OpenGL glGeneratemipmap and Framebuffers

Question

I'm wrapping my head around generating mipmaps on the fly, and reading this bit with this code: http://www.g-truc.net/post-0256.html

//Create the mipmapped texture
glGenTextures(1, &ColorbufferName);
glBindTexture(ColorbufferName);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 512, 512, 0, GL_UNSIGNED_BYTE, NULL);
glGenerateMipmap(GL_TEXTURE_2D); // /!\ Allocate the mipmaps /!\
...
//Create the framebuffer object and attach the mipmapped texture
glBindFramebuffer(GL_FRAMEBUFFER, FramebufferName);
glFramebufferTexture2D(
GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, ColorbufferName, 0);
...
//Commands to actually draw something
render();
...
//Generate the mipmaps of ColorbufferName
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, ColorbufferName);
glGenerateMipmap(GL_TEXTURE_2D);

My questions:

1. Why does glGenerateMipmap needs to be called twice in the case of render to texture?
2. Does it have to be called like this every frame?

If I for example import a diffuse 2d texture I only need to call it once after I load it into OpenGL like this:

GLCALL(glGenTextures(1, &mTexture));
GLCALL(glBindTexture(GL_TEXTURE_2D, mTexture));

GLint format = (colorFormat == ColorFormat::COLOR_FORMAT_RGB ? GL_RGB : colorFormat == ColorFormat::COLOR_FORMAT_RGBA ? GL_RGBA : GL_RED);
GLCALL(glTexImage2D(GL_TEXTURE_2D, 0, format, textureWidth, textureHeight, 0, format, GL_UNSIGNED_BYTE, &textureData[0]));
    
GLCALL(glGenerateMipmap(GL_TEXTURE_2D));
GLCALL(glBindTexture(GL_TEXTURE_2D, 0));

I suspect it is because the textures are redrawn every frame and the mipmap generation uses its content in the process but I want confirmation of this.

3 - Also, if I render to my gbuffer and then immediately glBlitFramebuffer it to the default FBO, do I need to bind and glGenerateMipmap like this?

GLCALL(glBindTexture(GL_TEXTURE_2D, mGBufferTextures[GBuffer::GBUFFER_TEXTURE_DIFFUSE]));
GLCALL(glGenerateMipmap(GL_TEXTURE_2D));
GLCALL(glReadBuffer(GL_COLOR_ATTACHMENT0 + GBuffer::GBUFFER_TEXTURE_DIFFUSE));
GLCALL(glBlitFramebuffer(0, 0, mWindowWidth, mWindowHeight, 0, 0, mWindowWidth, mWindowHeight, GL_COLOR_BUFFER_BIT, GL_LINEAR));

Answer 1

1. As explained in the post you link to, "[glGenerateMipmap] does actually two things which is maybe the only issue with it: It allocates the mipmaps memory and generate the mipmaps."

   Notice that what precedes the first glGenerateMipmap call is a glTexImage2D call with a NULL data pointer. Those two calls combined will simply allocate the memory for all of the texture's levels. The data they contain at this point is garbage.

   Once you have an image loaded into the texture's first level, you will have to call glGenerateMipmap a second time to actually fill the smaller levels with downsampled images.

2. Your guess is right, glGenerateMipmap is called every frame because the image rendered to the texture's first level changes every frame (since it is being rendered to). If you don't call the function, then the smaller mipmaps will never be modified (if you were to map such a texture, you would see your uninitialized smaller mipmap levels when far enough away).

3. No. Mipmaps are only needed if you intend to map the texture to triangles with a texture filtering mode that uses mipmaps. If you're only dealing with the first level of the texture, you don't need to generate the mipmaps. In fact, if you never map the texture, you can use a renderbuffer instead of a texture in your framebuffer.