Read pixels from off-screen OpenGL pixel buffer in iOS (OopenGL-ES)

问题

I want to read pixels from an off-screen (not backed by a CAEAGLLayer) Framebuffer. My code to create the buffer looks like:

glGenFramebuffersOES(1, &_storeFramebuffer);
glGenRenderbuffersOES(1, &_storeRenderbuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, _storeFramebuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, _storeRenderbuffer);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES,   GL_RENDERBUFFER_OES, _storeRenderbuffer);
glRenderbufferStorageOES(GL_RENDERBUFFER_OES, GL_RGBA8_OES, w, h);

I read raw pixels with:

glBindFramebufferOES(GL_FRAMEBUFFER_OES, _storeFramebuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, _storeRenderbuffer); 
glReadPixels(0, 0, _videoDimensions.width, _videoDimensions.height, GL_BGRA_EXT, GL_UNSIGNED_BYTE, CVPixelBufferGetBaseAddress(outPixelBuffer)); 

This works well. I can render to this buffer, and copy from it to the screen. But I can't get raw pixels. glReadPixels always returns zeros, and glReadBuffer seems not to exist. I can read from the on-screen frame buffer with glReadPixels. Any ideas?

回答1:

Solved. RGBA to BGRA conversion is not supported by glReadPixels on iOS. Changing

glReadPixels(0, 0, _videoDimensions.width, _videoDimensions.height, GL_BGRA_EXT, GL_UNSIGNED_BYTE, CVPixelBufferGetBaseAddress(outPixelBuffer)); 

to

glReadPixels(0, 0, _videoDimensions.width, _videoDimensions.height, GL_RGBA, GL_UNSIGNED_BYTE, CVPixelBufferGetBaseAddress(outPixelBuffer));

Solves the problem. glGetError is my new friend ;)

来源：https://stackoverflow.com/questions/6880487/read-pixels-from-off-screen-opengl-pixel-buffer-in-ios-oopengl-es