A short detour along the way…

I wanted to improve the Stencil Shadow Volumes code a little bit and enable it for the Programmable Pipeline in Vortex, however, I had to take a small detour to fix an issue related to mobile device support.

It turns out that OpenGL ES, the 3D library that Vortex uses to render its graphics on mobile devices, does not support rendering indexed geometry for indices larger than 16 bits. Keep this in mind when developing for mobile devices such as the iPhone or iPad.

I can completely understand the reasoning behind this decision. 32-bit indices could be considered too much data to submit to the GPU in a mobile device. Furthermore, they are not strictly necessary, as they could be replaced (if needed) by splitting the geometry into two groups defined by 16-bit indices.

The solution I devised, which is now part of Vortex 2.0, is to allow the user to specify the data size for the indices when defining the geometry. This provides the flexibility to use 32, 16 or 8 bit indices. You can even have several geometric objects with different index sizes in a scene.

The advantage of leveraging this mechanism is that now it is very easy to fine-tune the number of bytes used for index representation for improving performance. For example, using 16-bit indices instead of 32-bit indices would make no difference for representing models composed of less than 65536 vertices, while requiring a copy of just half the number of bytes to the GPU.

In the extreme case of 8-bit indices we would be constrained to only 256 vertices, but we would be sending only one fourth of the data to the GPU.

This was mostly plumbing work, so no new picture this time. Stay tuned for more updates!