Multivolume voxel cone tracing

Another feature experiment I nearly forgot to show you is voxel cone tracing with multiple voxel volumes. There are different approaches to support large scenes with voxel cone tracing:

* Use a single volume texture and scale it to the scene's bounds. This increases a single voxel's world extents, which leads to coarser lighting and even more light leaking.
* Use sparse octree voxelization with a buffer instead of a volume texture. This is a way more complicated implementation. Additionally, the performance hit for lighting evaluation is quite big, compared to hardware-filtered volume texel fetching.
* Use cascaded voxel cone tracing, a similar approach to cascaded shadow maps. Revoxelization of the whole scene (or what's visible for the player) is quite demanding - implementations that only revoxelize objects on the border of the cascades are way more complex than the traditional, non-cascaded approach. Not using such an approach and revoxelizing everything every frame leads to flickering in the voxelization step, which can't be eliminated complettely due to the "binary" nature" of voxels (or at least I didn't manage to achieve it).

My implementation goes a different way, that doesn't suffer from the above problems, by introducing world space voxel volumes. Instead of a single big one, there are many smaller ones. There are many advantages now:

* Not all voxel volumes have to be updated every frame - one can update the nearest n volumes per frame, depending on the given hardware specs.
* There can be higher resolutions where needed and less resolution where coarse illumination is sufficient.
* Since everything is in world space, no flickering on revoxelization - at least when materials change. For dynamic objects, one still has to do some tricks or use temporal filtering with multiple bounces or sth.
* Theoretically, the voxel data could be precalculated and streamed in.


I put a sized list of VoxelGrid entries into a generic buffer that my avaluation compute shaders can read. My VoxelGrid data structure is as simple as the following.

struct VoxelGrid {
   int albedoGrid;
   int normalGrid;
   int grid;
   int grid2;
   int resolution;
   int resolutionHalf;
   int dummy2;
   int dummy3;
   mat4 projectionMatrix;
   vec3 position;
   float scale;
   uvec2 albedoGridHandle;
   uvec2 normalGridHandle;
   uvec2 gridHandle;
   uvec2 grid2Handle;
 };

Tracing

    vec4 accum = vec4(0.0);
    float alpha = 0;
    float dist = 0;
    vec3 samplePos = origin;// + dir;

    while (dist <= maxDist && alpha < 1.0)
    {
        float minScale = 100000.0;
        int canditateIndex = -1;
        VoxelGrid voxelGrid;
        for(int voxelGridIndex = 0; voxelGridIndex < voxelGridArray.size; voxelGridIndex++) {
            VoxelGrid candidate = voxelGridArray.voxelGrids[voxelGridIndex];
            if(isInsideVoxelGrid(samplePos, candidate) && candidate.scale < minScale) {
                canditateIndex = voxelGridIndex;
                minScale = candidate.scale;
                voxelGrid = candidate;
            }
        }

        float minVoxelDiameter = 0.25f*voxelGrid.scale;
        float minVoxelDiameterInv = 1.0/minVoxelDiameter;
        vec4 ambientLightColor = vec4(0.);
        float diameter = max(minVoxelDiameter, 2 * coneRatio * (1+dist));
        float increment = diameter;

        if(canditateIndex != -1) {
            sampler3D grid;
            // sample grid here
        }

        dist += increment;
        samplePos = origin + dir * dist;
        increment *= 1.25f;
    }
 return vec4(accum.rgb, alpha);

coarse and fine voxel volume side by side