OpenGL ES Render to Texture

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.

Answer 2

To render the scene to a texture you must use a framebuffer associated with a texture. Here is a method that i created to simplify it :

void glGenTextureFromFramebuffer(GLuint *t, GLuint *f, GLsizei w, GLsizei h) {     glGenFramebuffers(1, f);     glGenTextures(1, t);      glBindFramebuffer(GL_FRAMEBUFFER, *f);      glBindTexture(GL_TEXTURE_2D, *t);     glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);     glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);     glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);     glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);     glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, w, h, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);     glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, *t, 0);      GLuint depthbuffer;     glGenRenderbuffers(1, &depthbuffer);         glBindRenderbuffer(GL_RENDERBUFFER, depthbuffer);     glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT16, w, h);     glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthbuffer);      GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER);     if(status != GL_FRAMEBUFFER_COMPLETE)         NSLog(@"Framebuffer status: %x", (int)status); } 

You can create the frame buffer and the texture easily :

GLuint _texture, _framebuffer; GLsizei w,h; float scale = [UIScreen mainScreen].scale; w = self.view.bounds.size.width * scale; h = self.view.bounds.size.height * scale; glGenTextureFromFramebuffer(&_texture, &_framebuffer, w, h); 

You can later use _framebuffer to render the scene into _texture in your draw method :

glBindFramebuffer(GL_FRAMEBUFFER, _framebuffer); //draw here the content you want in the texture //_texture is now a texture with the drawn content  //bind the base framebuffer glBindFramebuffer(GL_FRAMEBUFFER, 0); //or if you use GLKit [view bindDrawable]; //draw normaly 

Now you can do what you want with the texture. If you want to do some post processing (blur, bloom, shadow, etc...) you can !