How to determine the width and height of a GL framebuffer object given only the corresponding id
I want to determine the size (width, height) of a framebuffer object.
I created a framebuffer object via
// create the FBO.
glGenFramebuffers(1, &fboId);
How can I get the size of the first color attachment given only the framebuffer object id (fboId)?
Is this possible or do I have tor store the size of the color attachment in an external variable to know later the size of the FBO?
Answer
Your question is somewhat confused, as you ask for two different things.
Here's the easy question:
How can I get the size of the first color attachment given only the framebuffer object id (fboId)?
That's simple: get the texture/renderbuffer attached to that attachment, get what mipmap level and array layer is attached, then query the texture/renderbuffer for how big it is.
The first two steps are done with glGetFramebufferAttachmentParameter (note the key word "Attachment") for GL_COLOR_ATTACHMENT0. You query the GL_FRAMEBUFFER_ATTACHMENT_OBJECT_TYPE to get whether it's a renderbuffer or a texture. You can get the renderbuffer/texture name with GL_FRAMEBUFFER_ATTACHMENT_OBJECT_NAME.
If the object is a renderbuffer, you can then bind the renderbuffer and use glGetRenderbufferParameter to fetch the renderbuffer's GL_RENDERBUFFER_WIDTH and GL_RENDERBUFFER_HEIGHT.
If the object is a texture, you'll need to do more work. You need to query the attachment parameter GL_FRAMEBUFFER_ATTACHMENT_TEXTURE_LEVEL to get the mipmap level.
Of course, now you need to know how to bind it. If you're using OpenGL versions before 4.4 or without certain extensions, then this is complicated. See, you need to know which texture target type to use. Silly and annoying as this may seem, the only way to determine the target from just the texture object name is to... try everything. Go through each target and check glGetError. The one for which GL_INVALID_OPERATION isn't returned is the right one.
If you have GL 4.4 or ARB_multi_bind available, you can just use glBindTextures (note the "s"), which does not require that you specify the target. And if you have 4.5 or ARB_direct_state_access, you don't need to bind the texture at all. The DSA-style functions don't need the texture target, and it also provides glBindTextureUnit, which binds a texture to its natural internal target.
Once you have the texture bound and it's mipmap level, you use glGetTexLevelParameter to query the GL_TEXTURE_WIDTH and GL_TEXTURE_HEIGHT for that level.
Now, that's the easy problem. The hard problem is what your title asks:
I want to determine the size (width, height) of a framebuffer object.
The size of the renderable area of an FBO is not the same as the size of GL_COLOR_ATTACHMENT0. The renderable area of an FBO is the intersection of all of the sizes of all of the images attached to the FBO.
Unless you have special knowledge of this FBO, you can't assume that the FBO contains only one image or that all of the images have the same size (and if you have special knowledge of the FBO, then quite frankly you should also have special knowledge of how big it is). So you'll need to repeat the above procedure for every attachment (if the type is GL_NONE, then nothing is attached). Then take the intersection of the returned values (ie: the smallest width and height).
In general, you shouldn't have to ask an FBO that you created how big it is. Just as you don't have to ask textures how big they are. You made them; by definition, you know how big they are. You put them in the FBO, so again you know how big it is.