Retaining state is a pain and causes bugs. Trying to get fancy a la react and diffing the tree for changes makes not sense. That was a performance hack because changing the DOM in JS used to be slow as hell. You don't need that.
Just redraw the whole thing every frame. Great performance, simple, less bugs.
What about when you're embedding your GUI into an existing application? or for use on an already taxed system? (Audio plugins come to mind)
What if something is costly, that you need to compute dynamically, but not often, makes it into the frame? Do you separately now create a state flag for that one render object?
Retained mode is more optimal when not much changes but if a lot of stuff changes at once it can be worse. So for real time applications like your audio example or games you want immediate mode. Retained mode is better for saving battery though or can be.
The point of immediate mode UIs is not necessarily that there is no state specific to the UI, but rather that the state is owned by user code. You can (and, in these more complex cases, should) retain state between frames. The main difference is that the state is still managed by your code, rather than the UI system ("library", whatever).
That can be a reasonable choice sometimes. Note that the point is that you introduce state where necessary, rather than stateful UI being the default as with retained mode.
I've used ImGui in exactly these kinds of projects. Game engines already render graphics, so it is just part of the same pipeline. Rendering the gui is instant, how many fps you want to render is up to you.
You should check out the gamedev scene. It's soft real-time, and yet dearIMGUI is the choice for tooling. Immediate-mode is an API-design, not the implementation details. All Immediate-mode GUIs retain data some data, and for that reason they each have their own APIs for retaining data in various capacities. Usually something really simple like hashing and component-local state.
> This works for simple apps, utilities, and demos/mvps. Not great for actual applications.
Respectfully, I don't think you're informed on this. Probably the most responsive debugger out there is RAD Debugger and it's built with an IMGUI.
If you just have a lot of text and a few rectangles and no animation, immediate mode would work well...
But if you have a lot of images, animation etc ... You'd anyway have to track all the textures uploaded to the GPU to not reupload them. Might as well retain as much of the state as possible? (Eg. QtQuick)
The more dynamic/animated an UI is, the less there's a difference between a retained- and immediate-mode API, since the UI needs to be redrawn each frame anyway. Immediate mode UIs might even be more efficient for highly dynamic UIs because they skip a lot of internal state update code - like creating/destroying/showing/hiding/moving widget objects).
Immediate-mode UIs can also be implemented to track changes and retain the unchanged parts of the UI in baked textures, it's just usually not worth the hassle.
The key feature of immediate mode UIs is that the application describes the entire currently visible state of the UI for each frame which allows the UI code to be 'interleaved' with application state changes (e.g. no callbacks required), how this per-frame UI description is translated into pixels on screen is more or less an implementation detail.
That depends on the kind of animations - typically for user interfaces, it's just moving, scaling, playing with opacity etc.. that's just updating the matrices once.
So you describe the scene graph once (this rectangle here, upload that texture there, this border there) using DOM, QML etc..., and then just update the item properties on it.
As far as the end user/application developer is concerned , this is retained mode. As far as the GPU is considered it can be redrawing the whole UI every frame..
...any tiny change like this will trigger a redraw (e.g. the GPU doing work) that's not much different from a redraw in an immediate mode system.
At most the redraw can be restricted to a part of the visible UI, but here the question is whether such a 'local' redraw is actually any cheaper than just redrawing everything (since figuring out what needs to be redrawn might be more expensive than just rendering everything from scratch - YMMV of course).
Something like a lot of text ? Probably easier to redraw everything in immediate mode.
Something like a lot of images just moving, scaling, around? Easier to retain that state in GPU and just update a few values here and there...
It's really not that trivial to estimate, especially on high-dpi displays.
Rendering a texture with a 'baked UI' to the framebuffer might be "just about as expensive" as rendering the detailed UI elements directly to the framebuffer.
Processing a pixel isn't inherently cheaper than processing a vertex, but there are a lot more pixels than vertices in typical UIs (a baked texture might still win when there's a ton of alpha-blended layers though).
Also, of course you'd also need to aggressively batch draw calls (e.g. Dear ImGui only issues a new render command when the texture or clipping rectangle changes, e.g. a whole window will typically be rendered in one or two draw calls).
And in low power applications? Like on a smartphone?
When the UI is static and only needs to change on user input, an immediate mode UI can 'stop' too until there's new input to process.
For further low-power optimizations, immediate mode UI frameworks could skip describing parts of the UI when the application knows that this part doesn't need to change (contrary to popular belief, immediate mode UI frameworks do track and retain state between frames, just usually less than retained mode UIs - but how much state is retained is an internal implementation detail).
However ...
When you have a listbox of 10,000 rows and you only draw the visible rows, then the others will lose their state because of this.
Of course there are ways around that but it becomes messy. Maybe so messy that retained mode becomes attractive.
It seems you assume some sort of OO model.
> When you have a listbox of 10,000 rows and you only draw the visible rows, then the others will lose their state because of this.
Well keep the state then.
Immediate mode really just means you have your data as an array of things or whatever and the UI library creates the draw calls for you. Drawing and data are separate.
This is a bit oversimplified. For instance Dear ImGui needs to store at least the window positions between frames since the application code doesn't need to track window positions.
https://github.com/ocornut/imgui/wiki/About-the-IMGUI-paradi...
At the earlist in the first frame the application UI description code doesn't mention an UI item (that means UI items need a persistent id, in Dear ImGui this is a string hash, usually created from the item's label which can have a hidden `##` identifier to make it unique, plus a push/pop-id stack for hierarchical namespacing.
> then the others will lose their state because of this
Once an item is visible, the state must have been provided by the application's UI description code, when the item is invisible, that state becomes irrelevant.
What happens when the item moves out of view, e.g. because the user scrolls down?
State should be preserved, because the user might scroll back up.
E.g. pseudocode:
for (firstVisibleItemIndex .. lastVisibleItemIndex) |itemIndex| {
ui_list_item(itemIndex, listItemValues[itemIndex]);
}
How does the immediate mode system know that the corresponding state can be deleted too?
Does the system provide tools for that or does the burden lie on my application code?
Doesn't make a difference. If the page is static, there is no redraw happening. If the page is dynamic, the redraw is happening at the frequency of the change (once per second, or once per frame, or whatever).
Whether you're doing a diff of the DOM or redrawing the whole DOM, typical pages (i.e. not two-sigmas past the median) aren't going to redraw something on every frame anyway.
In the initial crappy implementation the code was assigning the same class over and over to the text input, rather than only when required. Despite that being an obvious bug, I could literally feel the difference in typing speed and how that was hammering the page.
Once the bug was fixed, and it only assigned it once correctly, the problem went away.
"redraw everything the whole frame" and "don't do any diffing" sound insane in this regard.
You need to consider that a web browser with its millions of lines of code in the DOM and rendering engine is pretty much the worst case for "redrawing a complex UI each frame", especially since the DOM had been designed for mostly static 'documents' and not highly dynamic graphical UIs.
Add React on top and the whole contraption might still be busy with figuring out what has changed and needs to be redrawn at the time an immediate mode UI sitting directly on top of a 3D API is already done rendering the entire UI from scratch.
A native immediate mode UI will easily be several hundred times less code (for instance Dear ImGui is currently just under 50kloc 'orthodox C++').