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Storing triangles in a super efficient data structure is precisely what I mean by traversing the scene. Do you really think that's a free operation when you hav

Storing triangles in a super efficient data structure is precisely what I mean by traversing the scene. Do you really think that's a free operation when you have billions of triangles inside a data structure that must be dynamic by nature? You hit all sorts of arcane memory issues. I hit the same thing just developing a basic kd-tree for 2D culling. The approximation algorithms I pointed out are exactly how raytracing is done backwards, but actually still must use a combination of camera rays and light source rays in a stocastic method in order to determine a path from the camera to the light source. Brute force raytracing is, in fact, just shooting out rays from the light sources. That's obviously inefficient, which is why we have the backwards lighting attempts, as discussed in the article. Raytracing stars is not as simply as you think, not when you can visit them. They can no longer be point sources at infinite distance because the object itself is not infinitely far away, so you therefore can only substitute an infinite light source in when its essentially a point, which gets really nasty when you are dealing with entire galaxies as light sources, and if it isn't you must deal with all the lighting effects in between. Materials are the most important part of computer graphics. If you don't think this, you have absolutely no idea what you are talking about.

vilya 14y ago
Re: raytracing stars, the obvious answer is to use a lower detail representation when the stars are far away. In this case all the photons from the faraway galaxy will be arriving from almost exactly the same direction, so there won't be any noticeable loss of accuracy. This fits in pretty naturally with the hierarchical data structure you'll already be using (in practice) to hold the scene. Regarding the data structure: the types of hierarchical structure commonly used to hold scene data for ray tracing actually lend themselves pretty well to level-of-detail, caching and streaming from disk; so it can be a help rather than a hindrance for managing memory. There's been some really promising work in the last few years on Sparse Voxel Octrees, for example.
Geee 14y ago
I don't think anyone has ever done purely a from light source tracing, because it doesn't make sense. Tracing from camera is not an approximation. Brute-force path tracing is random sampling from camera to every direction and bouncing/interacting until light source is hit. This is the standard Monte Carlo path tracing method which is unbiased and arrives at the accurate solution given enough time. This is unbiased, accurate and very simple to implement, but very slow to compute. Metropolis and photon mapping are bidirectional methods where rays are selectively traced back from light source to forward direction, to speed up the path tracing computation.

vilya 14y ago

Re: raytracing stars, the obvious answer is to use a lower detail representation when the stars are far away. In this case all the photons from the faraway galaxy will be arriving from almost exactly the same direction, so there won't be any noticeable loss of accuracy. This fits in pretty naturally with the hierarchical data structure you'll already be using (in practice) to hold the scene. Regarding the data structure: the types of hierarchical structure commonly used to hold scene data for ray tracing actually lend themselves pretty well to level-of-detail, caching and streaming from disk; so it can be a help rather than a hindrance for managing memory. There's been some really promising work in the last few years on Sparse Voxel Octrees, for example.

Geee 14y ago

I don't think anyone has ever done purely a from light source tracing, because it doesn't make sense. Tracing from camera is not an approximation. Brute-force path tracing is random sampling from camera to every direction and bouncing/interacting until light source is hit. This is the standard Monte Carlo path tracing method which is unbiased and arrives at the accurate solution given enough time. This is unbiased, accurate and very simple to implement, but very slow to compute. Metropolis and photon mapping are bidirectional methods where rays are selectively traced back from light source to forward direction, to speed up the path tracing computation.