I am developing a large game that streams in level data (including shaders) as you move through the game world. I do not want to have hitches in my frame rate as shaders are compiled/linked or on the first time they are used.
I have my shader compilation and linking working on a separate thread with its own open-gl context. But I have not been able to get the prewarming of the shaders to work on the separate thread (so that there is no performance hit when the shader is first used).
Prewarming is really not mentioned anywhere in the iOS or OpenGL docs. It is however mentioned in the OpenGL ES Analyzer (one of the instruments available when profiling from xcode). In this tool I get a "Shader Compiled Outside of Prewarming Phase" warning each time something is rendered with a shader that has not been used to render something before. The "Extended detail" says this:
"OpenGL ES Analyzer detected a shader compilation that is not part of an initial prewarming phase. Shader compilation can be a time consuming operation. To avoid them, prewarm all shaders used for rendering. To do this, make a prewarming passwhen your application launches and execute a drawing call with each of the shader programs to be used, using any gl state settings the shader program will be used in conjunction with. States such as blending, color mask, logic ops, multisamping, texture formats, and point primitive state can all affect shader compilation."
The term "compilation" is a little confusing here. The vertex and fragment shaders have already been compiled and the program has been linked. But the first time something is rendered with a given OpenGL state it does some more work on the shader to optimize it for that state I guess.
I have code to pre-warm the shaders by rendering a zero sized triangle before it's first use.
If I compile, link and pre-warm the shaders on the main thread with the same Open GL context as the normal rendering then it w