Why doesn't glBlitNamedFramebuffer() work properly? - opengl

Question

I am playing a bit with bit blitting in OpenGL. When I run the code looking like this:
GLuint fbo = CreateFBO(); // creates fbo and attaches a texture to it
glBindFramebuffer(GL_FRAMEBUFFER, 0);
GLfloat some_color[] = {0.8, 0.4, 0.6, 1.0};
glClearBufferfv(GL_COLOR, 0, some_color);
glBlitNamedFramebuffer(0, fbo, 0, 0, 100, 100, 0, 0, 100, 100, GL_COLOR_BUFFER_BIT, GL_NEAREST);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT |GL_STENCIL_BUFFER_BIT);
glBlitNamedFramebuffer(fbo, 0, 0, 0, 100, 100, 0, 0, 100, 100, GL_COLOR_BUFFER_BIT, GL_NEAREST);
It yields a black screen, that is, the screen was cleared, although it should have the color of some_color. But if I change the "named" function to the "unnamed" counterpart, then it works as intended. The new piece of code looks like:
GLuint fbo = CreateFBO(); // creates fbo and attaches a texture to it
glBindFramebuffer(GL_FRAMEBUFFER, 0);
GLfloat some_color[] = {0.8, 0.4, 0.6, 1.0};
glClearBufferfv(GL_COLOR, 0, some_color);
glBindFramebuffer(GL_READ_FRAMEBUFFER, 0);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, fbo_);
glBlitFramebuffer(0, 0, 100, 100, 0, 0, 100, 100, GL_COLOR_BUFFER_BIT, GL_NEAREST);
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT |GL_STENCIL_BUFFER_BIT);
glBindFramebuffer(GL_READ_FRAMEBUFFER, fbo_);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
glBlitFramebuffer(0, 0, 100, 100, 0, 0, 100, 100, GL_COLOR_BUFFER_BIT, GL_NEAREST);
And executing this code, I get the expected result, namely the screen having the color of some_color. What am I doing wrong employing bit blitting? Or if I am using it correctly, why doesn't it work properly?
UPD: I mixed up blit functions in the second code sample.

Answer 1

It yields a black screen, that is, the screen was cleared, although it should have the color of some_color.
No it shouldn't. The first parameter to glBlitNamedFramebuffer is the read framebuffer. So your blit reads from the default framebuffer and writes to the FBO. You clear the screen. Then you reverse this operation. This copies from the FBO to the default framebuffer. Thus overwriting the clear.
Whatever it is you see on the screen, it should be anything but some_color.