4 ms·

In principle, "voxels" and "polygons" are the same algorithm - you project some points on the surface to the screen and eliminate invisible ones with the z-buff

In principle, "voxels" and "polygons" are the same algorithm - you project some points on the surface to the screen and eliminate invisible ones with the z-buffer. Polygons allow to use sparser points and fill space between them with interpolation, voxels rely on the points density or blowing up the point size. Interpolation is quite complicated and voxels, on the other hand, could be arranged so it would be very easy to draw on a traditional CPU. So one could write a really fast voxel renderer on something like Pentium 200, which would produce better looking graphics than a polygonal renderer at the same speed (it would not be as versatile but when you do a demo you are free to chose scenes where voxels will do the job better). On GPUs interpolation is not just cheap but it's the only way to exploit massive parallelism since you can draw all interpolated points simultaneously. Drawing points, on the other hand, is sequential (because of z-buffer all primitives have to be drawn in the fixed order) and is already super slow by itself. GPUs are also optimized for drawing triangles so they do a lot of things with them they cannot do for a list of points (which would be the primitive you'd use to draw voxels). Of course you can do your voxels on compute but then you are not using color/depth buffer hardware so either way you are disadvantaged.

dkersten 8y ago
It was my understanding that a large reason why early voxel engines had great performance is because you only draw each pixel once (kinda like how deferred lighting only calculates lights for each pixel once) eliminating overdraw and therefore doing work that wasn’t ever going to be visible. But now GPUs can crunch that data super fast (as you say) and memory bandwidth is often a bigger issue. So while voxels still get used in some places, they’re avoided in favour of more compressed data formats and rendering techniques. At least, that’s what it sounded like to me, I’m no graphics expert.
- JKCalhoun 8y ago
  Early voxel engines (referred to as height-map engines in the article) were narrow, optimized, limited ray tracers essentially.
- pandaman 8y ago
  >because you only draw each pixel once It depends on the algorithm, if you did heightfield then yes, you could easily eliminate invisible pixels without testing each of them against z-buffer. However you could not do complex models with heightfields. A popular general voxel algorithm in 90s was "chains", where an object was represented as a sequence of voxels, where position of each was encoded as an offset from the previous one and voxels were on a regular grid so there were only 26 possible offsets. If you did not do perspective projection then you could have just projected all 26 deltas into the screen space and compute each voxel's position with 3 additions. With the offset taking 5 bits you could fit each voxel in a 16-bit dword together with some ramp-lit material for per-pixel lighting. It was a very fast loop even on a CPU without division. On a modern CPU or GPU, of course, this is painfully slow, since each position depends on the previous one and ensures strict order on the loop. Memory-bandwidth wise this is quite nice because of the very compact input data. E.g. you could put position and texture deltas into 8 bit voxels and had more compact representation for a smaller model than using popular 16/32 byte vertices.
  - dkersten 8y ago
    Oh that’s interesting, thanks for the comment! Must read up on it more :) Do you know how modern voxel algorithms work? Do they just blindly render point clouds to make use of the GPU parallelism and let the z-buffer remove invisible voxels (and a spatial index to render only the ones in view) or is there more sophistication? I assume there’s some clever parallel algorithms available nowadays, or?
    - pandaman 8y ago
      I am not closely following voxel developments so I could be wrong, but my impression is that the practical implementations focus on the representation and its compression. They all render with some type of raytracing using compute. There had been approaches to generate a triangle mesh from a voxel representation but I am not sure how far did they get.
      - dkersten 8y ago

dkersten 8y ago

It was my understanding that a large reason why early voxel engines had great performance is because you only draw each pixel once (kinda like how deferred lighting only calculates lights for each pixel once) eliminating overdraw and therefore doing work that wasn’t ever going to be visible. But now GPUs can crunch that data super fast (as you say) and memory bandwidth is often a bigger issue. So while voxels still get used in some places, they’re avoided in favour of more compressed data formats and rendering techniques. At least, that’s what it sounded like to me, I’m no graphics expert.

JKCalhoun 8y ago
Early voxel engines (referred to as height-map engines in the article) were narrow, optimized, limited ray tracers essentially.
pandaman 8y ago
>because you only draw each pixel once It depends on the algorithm, if you did heightfield then yes, you could easily eliminate invisible pixels without testing each of them against z-buffer. However you could not do complex models with heightfields. A popular general voxel algorithm in 90s was "chains", where an object was represented as a sequence of voxels, where position of each was encoded as an offset from the previous one and voxels were on a regular grid so there were only 26 possible offsets. If you did not do perspective projection then you could have just projected all 26 deltas into the screen space and compute each voxel's position with 3 additions. With the offset taking 5 bits you could fit each voxel in a 16-bit dword together with some ramp-lit material for per-pixel lighting. It was a very fast loop even on a CPU without division. On a modern CPU or GPU, of course, this is painfully slow, since each position depends on the previous one and ensures strict order on the loop. Memory-bandwidth wise this is quite nice because of the very compact input data. E.g. you could put position and texture deltas into 8 bit voxels and had more compact representation for a smaller model than using popular 16/32 byte vertices.
- dkersten 8y ago
  Oh that’s interesting, thanks for the comment! Must read up on it more :) Do you know how modern voxel algorithms work? Do they just blindly render point clouds to make use of the GPU parallelism and let the z-buffer remove invisible voxels (and a spatial index to render only the ones in view) or is there more sophistication? I assume there’s some clever parallel algorithms available nowadays, or?
  - pandaman 8y ago
    I am not closely following voxel developments so I could be wrong, but my impression is that the practical implementations focus on the representation and its compression. They all render with some type of raytracing using compute. There had been approaches to generate a triangle mesh from a voxel representation but I am not sure how far did they get.
    - dkersten 8y ago