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macosopenglcore-graphicsfborender-to-texture

How to render to an FBO on a shared context?

I have an application where I need to do the following:

  1. load a texture from disk into a GL texture
  2. run an image filter on it (render it onto another texture via an FBO)
  3. display the resulting texture on screen

I've got that much working.

Next, I'd like to be able to move step #2 to a separate shared GL context.

On initialization, I create a shared context:

rootContext = CGLGetCurrentContext();
CGLPixelFormatObj pf = CGLGetPixelFormat(rootContext);
CGLCreateContext(pf, rootContext, &childContext);

...then make it current and set up the framebuffer on it...

CGLSetCurrentContext(childContext);
glGenTextures(1, &childTexture);
glBindTexture(GL_TEXTURE_2D, childTexture);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, image.width(), image.height(), 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
glGenFramebuffers(1, &childFramebuffer);
glBindFramebuffer(GL_FRAMEBUFFER, childFramebuffer);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, childTexture, 0);

Then, when it's time to render each frame, I make childContext current and render to it:

CGLSetCurrentContext(childContext);
glBindFramebuffer(GL_FRAMEBUFFER, childFramebuffer);
glUseProgram(childProgram);

glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, inputTexture);
glUniform1i(childTextureUniform, 0);

glBindBuffer(GL_ARRAY_BUFFER, childQuadPositionBuffer);
glVertexAttribPointer(childPositionAttribute, 2, GL_FLOAT, GL_FALSE, sizeof(GLfloat)*2, (void*)0);
glEnableVertexAttribArray(childPositionAttribute);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, childQuadElementBuffer);
glDrawElements(GL_TRIANGLE_STRIP, 4, GL_UNSIGNED_SHORT, (void*)0);
glDisableVertexAttribArray(childPositionAttribute);

glUseProgram(0);
glBindFramebuffer(GL_FRAMEBUFFER, 0);

...then I make rootContext current and render the FBO's texture to screen:

CGLSetCurrentContext(rootContext);
glUseProgram(rootProgram);

glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, childTexture);  // This texture was created and populated on childContext.
glUniform1i(rootTextureUniform, 0);

glBindBuffer(GL_ARRAY_BUFFER, rootQuadPositionBuffer);
glVertexAttribPointer(rootPositionAttribute, 2, GL_FLOAT, GL_FALSE, sizeof(GLfloat)*2, (void*)0);
glEnableVertexAttribArray(rootPositionAttribute);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, rootQuadElementBuffer);
glDrawElements(GL_TRIANGLE_STRIP, 4, GL_UNSIGNED_SHORT, (void*)0);
glDisableVertexAttribArray(rootPositionAttribute);

glUseProgram(0);

This works perfectly if I comment out the CGLSetCurrentContext(childContext); calls.

But if I switch to the shared context to render to the FBO, I see glitchy garbage rendered onscreen, as though nothing ever gets rendered onto childTexture:

glitchy garbage rendered onscreen when using shared contexts

...which is really cool, but I'm going for a more of a realist aesthetic here.

Any ideas about how I can get this working when using shared contexts?

I've already tried

  1. Checking for CGLErrors when creating the shared context — there are no errors; it's created successfully.
  2. Checking glGetError() — there are no errors.
  3. Checking glCheckFramebufferStatus() — it's GL_FRAMEBUFFER_COMPLETE.
  4. Calling glBindTexture(GL_TEXTURE_2D, 0); after each glDisableVertexAttribArray() call, as suggested by @datenwolf. No change.

Complete source code

I created 2 simple test apps which both exhibit the same problem:

Solution

  • I got it working, with 2 changes to the sample code above:

    • call glViewport() on childContext once after creating it
    • call glFlushRenderAPPLE(); after rendering to the FBO each frame