Reading data using glReadPixel() with multisampling - ios

Question

Presently I am trying to read the pixel data from the frame Buffer in order to capture the screen in IOS. GlreadPixels command works fine when using the following code to setup frame buffer :-
//buffers
// Create a depth buffer that has the same size as the color buffer.
glGenRenderbuffersOES(1, &m_depthRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, m_depthRenderbuffer);
glRenderbufferStorageOES(GL_RENDERBUFFER_OES, GL_DEPTH_COMPONENT24_OES, width, height);
// Create the framebuffer object.
glGenFramebuffersOES(1, &framebuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, framebuffer);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES,
GL_RENDERBUFFER_OES, m_colorRenderbuffer);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_DEPTH_ATTACHMENT_OES,
GL_RENDERBUFFER_OES, m_depthRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, m_colorRenderbuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, framebuffer);
but when I use depth buffer and colorbuffer for Multi-Sampling glreadpixels() don't capture any pixel data as with earlier code ....for multi-sampling I use following code :-
glGenFramebuffersOES(1, &sampleFramebuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, sampleFramebuffer);
//GLuint sampleColorRenderbuffer;
glGenRenderbuffersOES(1, &sampleColorRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, sampleColorRenderbuffer);
glRenderbufferStorageMultisampleAPPLE(GL_RENDERBUFFER_OES, 4, GL_RGBA8_OES, width, height);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, sampleColorRenderbuffer);
//glRenderbufferStorageMultisampleAPPLE(GL_RENDERBUFFER_OES, 4, GL_RGBA8_OES, width, height);
//glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, sampleColorRenderbuffer);
GLuint sampleDepthRenderbuffer;
glGenRenderbuffersOES(1, &sampleDepthRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, sampleDepthRenderbuffer);
glRenderbufferStorageMultisampleAPPLE(GL_RENDERBUFFER_OES, 4, GL_DEPTH_COMPONENT24_OES, width, height);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_DEPTH_ATTACHMENT_OES, GL_RENDERBUFFER_OES, sampleDepthRenderbuffer);
//glBindRenderbufferOES(GL_RENDERBUFFER_OES,sampleColorRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, m_colorRenderbuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, sampleFramebuffer);
I use the follwing code to read pixel data :-
CGRect screenBounds = [[UIScreen mainScreen] bounds];
int backingWidth = screenBounds.size.width;
int backingHeight =screenBounds.size.height;
glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_WIDTH_OES, &backingWidth);
glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_HEIGHT_OES, &backingHeight);
NSInteger myDataLength = backingWidth * backingHeight * 4;
buffer= (GLuint *) malloc(myDataLength);
glReadPixels(0, 0, backingWidth, backingHeight, GL_RGBA, GL_UNSIGNED_BYTE, buffer);
Any idea how to capture correct pixel data with multi-Sampling technique....or Am I doing something wrong? Please guide me in right direction.
Thanks

Answer 1

When using multisampled FBOs you cannot just read the sample buffer (as it doesn't contain simple pixels). You first need to resolve the sample buffers into a single buffer.
You do this by creating another non-multisampled FBO (let's call it resultFramebuffer) with the neccessary renderbuffer storage you want to read and then calling:
glBindFramebuffer(GL_READ_FRAMEBUFFER, sampleFramebuffer);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, resultFramebuffer);
glBlitFramebuffer(0, 0, width, height, 0, 0, width, height, GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT, GL_NEAREST);
And then you read from the result buffer (of course the actual constant and function names may contain OES or APPLE). If you don't need the final depth values, the result buffer doesn't need a depth renderbuffer.
EDIT: As you wrote in your comment and what I searched, there is a dedicated function glResolveMultisampleFramebufferAPPLE you have to use instead of glBlitFramebuffer. The rest stays the same.