doesn't animate.

> - Limited layout features, intricate layouts are typically crafted in code.

It may not replace retained mode GUI toolkits, but it can certainly make the life of devs easier. If all you need is to quickly hack together an internal tool, or some quick debugging interface, keep ImGui in mind.

I have seem other programs doing stuff like that before, and I have also done some of that in my own programming (although not with this or any other library). I did not know what it is called, until I read this today. It look like good to me. Also, you will still need to add some extra variables if you are doing such thing as tab to focus, I think.

Anyone experienced with ImGUI ever use Rebol and Red's DRAW DSL?

I believe Rebol's GUI support is even easier to use than ImGUI, but of course it can't be embedded and used in the same way as ImGUI either. I wonder if non Red projects could possibly hook into Red's system once Red/System gets closer to C level performance?

> ... people scroll almost constantly in which case the ImGUI wins by a landslide

I think this is a misrepresentation of how fast scrolling is usually implemented.

For fast scrolling, you render the page (which is larger than the viewport = "what fits on the monitor") ONCE onto a GPU texture, and then all scolling happens on the GPU side (the CPU just tells the GPU the offsets into that texture).

Immediate mode has to recreate the texture every frame, instead of once for multiple frames. So "It might use more CPU" is quite certainly true.

You can also do some simple math to arrive at a justification:

Assume a 4k x 2k display at 60 FPS.

Compute the throughput needed (Bytes per second) to draw an RGB framebuffer.

That is: 8M pixels x 3 Bytes x 60 fps = 1.44 GB/s

Note how we haven't done any computation yet to decide what the colours should be, this is just the CPU effort to do IO to tell the GPU about the new colours to show.

This would incur significant CPU usage, and your device would get hot quickly. In contrast, if you let the GPU scroll, you have to send two floats (for X and Y offset) per frame, and the GPU just does a hardware-parallelised lookup.

This is why we have GPUs, and why scrolling immediate-mode would make your device burning hot while a GPU does the task with minimal energy usage.

Following the same logic that you use... have you calculated how much memory do you need to store the memory of a single treeview, or a single scrolling document of just several pages?

Two floats? You are using a memory that is a CPU memory, a big chunk of memory. That does not exist in the GPU. In the GPU the memory is distributed so it can be used in parallel.

Immediate GUI exist because of GPUs, because with GPUs you can draw frames fast. If you look at Imgui code it uses GPU drawing for everything. In fact it uses only two drawing functions copying small rectangles in the screen.

It is drawing a single big chunk of memory what is extremely slow, and you need to do that before you do offsetting.

And if you work with variable data, like a treeview, you need to allocate for a finite amount of memory in the GPU buffers.

I think you're conflating the IMGUI versus RMGUI question with whether rendering is done on the CPU or GPU. The two questions are independent of one another.

I don't think so: Full immediate mode rendering on the GPU would require scene graph and application logic on the GPU.

People don't do that stuff in shaders or CUDA, because they were neither designed for it, nor is it fast, nor is it pleasant to code.

An immediate mode GUI doesn't have to use immediate mode rendering. For example, the Dear ImGui library "outputs optimized vertex buffers that you can render anytime in your 3D-pipeline enabled application." [0]

[0] https://github.com/ocornut/imgui

If you have to create a big texture for the GPU, this is the definition of slow.

One of the big advantages of drawing for every frame is that you only need to draw what is visible. And do it in real time, with no perceived lag.

For a text editor I use I only draw at any given time 1/10.000 of what is actually there. In fact, you don't need to draw textures at all with the GPU, Apple or Microsoft or Google does not do that for drawing text because it is extremely slow generating this texture and it is not flexible.

Those companies in the bleeding edge, they draw directly in the screen from curves like bezier approximations.

The GPU doesn't need to read texels or calculate fragments that aren't visible. So, every frame it is only copying the visible chunk of the scrollable buffer to the framebuffer. GPUs are efficient at that. More expensive rendering to the scrollable buffer, when it is either static or infrequently changing, can be as-needed instead of every frame.

That's a really well-favored set of trade offs for many cases where you need smooth scrolling.

I'm having a hard time to understand what is ImGUI (and what is the opposite RmGUI)... any could help me with ELI5? It sounds like ImGUI is reactive while RmGUI is not?

Yeah I'd appreciate an ELI5 source for this also. The article mostly assumes that you already understand the concept and the linked video is incredibly dry. Some visuals alongside the code would be nice.

Games (normally) re-render the whole scene every frame.

ImGUI exposes that to their API users: you have to re-render and check for clicks on every frame. The code looks like React, (but not as optimized, it re-renders every frame!), and normally you have to keep state by yourself. Code example: [1].

Retained Mode is closer to the DOM, Cocoa or WPF: you create objects and there's an abstraction between the API and the renderer: they get re-rendered every frame for you. Componentes normally have events and state by themselves. Sometimes there's a visual editor too.

The main difference is the API. One is lower level than the other. In practice, the APIs aren't that different, except when it comes to event handling.

[1] - https://docs.unity3d.com/Manual/gui-Basics.html

> Games (normally) re-render the whole scene every frame

They don't re-upload the whole scene to the GPU every frame, they don't recreate objects all the time. Immediate mode was only used in, like, the GL 1.x times.

But I never said they did?

A retained mode GUI is defined by writing initialization code that sets up a model of the UI elements in memory, attaching data and linking callbacks to interactive elements like buttons. The UI library uses the model in memory to draw the UI and handle user interactions.

An immediate mode GUI is defined by writing an update function that draws the UI, passing in data and conditionally running callbacks for interactive elements like buttons. There is no model saved in memory, rather the structure of the UI is implied through the code path taken by the update function. The UI library uses this function to both draw the UI and handle user interactions.

It comes from 3D gaming and hardware acceleration.

In 3D you use hardware acceleration that is 100-200 times more energy efficient than drawing in the CPU. But you loose flexibility.

It is actually way cheaper to actually clear the screen and redraw it again each 1/60 of a second than to complicate the design of the drawing.

If you make it complex, the GPU could not draw it, because the GPU is not flexible, so only the CPU could draw it.

With GPUs, those tricks do not make sense at all.

In the past, there was a lot of optimization for things like drawing windows on the screen, things like circular buffers, "happy ideas" everywhere that made it efficient in the CPU at the expense of complexity, that the CPU could handle.

Imagine solving a mathematical equation. With matrices we fill a table with zeros that represents the elements of the table that do not exist. This is inefficient, but with a matrix a machine can solve it automatically without "thinking".

Immediate Gui uses the same concept, it draws without caring for last frames or states, making it way simpler.

Related recent thread: https://news.ycombinator.com/item?id=19744513

Great find! I haven't done much work GUIs myself, but I would love to read more about alternative design philosophies in human-centered computing and their downfalls.

React hooks is the closest you'll get in practice, once you factor in async, layout measurements and other practical things. But useState is pretty much just imgui with built in getters/setters rather than external state.

The single most important factor when it comes to UI is text rendering. Immediate mode UIs are based on text rasterization, ie polygon creation for each character of the text while retained mode text is usually featuring texture atlases containing all characters in upper and lower case. This of course introduces a limitation on the number of fonts available to the developer, font sizes, etc. My 3D engine is using an immediate mode UI for the editor and tools while it allows for creation of retained mode UI for in-game UI components by automatically generating texture atlases for the selected font types

Didn't Firefox put forward a big plan to basically implement the browser as an immediate mode UI designed to redraw everything on every frame?

WebRender is not just a "plan" but a real thing (I'm using it right now), and it's fully retained.

Immediate doesn't mean just redrawing everything on every frame, immediate means what's described in the article — not keeping state. WR aggressively relies on kept state to optimize rendering of each frame as much as possible.

Very interesting, thank you.

I hadn't heard of ImGUI, but after reading the definition realised that's exactly how you made custom menus in RM2k/3 :)

Say I want to make a program, X, that draws a 50x50px square over another program, Y, at (200,100) and only program Y. Do i need a low level stuff for this, can imGUI be used here? or is it possible with electronjs etc... Also, would program Y be able to detect, with administrator privileges that another program was targeting its pixel space and drawing over it?

>programmers find it more performant >plusses speed >minuses uses more CPU what?

Are any implementations of this approach available for high-level languages?

ImGUI is the past of GUIs.

That is how we used to do it on 8 bit and 16 bit platforms, before frameworks like Turbo Vision, GEM and Workbench came into the scene.

Could somebody please ELI5 this? How is this different from traditional UI approaches and why would I want to use this?

this feels more like drawing UI's using state charts in terms of expressiveness.

Isn't the future of GUIs to have no GUI?

It's important to point out why games use immediate mode GUIs:

1. The GUI needs to be overlaid on the game image (OpenGL/DirectX). This is difficult with traditional GUIs like QT.

2. The GUI needs to be updated in sync with the game, again, it's difficult to integrate traditional GUIs event loops into the game loop, especially with stuff like double/triple buffering.

3. The GUI needs to be as fast as possible, games are severely CPU bound.

A retained mode GUI is typically easier to use, convenience is not why people use immediate mode GUIs.

It's worth pointing out that the immediate/retained split doesn't apply only to the GUI - there are retained mode graphical APIs - DirectX used to have one. They are only used in low-demand games, they sacrifice a lot of speed for the convenience of using a retained mode.

I think 1, 2 and maybe 3 could all be done with a traditional retained mode GUI, and indeed there are retained mode GUI systems for games. The big traditional UI frameworks just have not been made to work well with games. Which is fair, game engines are alien environments that may as well be considered their own platforms.

> It's worth pointing out that the immediate/retained split doesn't apply only to the GUI

Indeed. Modern game engines, and in particular almost all 3D games, use what could be described as a “retained mode” system for the rendering of the game itself — the scene graph — so it is interesting that the UI doesn't always match that.

Generic retained mode is dead, but creating a scene graph driven by something other than imperative updates (eg constraint satisfaction via a physics engine) is alive and well. The problem is that you can’t overlay the former very well in the latter, so things like OGL retained mode are never used anymore.

Honestly, this is far too generous.

IMGUI is easy to implement so its the easiest to put in a custom game engine.

IMGUI is not particularly fast in practice. Unity's certainly isn't. However, they are simple, which approximates performance for small cases and it also lets you write your own tuned implementation.

Traditionally screen orientations for games were pretty simple so doing layouts in a single pass was feasible. These days, when you want to fit your game on every platform you need to do a lot of layout work. At that point you're just making an incomplete retained gui implementation.

I've written real-time game UIs before so I think I have some relevant experience here.

1. It is very possible to write a retained-mode GUI in a graphics API like DirectX or OpenGL. In fact, a retained GUI would typically wipe immediate GUIs in terms of performance in this context. In immediate mode, the GUI's vertex buffers need to be completely reconstructed from scratch every single frame, which is slow, CPU bound, and cannot be (easily) parallelized. It's like reconstructing the game world every frame -- that would be ludicrous for any non-trivial game.

2. I don't think there would be that much of a difference between the two UI models, since data updates can be dispatched from the event loop. It would be faster, too, because only UI components that need updating could be redrawn. This is far faster than updating the entire UI every single frame.

3. As mentioned earlier, immediate mode GUIs are going to be a lot slower than retained mode, when implemented properly. Immediate mode GUIs put most of the work on the CPU instead of offloading most of the work to the GPU like in the retained model.

I think developers that are using immediate mode GUIs are doing so because of their ease of use. I think retained mode is typically harder for a game developer to conceptualize because immediate mode is conceptually similar to a game loop. Also, I don't know of any free & open source retained mode GUIs for DirectX and OpenGL and the like.

Also, DirectX at least (and probably OpenGL) encourages a retained-like model for general rendering. The only way to get decent performance is to re-use vertex buffers between frames and only update them when something changes.

I didn't say that it's not possible to solve these problems in a retained GUI, just that existing ones, QT, Win32, WPF have these problems.

And since writing a full retained GUI is not exactly trivial, people just wrote mini-GUIs using immediate mode.

When I talked about retained mode APIs, I was thinking about scene graphs, where you say "addMesh" or "addSphere" and then just call "renderFrame". I'm aware that most game engines implement their own scene graph anyway, but it's game specific, not some generic one provided by the OpenGL/DX API.

Sorry if I misinterpreted your post but I don't think that changes my response much.

You can absolutely use Win32 Forms and WPF (and probably Qt) with a game. They can be overlaid on top of a DirectX or OpenGL window (with a transparent background) -- I've done it before! I don't think it would have any of the downsides you mentioned, either, except that it wouldn't be GPU accelerated or actually rendered inside the graphics context, which is why nobody actually does this in practice.

But that ignores the dozens of GUI middleware specifically designed for games. A cursory Google search reveals that most of these are going to be retained mode. There's a reason for that. Projects like ImGUI appeal to mostly indie devs who don't have the time or resources to write their own GUI library or license some third-party middleware. And it's probably going to be just fine for their use case. But it's definitely not a perfect solution and we definitely shouldn't throw away decades worth of knowledge and experience like the article is implying.

Qt's QML uses OpenGL and can at least render into an FBO in your existing OpenGL context more or less out of the box. Integrating it with a game render loop deeper should be possible too, but more effort.

Qt is moving into a 3D API agnostic backend.

Sure, for Qt 6, so at some not-yet known point in the next few years (I doubt they'll get that done next year). Whereas both the FBO method or a custom renderer are things you can do today.

Which GUI middleware libraries do you like ?

With Scaleform being one of the most common ones.

I've written real-time game UIs too. I think you underestimate just how ludicrously fast CPUs and GPUs are, and overestimate the complexity of your average GUI. What does your average screen's-worth of GUI consist of, after all? How many widgets are there? I double dare you to tell me that a modern computer or games console can't handle 500 widgets per frame. And I now triple dare you to tell me that your UI designer has put that many damn widgets on one stupid screen in the first place.

(Every UI I worked on actually did redraw everything every frame anyway. It's really not a big deal. Your average GPU can draw a monstrous amount of stuff, something quite ridiculous, and any sensible game UI that's actually usable will struggle to get anywhere near that limit.)

I'm sure many games can get away very well with an immediate mode GUI. I think the question is not can you, but rather should you. My last project used a custom immediate-mode GUI. At the absolute pinnacle of optimization, it was pushing 2,000+ FPS on my machine with something like 3-4k vertices, with heavy texture mapping and anti-aliasing. But the problem was that even with peak optimization, the CPU was spending 15-20% of its time every frame recreating the UI's vertex buffer. Now imagine if we had done a retained-mode GUI instead. That 15-20% overhead would be reduced to near 0% on a typical frame. For nearly any type of game, that kind of savings is really significant. Think of how many more vertices your artists can add, or cool gameplay elements you can add that you didn't have the CPU time available before, and how much better it will run on lower-end hardware.

Why settle for "good-enough" performance?

I don't understand the problem. 2000Hz = 0.5ms/frame; 20% of this = 0.1ms. That sounds like a great result. Your target frame rate is presumably 60-100Hz, assuming it's a PC game, meaning your frame budget is 10-16ms. If your UI takes 0.1ms, you've got >99% of your budget left.

(Also: 3-4K vertices for UI was about what you could expect to budget for a PS2 or Xbox game! - max throughput for PS2 was something like 250,000 vertices/frame at 60Hz, and this is <2% of that. I struggle to believe this is any kind of an issue for anything modern.)

I should mention that 2000 Hz when only running the UI -- with the full game running at 150 FPS. So really the CPU time is 6.66ms*0.2 = 1.33 ms just for the UI!

Of course, that's on my beefy machine with a octo-core overclocked CPU and a couple 1080Tis. But what about the players who are trying to play on their mobile CPUs with integrated graphics? That margin could be the difference between 45 and 60 FPS.

This is wrong, just because it uses 20% CPU when rendering at 2000Hz doesn't mean it will consume 20% CPU when running the game too. Running the game alongside the UI is what makes it drop to 150FPS from 2000FPS. So if the UI consumes the 20% of the frame time at 2000FPS, that is indeed 0.1ms and this number will not magically increase when you add the game. So when you're running at 150FPS your frame time is 6.66ms and you're using 0.1 for the UI. That seems pretty good for me to be honest.

> The CPU was spending 15-20% of its time every frame recreating the UI's vertex buffer.

Not saying it is easy, but it's possible to optimize and cache vertex buffers by using something similar to React's VDOM.

Doesn't your cache just become a limited retained mode with a somewhat hacky, opaque API?

Basically yes. If an immediate mode API is much easier to use, and a retained mode underlying implementation has much better performance, then putting a React-style VDOM layer in-between could get the best of both worlds, depending on how well the middle layer is implemented.

>Why settle for "good-enough" performance?

Because its good enough? Perfection is the enemy good.

I can't see 2000 FPS, 60 FPS is good enough for me. Similarly the 20% vertex buffer hit might not matter.

The catch is that games follow the law of "MOAR!": more details, more effects, more postprocessing, mpre abimation, more simulation... this means that the engine will almost always be pushed to max out the hardware long before the designers are happy with what they have.

Heck, I can think of pretty easy ways to improve sound and video quality of games in ways that are quite fundamental, but that would easily max out the best gaming rigs with either sound or graphics alone.

Bottom line: the need to optimize always comes sooner than you would expect.

Perhaps you're right. As you can probably tell from my prior comment, I don't really subscribe to the 'MOAR' philosophy.

Performance is not the only consideration, even in most demanding games. IM GUIs simplify UI code a lot, by not having to rely on message systems that make the code harder to follow and less predictable.

As most things in life, it's a matter of trade-offs.

Every game I've worked on didn't actually use an IMGUI for anything you'd ever show a player; (unless it's a style of game that really doesn't need much UI), it's just custom gui implementations.

A lot of modern games actually put the UI in with the scene graph of the game itself (IE: Unity's new GUI system)

This is definitely the way to go, especially with the more recent trend of embedding UI elements in world space, and VR/AR essentially requiring it.

Internal tools tend to be IMGUI, Winforms, or WPF from what I've seen. Essentially whatever the original author is most productive in. No need to be super performant because your only users are other devs with really high end systems and rarely an actual need for a constant 30/60fps.

Your engine likely already has a graph structure, event system, and input handling. The core pieces of a retained mode UI system are already there.

> This is difficult with traditional GUIs like QT.

That's not actually that hard to do. You just need to split the UI on layers.

Here is for example HTML/CSS is rendered below and on top of 3D scene: https://sciter.com/sciter-and-directx/

So basically go back to WM_PAINT

Not really, it is a go back to mode 13h.

No. ImGUI could not be the future of GUIs.

GUIs are multi-process, multi-system, multi-clock, multi-network entities, or at least they have the potential to be. Immediate Mode GUIs are almost non-scalable by design.

Imagine a multi-system asynchronous AR collaboration environment. Now imagine that as an Immediate Mode GUI. If we had enough horsepower to do that, we'd be doing something far better with it.

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