Optimizing the Vertex Pipeline

Entry posted by Josh · August 19, 2020

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In my work with NASA we visualize many detailed CAD models in VR. These models may consist of tens of millions of polygons and thousands of articulated sub-objects. This often results in rendering performance that is bottlenecked by the vertex rather than the fragment pipeline. I recently performed some research to determine how to maximize our rendering speed in these situations.

Leadwerks 4 used separate vertex buffers, but in Leadwerks 5 I have been working exclusively with interleaved vertex buffers. Data is interleaved and packed tightly. I always knew this could make a small improvement in speed, but I underestimated how important this is. Each byte in the data makes a huge impact. Now vertex colors and the second texture coordinate set are two vertex attributes that are almost never used. I decided to eliminate these. If required, this data can be packed into a 1D texture, applied to a material, and then read in a custom vertex shader, but I don't think the cost of keeping this data in the default vertex structure is justified. By reducing the size of the vertex structure I was able to make rendering speed in vertex-heavy scenarios about four times faster.

Our vertex structure has been cut down to a convenient 32 bytes:

struct Vertex
{
    Vec3 position;
    short texcoords[2];
    signed char normal[3];
    signed char displacement;
    signed char tangent[4];
    unsigned char boneweights[4];
    unsigned char boneindices[4];
};

I created a separate vertex buffer for rendering shadow maps, which only require position data. I decided to copy the position data into this and store it separately. This requires about 15% more vertex memory usage, but results in a much more compact vertex structure for faster shadow rendering. I may pack the vertex texture coordinates in there, since that would result in a 16-byte-aligned structure. I did not see any difference in performance on my Nvidia card and I suspect this is the same cost as a 12-byte structure on most hardware.

Using unsigned shorts instead of unsigned integers for mesh indices increases performance by 11%.

A vertex-limited scene is one in which our default setting of using an early Z-pass can be a disadvantage, so I added an option to disable this on a per-camera basis.

Finally, I found that vertex cache optimization tools can produce a significant performance increase. I implemented two different libraries. In order to do this, I added a new plugin function for filtering a mesh:

int FilterMesh(char* filtername, char* params, GMFSDK::GMFVertex*& vertices, uint32_t& vertex_count, uint32_t*& indices, uint32_t& indice_count, int polygonpoints);

This allows you to add new mesh processing routines such as flipping the indices of a mesh, calculating normals, or performing mesh modifications like bending, twisting, distorting, etc. Both libraries resulted in an additional 100% increase in framerate in vertex-limited scenes.

What will this help with? These optimizations will make a real difference when rendering CAD models and point cloud data.