glReadPixels on iOS with multisampling

Question

I'm trying to get pixels from framebuffer with multisampling. It returns only zeros. I do call glResolveMultisampleFramebufferAPPLE as suggested here and here, but I can not figure out whats the problem in my case.

first of all I create non-multisampled framebuffer with color attachment:

GLuint framebuffer, colorRenderbuffer;

glGenFramebuffersOES(1, &framebuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, framebuffer);

glGenRenderbuffersOES(1, &colorRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, colorRenderbuffer);
glRenderbufferStorageOES(GL_RENDERBUFFER_OES, GL_RGBA8_OES, w, h);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, colorRenderbuffer);

then create multisample framebuffer with color and depth attachment:

GLuint sampleFramebuffer, sampleColorRenderbuffer, sampleDepthRenderbuffer;

glGenFramebuffersOES(1, &sampleFramebuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, sampleFramebuffer);

glGenRenderbuffersOES(1, &sampleColorRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, sampleColorRenderbuffer);
glRenderbufferStorageMultisampleAPPLE(GL_RENDERBUFFER_OES, 4, GL_RGBA8_OES, w, h);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, sampleColorRenderbuffer);

glGenRenderbuffersOES(1, &sampleDepthRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, sampleDepthRenderbuffer);
glRenderbufferStorageMultisampleAPPLE(GL_RENDERBUFFER_OES, 4, GL_DEPTH_COMPONENT16_OES, w, h);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_DEPTH_ATTACHMENT_OES, GL_RENDERBUFFER_OES, sampleDepthRenderbuffer);

then clear framebuffers:

glBindFramebufferOES(GL_FRAMEBUFFER_OES, framebuffer);
glClear(GL_COLOR_BUFFER_BIT);

glBindFramebufferOES(GL_FRAMEBUFFER_OES, sampleFramebuffer);
glViewport(0, 0, w, h);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

do my drawing (this is Cocos3D drawing code):

[cc3Layer visit];

then resolve buffers:

glBindFramebufferOES(GL_DRAW_FRAMEBUFFER_APPLE, framebuffer);
glBindFramebufferOES(GL_READ_FRAMEBUFFER_APPLE, sampleFramebuffer);
glResolveMultisampleFramebufferAPPLE();

glBindFramebufferOES(GL_READ_FRAMEBUFFER_APPLE, framebuffer);

and then get all zeros:

glReadPixels(0, 0, w, h, GL_RGBA, GL_UNSIGNED_BYTE, buffer);

glBindRenderbufferOES(GL_RENDERBUFFER_OES, colorRenderbuffer);

I skipped two gl checks for success with creating framebuffer, since they are creating successfully. Where is the error in my code?

Answer 1

You should bind the non-multisample color-renderbuffer BEFORE read pixels.

like that:

glResolveMultisampleFramebufferAPPLE()
glBindFramebufferOES(GL_READ_FRAMEBUFFER_APPLE, framebuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, colorRenderbuffer);
glReadPixels(0, 0, w, h, GL_RGBA, GL_UNSIGNED_BYTE, buffer);