GLSL：iPhone应用程序无法访问内置属性？

Question

I am getting really desperate here. I working with Xcode, trying to implement some OpenGL stuff on the iPhone. I have to write a Shader for Phong-Lighting. I got as far as declaring my geometry (vertices, indices, calculating etc.) and passing the respective arguments as attributes to the shader (written in GLSL). Using these attributes works fine, some really basic shader programs compile correctly and give the expected output.

Now I'm trying to start with some more advanced calculations, for which I need to use some of the built-in attributes of GLSL, namely ie the "gl_NormalMatrix", but whenever I try that, the program crashes and I get an "ERROR: 0:3: Use of undeclared identifier 'gl_NormalMatrix'". It happens identically whenever use any of the built-in attributes such as gl_Vertex, gl_Normal etc.

Are these attributes not available in GLSL on the iPhone or am I missing something? Maybe I haven't fully understood how this works. As mentioned, passing my own vertex attributes into the shader works fine, and I am also wondering how the program should "know" the correct values "on its own" - so the whole concept of built-in attributes is still a bit confusing to me. But whenever I try to run some shaders I found online to see if something happens, I get these errors thrown, even though everyone else seems to use built-in attributes extensively when writing shaders.

I really hope someone here can shed some light on this. Thousand thanks in advance!

Julia

Answer 1

Those attributes aren't available in GL ES, whether on the iPhone or elsewhere. And the same goes for WebGL. You need to write your own matrix code, or use GLKit if you're supporting only iOS 5+, and supply attributes for yourself. See Kronos' reference card for an incredibly concise summary of what made it into ES 2.0 — amongst other things it lists all the available built-in special variables as:

• gl_Position
• gl_PointSize
• gl_FragCoord
• gl_FrontFacing
• gl_PointCoord
• gl_FragColor
• gl_FragData[n]

GLKit's maths stuff is really good because it inlines and uses the ARM's NEON SIMD unit. I consider it a sufficient reason to specify iOS 5 as a minimum in all new GL projects.