In OpenGL rendering pipeline, the geometry data and textures are transformed and passed several tests, and then finally rendered onto a screen as 2D pixels. The final rendering destination of the OpenGL pipeline is called framebuffer. Framebuffer is a collection of 2D arrays or storages utilized by OpenGL; colour buffers, depth buffer, stencil buffer and accumulation buffer. By default, OpenGL uses the framebuffer as a rendering destination that is created and managed entirely by the window system. This default framebuffer is called window-system-provided framebuffer.

The OpenGL extension, GL_EXT_framebuffer_object provides an interface to create additional non-displayable framebuffer objects (FBO). This framebuffer is called application-created framebuffer in order to distinguish from the default window-system-provided framebuffer. By using framebuffer object (FBO), an OpenGL application can redirect the rendering output to the application-created framebuffer object (FBO) other than the traditional window-system-provided framebuffer. And, it is fully controlled by OpenGL.

Similar to window-system-provided framebuffer, a FBO contains a collection of rendering destinations; color, depth and stencil buffer. (Note that accumulation buffer is not defined in FBO.) These logical buffers in a FBO are called framebuffer-attachable images, which are 2D arrays of pixels that can be attached to a framebuffer object.

There are two types of framebuffer-attachable images; texture images and renderbuffer images. If an image of a texture object is attached to a framebuffer, OpenGL performs "render to texture". And if an image of a renderbuffer object is attached to a framebuffer, then OpenGL performs "offscreen rendering".

By the way, renderbuffer object is a new type of storage object defined in GL_EXT_framebuffer_object extension. It is used as a rendering destination for a single 2D image during rendering process.

Connectivity among FBO, texture and Renderbuffer

The following diagram shows the connectivity among the framebuffer object, texture object and renderbuffer object. Multiple texture objects or renderbuffer objects can be attached to a framebuffer object through the attachment points.

There are multiple color attachment points (GL_COLOR_ATTACHMENT0_EXT,..., GL_COLOR_ATTACHMENTn_EXT), one depth attachment point (GL_DEPTH_ATTACHMENT_EXT), and one stencil attachment point (GL_STENCIL_ATTACHMENT_EXT) in a framebuffer object. The number of color attachment points is implementation dependent, but each FBO must have at least one color attachement point. You can query the maximum number of color attachement points with GL_MAX_COLOR_ATTACHMENTS_EXT, which are supported by a graphics card. The reason that a FBO has multiple color attachement points is to allow to render the color buffer to multiple destinations at the same time. This "multiple render targets"(MRT) can be accomplished by GL_ARB_draw_buffers extension. Notice that the framebuffer object itself does not have any image storage(array) in it, but, it has only multiple attachment points.

Framebuffer object (FBO) provides an efficient switching mechanism; detach the previous framebuffer-attachable image from a FBO, and attach a new framebuffer-attachable image to the FBO. Switching framebuffer-attachable images is much faster than switching between FBOs. FBO providesglFramebufferTexture2DEXT() to switch 2D texture objects, and glFramebufferRenderbufferEXT()to switch renderbuffer objects.

Creating Frame Buffer Object (FBO)

Creating framebuffer objects is similar to generating vertex buffer objects (VBO).

glGenFramebuffersEXT()

void glGenFramebuffersEXT(GLsizei n, GLuint* ids)
void glDeleteFramebuffersEXT(GLsizei n, const GLuint* ids)

glGenFramebuffersEXT() requires 2 parameters; the first one is the number of framebuffers to create, and the second parameter is the pointer to a GLuint variable or an array to store a single ID or multiple IDs. It returns the IDs of unused framebuffer objects. ID 0 means the default framebuffer, which is the window-system-provided framebuffer.
And, FBO may be deleted by calling glDeleteFramebuffersEXT() when it is not used anymore.

glBindFramebufferEXT()

Once a FBO is created, it has to be bound before using it.

void glBindFramebufferEXT(GLenum target, GLuint id)

The first parameter, target, should be GL_FRAMEBUFFER_EXT, and the second parameter is the ID of a framebuffer object. Once a FBO is bound, all OpenGL operations affect onto the current bound framebuffer object. The object ID 0 is reserved for the default window-system provided framebuffer. Therefore, in order to unbind the current framebuffer (FBO), use ID 0 in glBindFramebufferEXT().

Renderbuffer Object

In addition, renderbuffer object is newly introduced for offscreen rendering. It allows to render a scene directly to a renderbuffer object, instead of rendering to a texture object. Renderbuffer is simply a data storage object containing a single image of a renderable internal format. It is used to store OpenGL logical buffers that do not have corresponding texture format, such as stencil or depth buffer.

glGenRenderbuffersEXT()

void glGenRenderbuffersEXT(GLsizei n, GLuint* ids)
void glDeleteRenderbuffersEXT(GLsizei n, const Gluint* ids)

Once a renderbuffer is created, it returns non-zero positive integer. ID 0 is reserved for OpenGL.

glBindRenderbufferEXT()

void glBindRenderbufferEXT(GLenum target, GLuint id)

Same as other OpenGL objects, you have to bind the current renderbuffer object before referencing it. The target parameter should be GL_RENDERBUFFER_EXT for renderbuffer object.

glRenderbufferStorageEXT()

void glRenderbufferStorageEXT(GLenum target, GLenum internalFormat,
                              GLsizei width, GLsizei height)

When a renderbuffer object is created, it does not have any data storage, so we have to allocate a memory space for it. This can be done by using glRenderbufferStorageEXT(). The first parameter must be GL_RENDERBUFFER_EXT. The second parameter would be color-renderable (GL_RGB, GL_RGBA, etc.), depth-renderable (GL_DEPTH_COMPONENT), or stencil-renderable formats (GL_STENCIL_INDEX). The width and height are the dimension of the renderbuffer image in pixels.

The width and height should be less than GL_MAX_RENDERBUFFER_SIZE_EXT, otherwise, it generates GL_INVALID_VALUE error.

glGetRenderbufferParameterivEXT()

void glGetRenderbufferParameterivEXT(GLenum target, GLenum param, GLint* value);

You also get various parameters of the currently bound renderbuffer object. target should be GL_RENDERBUFFER_EXT, and the second parameter is the name of parameter. The last is the pointer to an integer variable to store the returned value. The available names of the renderbuffer parameters are;

GL_RENDERBUFFER_WIDTH_EXT
GL_RENDERBUFFER_HEIGHT_EXT
GL_RENDERBUFFER_INTERNAL_FORMAT_EXT
GL_RENDERBUFFER_RED_SIZE_EXT
GL_RENDERBUFFER_GREEN_SIZE_EXT
GL_RENDERBUFFER_BLUE_SIZE_EXT
GL_RENDERBUFFER_ALPHA_SIZE_EXT
GL_RENDERBUFFER_DEPTH_SIZE_EXT
GL_RENDERBUFFER_STENCIL_SIZE_EXT

Attaching images to FBO

FBO itself does not have any image storage(buffer) in it. Instead, we must attach framebuffer-attachable images (texture or renderbuffer objects) to the FBO. This mechanism allows that FBO quickly switch (detach and attach) the framebuffer-attachable images in a FBO. It is much faster to switch framebuffer-attachable images than to switch between FBOs. And, it saves unnecessary data copies and memory consumption. For example, a texture can be attached to multiple FBOs, and its image storage can be shared by multiple FBOs.

Attaching a 2D texture image to FBO

glFramebufferTexture2DEXT(GLenum target,
                          GLenum attachmentPoint,
                          GLenum textureTarget,
                          GLuint textureId,
                          GLint  level)

glFramebufferTexture2DEXT() is to attach a 2D texture image to a FBO. The first parameter must be GL_FRAMEBUFFER_EXT, and the second parameter is the attachment point where to connect the texture image. A FBO has multiple color attachment points (GL_COLOR_ATTACHMENT0_EXT, ..., GL_COLOR_ATTACHMENTn_EXT), GL_DEPTH_ATTACHMENT_EXT, and GL_STENCIL_ATTACHMENT_EXT. The third parameter, "textureTarget" is GL_TEXTURE_2D in most cases. The fourth parameter is the identifier of the texture object. The last parameter is the mipmap level of the texture to be attached.

If the textureId parameter is set to 0, then, the texture image will be detached from the FBO. If a texture object is deleted while it is still attached to a FBO, then, the texture image will be automatically detached from the currently bound FBO. However, if it is attached to multiple FBOs and deleted, then it will be detached from only the bound FBO, but will not be detached from any other un-bound FBOs.

Attaching a Renderbuffer image to FBO

void glFramebufferRenderbufferEXT(GLenum target,
                                  GLenum attachmentPoint,
                                  GLenum renderbufferTarget,
                                  GLuint renderbufferId)

A renderbuffer image can be attached by calling glFramebufferRenderbufferEXT(). The first and second parameters are same as glFramebufferTexture2DEXT(). The third parameter must be GL_RENDERBUFFER_EXT, and the last parameter is the ID of the renderbuffer object.

If renderbufferId parameter is set to 0, the renderbuffer image will be detached from the attachment point in the FBO. If a renderbuffer object is deleted while it is still attached in a FBO, then it will be automatically detached from the bound FBO. However, it will not be detached from any other non-bound FBOs.

Checking FBO Status

Once attachable images (textures and renderbuffers) are attached to a FBO and before performing FBO operation, you must validate if the FBO status is complete or incomplete by usingglCheckFramebufferStatusEXT(). If the FBO is not complete, then any drawing and reading command (glBegin(), glCopyTexImage2D(), etc) will be failed.

GLenum glCheckFramebufferStatusEXT(GLenum target)

glCheckFramebufferStatusEXT() validates all its attached images and framebuffer parameters on the currently bound FBO. And, this function cannot be called within glBegin()/glEnd() pair. The target parameter should be GL_FRAMEBUFFER_EXT. It returns non-zero value after checking the FBO. If all requirements and rules are satisfied, then it returns GL_FRAMEBUFFER_COMPLETE_EXT. Otherwise, it returns a relevant error value, which tells what rule is violated.

The rules of FBO completeness are:

• The width and height of framebuffer-attachable image must be not zero.
• If an image is attached to a color attachment point, then the image must have a color-renderable internal format. (GL_RGBA, GL_DEPTH_COMPONENT, GL_LUMINANCE, etc)
• If an image is attached to GL_DEPTH_ATTACHMENT_EXT, then the image must have a depth-renderable internal format. (GL_DEPTH_COMPONENT, GL_DEPTH_COMPONENT24_EXT, etc)
• If an image is attached to GL_STENCIL_ATTACHMENT_EXT, then the image must have a stencil-renderable internal format. (GL_STENCIL_INDEX, GL_STENCIL_INDEX8_EXT, etc)
• FBO must have at least one image attached.
• All images attached a FBO must have the same width and height.
• All images attached the color attachment points must have the same internal format.

Note that even though all of the above conditions are satisfied, your OpenGL driver may not support some combinations of internal formats and parameters. If a particular implementation is not supported by OpenGL driver, then glCheckFramebufferStatusEXT() returns GL_FRAMEBUFFER_UNSUPPORTED_EXT.