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Comment by benj111 | Hacker News Reader

>raytracing in real time games is legitimately cool

I may be (read am) an old git, and ray tracing just makes me think Wolfenstein 3D. I assume this is a different raytracing? Is it the origin of the rays rather than coming from the 'eye', coming from the light source? Isn't that inefficient? Or something else???

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I'm simplifying, but Wolfenstein's technique is called Ray Casting, which calculated the player's view by drawing lines from the player's "eye" to objects and walls in the field of view and use that information to calculate distance and perspective. Ray tracing has been around for forever, but was too resource-intensive to use in real-time until recently. It calculates the path a pixel/light source takes, allowing for realistic lighting (including bouncing off of surfaces) and perfect reflections.

Wolfenstein 3D was ray-casting, not ray-tracing. There is quite a difference between the two techniques.

Ok thanks.

According to the wikipedia ray casting article, the two were used interchangeably "in early computer graphics literature", just to make me feel even older.

One of the differences nobody mentioned yet is that Wolfenstein 3D was based on 2D ray casting. It’s much less common to use the term “ray tracing” for two-dimensional rays, and more common to use something more generic like “ray casting” in that case. This is in addition to how the WP article mentions that “ray casting” is currently referring to visibility testing, and “ray tracing” is more often referring to a specific rendering algorithm.

The big difference is ray casting is just along a line, not on every pixel. Hence the thing with not being able to make overhangs.

> The big difference is ray casting is just along a line, not on every pixel.

I think you have it backwards. Ray tracing is O(#of pixels*number of light bounces). More common 3D rendering is based on O(# of objects in the scene). It's conceivable that ray tracing can be more efficient for scenes with many objects.

It's essentially the same process to begin with (I think), but every time the ray hits something you fire off secondary rays probabilistically from the hit point to see what that point is receiving light from, and so on ad infinitum. Usually you're calculating a BSDF[1] for each surface you hit, which tells you where to cast your next set of rays.

[1] https://en.wikipedia.org/wiki/Bidirectional_scattering_distr...

Ray tracing produces the best results for complex dynamic lighting. This is why it's used for example in movies. It's also normally done on the cpu which is slow rtx is supposed to do something with specialized hardware on chip which is much faster although games would have to build Nvidia specific features which doesn't bode well.

> although games would have to build Nvidia specific features which doesn't bode well.

Actually, nVidia's raytracing stuff is—somewhat-uncharacteristically for the company—all done in open APIs.

I'm past the edit window, but tiny self-correction: I probably should have said "hardware agnostic APIs" instead than "open APIs". DirectX isn't "open" per se, although it depends on your definition.

https://youtu.be/SrF4k6wJ-do?t=543 This video is one of the best explanations on ray casting/tracing I have come across.

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