Write to OpenGL multiple renderbuffer failed

Question

Not sure what is the problem, it failed to read the pixels when writing to multiple renderbuffer. Here is the code:

setup code:

// gen framebuffer object
glGenFramebuffers(1, &fbo);
glBindFramebuffer(GL_FRAMEBUFFER, fbo);

// gen renderbuffer 0
glGenRenderbuffers(1, &rbo_color0);
glBindRenderbuffer(GL_RENDERBUFFER, rbo_color0);
glRenderbufferStorage(GL_RENDERBUFFER, GL_RGB, w, h);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, rbo_color0);

// gen renderbuffer 1
glGenRenderbuffers(1, &rbo_color1);
glBindRenderbuffer(GL_RENDERBUFFER, rbo_color1);
glRenderbufferStorage(GL_RENDERBUFFER, GL_RED_INTEGER, w, h);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT1, GL_RENDERBUFFER, rbo_color1);

// gen depth buffer
glGenRenderbuffers(1, &rbo_depth);
glBindRenderbuffer(GL_RENDERBUFFER, rbo_depth);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT, w, h);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, rbo_depth);

glBindFramebuffer(GL_FRAMEBUFFER, 0);

render and read pixel code:

// setup shader and uniforms...
// bind framebuffer and clear color/depth buffer
glBindFramebuffer(GL_FRAMEBUFFER, fbo);
glEnable(GL_DEPTH_TEST);
glClearColor(0.f, 0.f, 0.f, 0.1f);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

const GLuint buffers[] = {GL_COLOR_ATTACHMENT0, GL_COLOR_ATTACHMENT1};
glDrawBuffers(2, buffers);
glBindVertexArray(vao);
glDrawElements(GL_TRIANGLES, index_count, GL_UNSIGNED_INT, (void*)0);
glReadBuffer(GL_COLOR_ATTACHMENT0);
glReadPixels(x, y, 1, 1, GL_RGB, GL_FLOAT, data);
glReadBuffer(GL_COLOR_ATTACHMENT1);
glReadPixels(x, y, 1, 1, GL_RED_INTEGER, GL_INT, (int*)(data) + 4);
glBindVertexArray(0);
glBindFramebuffer(GL_FRAMEBUFFER, 0);

I used layout qualifier in the shader for the outputs, shouldn't be any problems there. So I think probably there is something wrong the the buffer setup or render code. I've succeed in reading the pixels when there is only one renderbuffer(rbo_color0). The main difference is:

no generation code for renderbuffer1;
use glDrawBuffer(rbo_color0) when rendering;
the format of rbo_color0 is GL_RGBA(double checked the format setting in corresponding glRenderbufferStorage/glReadBuffer calls, no problem here);
no read pixel call for rbo_color1.

Any helps?

good idea to have more error check, something like glGetError — Joey
– Joey, Commented Sep 24, 2018 at 1:22

Rabbid76 · Accepted Answer · 2018-09-23 14:41:33Z

3

GL_RED_INTEGER is not a proper internal format, as it can be used for the 2nd parameter of glRenderbufferStorage. A proper enumerator constant for the internal format would be GL_R32I.
See OpenGL 4.6 API Compatibility Profile Specification; 8.26. TEXTURE IMAGE LOADS AND STORES; page 334

If you would check for OpenGL erros (glGetError), then you would get an INVALID_ENUM error.

Change the render buffer storage specification, to solve the issue:

glRenderbufferStorage(GL_RENDERBUFFER, GL_R32I, w, h);

Note, you should check the framebuffer completeness by glCheckFramebufferStatus:

GLenum fb_status = glCheckFramebufferStatus( GL_FRAMEBUFFER );
if ( fb_status != GL_FRAMEBUFFER_COMPLETE )
{
  // error handling
}

edited Sep 23, 2018 at 14:41

answered Sep 23, 2018 at 13:57

Rabbid76

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3 Comments

derhass Over a year ago

One could add that GL_RED_INTEGER is also not valid for glReadPixels.

Nicol Bolas Over a year ago

@derhass: Except that it very much is valid. GL_RED, GL_INT will read floating point values, converted into normalized, 32-bit signed integers. If you want to read integer values from an integer texture, you have to use the _INTEGER formats. The specification makes this clear; the format parameter of glReadPixels is defined in section 8.4.4, and nothing in there says that _INTEGER is forbidden for glReadPixels, no matter what those HTML pages say.

Joey Over a year ago

tested, GL_RED_INTEGER works in glReadPixels, thank you guys.

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