Binding to framebuffer 0 may cause a blank screen

Today, I encountered an interesting issue that took away a few hours of my (not so) precious life, to debug and understand. I was implementing some basic shadow mapping, which requires you to create a new render target (meaning you need to render to a separate buffer other than the screen). So, we have to switch back and forth between framebuffers. Most OpenGL tutorials out there will simply ask you to bind back to the default framebuffer 0.

Here’s a code excerpt from site (as of 11/16/2018) from their article on shadow mapping (I love this site, and this in no way a criticism of their content, just using it to point to a probably bug).

// 1. first render to depth map
glBindFramebuffer(GL_FRAMEBUFFER, depthMapFBO);
glBindFramebuffer(GL_FRAMEBUFFER, 0);
// 2. then render scene as normal with shadow mapping (using depth map)
glViewport(0, 0, SCR_WIDTH, SCR_HEIGHT);
glBindTexture(GL_TEXTURE_2D, depthMap);

Binding back to framebuffer 0, as mentioned here, simply output a blank screen for me. And I went through a period of commenting out all my rendering code and enabling line by line (costing me half a day or more) to see what was going wrong. I found the culprit in the line: glBindFramebuffer(GL_FRAMEBUFFER, 0);. Then I suddenly got an idea and executed these lines to find my actual default framebuffer (after disabling any shadow mapping code and simply doing a simple setup for single pass rendering):

glGetIntegerv(GL_DRAW_FRAMEBUFFER_BINDING, &default_draw_fbo_);
glGetIntegerv(GL_READ_FRAMEBUFFER_BINDING, &default_read_fbo_);

And to my surprise, the answer was 3. With allocating a new framebuffer for shadow mapping, this went up to 4 (weird!). I’m using Qt 5.11 as my application framework, and I’m not sure whether this is a bug/feature or what (maybe they use framebuffer 0 to render their own stuff). But, it seems that the default framebuffer cannot be assumed to be 0.

So, if you’re experiencing a blank screen when trying to a render anything that causes you to switch between framebuffers, make sure you find out what exactly you’re default framebuffer ID is. Then, just switch back to this known number, and all will be well.

// also GL_FRAMEBUFFER is deprecated now, simply use
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, default_draw_fbo_);



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