Frame Buffer Object

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Frame Buffer Object

• CG2 Repetitorium 2, SS08Veronika Solteszova

Institute of Computer Graphics and
Algorithms Vienna University of Technology
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What for?

• What is normally rendered to the screen
• ?
• Save as a texture (with color and depth
  attachments)
• Use the texture in further passes (shadow map,
  mini-map, showing different views of the scene

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What is an FBO actually ?

• FBO is an encapsulation of attachments
• Attachments can be color or renderbuffers
• Renderbuffers are objects that support off-screen
  rendering without an assigned texture
• Depth buffer and stencil buffer
• There can be up to 8 color attachments
• Number depends on your HW
• More than one is advanced stuff
• Wont deal with it too much

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Setup FBO

• Some code you have to use for setup
• Create and validate its handle
• Bind it

GLuint fbo glGenFramebuffersEXT(1, fbo)
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fbo)
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Setup renderbuffer (depth buffer)

• Attaching renderable objects (like depth buffer -
  DB)
• Create its handle
• Bind it

GLuint depthbuffer glGenRenderbuffersEXT(1,
depthbuffer)
glBindRenderbufferEXT(GL_RENDERBUFFER_EXT,
depthbuffer)
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Creating storage space

• We define its size
• Last creators job attaching DB to FBO

glRenderbufferStorageEXT(GL_RENDERBUFFER_EXT,
GL_DEPTH_COMPONENT, width, height)
glFramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT,
GL_DEPTH_ATTACHMENT_EXT, GL_RENDERBUFFER_EXT,
depthbuffer)
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Attaching a texture T to render to

• Assumption a handle to a texture
• NOTE width and height are the same as those of
  FBO!

GLuint img glGenTextures(1, img)
glBindTexture(GL_TEXTURE_2D, img)
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, width,
height, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL)
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Attaching a texture T to render to

• We set the target of the FBO

glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT,
GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, img, 0)
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Status checking

• If everything worked out correctly

GLenum status glCheckFramebufferStatusEXT(GL_FRA
MEBUFFER_EXT)
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Rendering to the texture T

• Bind render unbind
• // render scene
• Note keep the same height and width of T for the
  viewport size

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fbo)
glPushAttrib(GL_VIEWPORT_BIT) glViewport(0,0,wid
th, height)
glPopAttrib() glBindFramebufferEXT(GL_FRAMEBUFFE
R_EXT, 0)
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Using the texture T

• Just bind it like a regular texture
• Note dont forget about texture filtering stuff!

glBindTexture(GL_TEXTURE_2D, img)
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And not only that

• With an FBO, you can render into more than 1
  textures simultaneously
• Check the tutorials about more color attachments
  and DrawBuffers extension at www.gamedev.net
• http//www.gamedev.net/reference/programming/featu
  res/fbo2/

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Cleaning up (the mess?)

• Clean up, unbinding, deleting is done by
• Cleaning of render buffers
• Delete also all textures associated with FBO
• Source www.gamedev.net

glDeleteFramebuffersEXT(1, fbo)
glDeleteRenderbuffersEXT(1, depthbuffer)