Question

I have been having trouble finding straightforward code to render a scene to a texture in OpenGL ES (specifically for the iPhone, if that matters). I am interested in knowing the following:

1. How do you render a scene to a texture in OpenGL ES?
2. What parameters must you use to create a texture that is capable of being a render target in OpenGL ES?
3. Are there any implications with applying this rendered texture to other primitives?

Answer 1

This is how I’m doing it.

I define a texture variable (I use Apple’s Texture2D class, but you can use an OpenGL texture id if you want), and a frame buffer:

Texture2d * texture;
GLuint textureFrameBuffer;

Then at some point, I create the texture, frame buffer and attach the renderbuffer. This you only need to do it once:

texture = [[Texture2D alloc] initWithData:0 
                             pixelFormat:kTexture2DPixelFormat_RGB888
                             pixelsWide:32
                             pixelsHigh:32
                             contentSize:CGSizeMake(width, height)];

// create framebuffer
glGenFramebuffersOES(1, &textureFrameBuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, textureFrameBuffer);

// attach renderbuffer
glFramebufferTexture2DOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_TEXTURE_2D, texture.name, 0);

// unbind frame buffer
glBindFramebufferOES(GL_FRAMEBUFFER_OES, 0);

Every time I want to render to the texture, I do:

glBindFramebufferOES(GL_FRAMEBUFFER_OES, textureFrameBuffer);

...
// GL commands
...

glBindFramebufferOES(GL_FRAMEBUFFER_OES, 0);

About your question 3, that’s it, you can use the texture as if it is any other texture.