glReadPixels() sets GL_INVALID_OPERATION error

Question

I'm trying to implement color picking with FBO. I have multisampled FBO (fbo[0]) which I use to render the scene and I have non multisampled FBO (fbo[1]) which I use for color picking.

The problem is: when I try to read pixel data from fbo[1] everything goes well until glReadPixels call which sets GL_INVALID_OPERATION flag. I've checked the manual and can't find the reason why.

The code to create FBO:

glBindRenderbuffer(GL_RENDERBUFFER, rbo[0]);
glRenderbufferStorageMultisample(GL_RENDERBUFFER, numSamples, GL_RGBA8, resolution[0], resolution[1]);
glBindRenderbuffer(GL_RENDERBUFFER, rbo[1]);
glRenderbufferStorageMultisample(GL_RENDERBUFFER, numSamples, GL_DEPTH24_STENCIL8, resolution[0], resolution[1]);
glBindRenderbuffer(GL_RENDERBUFFER, rbo[2]);
glRenderbufferStorage(GL_RENDERBUFFER, GL_R32UI, resolution[0], resolution[1]);
glBindRenderbuffer(GL_RENDERBUFFER, rbo[3]);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH24_STENCIL8, resolution[0], resolution[1]);

glBindFramebuffer(GL_DRAW_FRAMEBUFFER, fbo[1]);   
glFramebufferRenderbuffer(GL_DRAW_FRAMEBUFFER, GL_DEPTH_STENCIL_ATTACHMENT, GL_RENDERBUFFER, rbo[3]);
glFramebufferRenderbuffer(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, rbo[2]);
OGLChecker::checkFBO(GL_DRAW_FRAMEBUFFER);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, fbo[0]);
glFramebufferRenderbuffer(GL_DRAW_FRAMEBUFFER, GL_DEPTH_STENCIL_ATTACHMENT, GL_RENDERBUFFER, rbo[1]);
glFramebufferRenderbuffer(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, rbo[0]);
OGLChecker::checkFBO(GL_DRAW_FRAMEBUFFER);

My checker stays silent so the FBOs are complete. Next the picking code

glBindFramebuffer(GL_DRAW_FRAMEBUFFER, fbo[1]);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
// bla, bla, bla
// do the rendering
unsigned int result;
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
glBindFramebuffer(GL_READ_FRAMEBUFFER, fbo[1]);
int sb;
glReadBuffer(GL_COLOR_ATTACHMENT0);
glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);
glGetIntegerv(GL_SAMPLE_BUFFERS, &sb);
//    glBindFramebuffer(GL_READ_FRAMEBUFFER, 0);
OGLChecker::getGlError();
std::cerr << "Sample buffers " << sb << std::endl;
glReadPixels(pos.x(), resolution.y() - pos.y(), 1, 1, GL_RED, GL_UNSIGNED_INT, &result);
OGLChecker::getGlError();
return result;

the output:

Sample buffers 0
OpenGL Error : Invalid Operation

The interesting fact that if I uncomment glBindFramebuffer(GL_READ_FRAMEBUFFER, 0); then no error happens and pixels are read from screen (but I don't need this).

What may be wrong here?

Answer 1

Your problem is the format parameter. For a texture that has a one-channel integer internal format the correct parameter isn't GL_RED, but GL_RED_INTEGER:

glReadPixels(pos.x(), resolution.y() - pos.y(), 1, 1, GL_RED_INTEGER, GL_UNSIGNED_INT, &result);

Look at the OpenGL documentation wiki (emphasis mine):

...

format

Specifies the format of the pixel data. For transfers of depth, stencil, or depth/stencil data, you must use GL_DEPTH_COMPONENT, GL_STENCIL_INDEX, or GL_DEPTH_STENCIL, where appropriate. For transfers of normalized integer or floating-point color image data, you must use one of the following: GL_RED, GL_GREEN, GL_BLUE, GL_RG, GL_RGB, GL_BGR, GL_RGBA, and GL_BGRA. For transfers of non-normalized integer data, you must use one of the following: GL_RED_INTEGER, GL_GREEN_INTEGER, GL_BLUE_INTEGER, GL_RG_INTEGER, GL_RGB_INTEGER, GL_BGR_INTEGER, GL_RGBA_INTEGER, and GL_BGRA_INTEGER. Even if no actual pixel transfer is made (data​ is NULL and no buffer is bound to GL_PIXEL_UNPACK_BUFFER), you must set this parameter correctly for the internal format of the destination image.

...

---

Note: the official reference page is incomplete/wrong.