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15-compositing.md

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Compositing

  • Both the Color Buffer and the Depth Buffer have limited resolution — both spatially and in depth
  • 2D buffer is a block of memory with nm k bit elements
  • Frame Buffer refers to the set of buffers the graphics system uses for rendering
  • At a given spatial location in the frame buffer, the k bits can include 32-bit RGBA colors, floats representing depths, or bits that can be used for masks
  • Compositing — when fragments from multiple objects can can contribute to the same color pixel

Opacity and Blending

  • The opacity of a surface is a measure of how much light penetrates through that surface
  • In RGBA mode, if blending is enabled, the value of A controls how the RGB values are written into the frame buffer
  • As a polygon is processed, pixel-sized fragments are computed and if they are visible are assigned colors based on the shading model in use
  • If we regard the fragment as the source pixel and the frame-buffer pixel as the destination, we can combine these values in various ways
  • s = [s_r, s_g, s_b, s_a] (source)
  • d = [d_r, d_g, d_b, d_a] (destination)
  • b = [b_r, b_g, b_b, b_a] (source blend factor)
  • c = [c_r, c_g, c_b, c_a] (destination blend factor)
  • d' = [b_r×s_r + c_r×d_r, b_g×s_g + c_g×d_g, b_b×s_b + c_b×d_b, b_a×s_a + c_a×d_a] (final destination)
  • Many applications use the source's alpha as the source blending factor, and those (1 - source alpha) for the destinatio factor — this ensure that neither colors nor opacities can saturate
  • Enabled with:
    • glEnable(GL_BLEND)
    • glBlendFunc(source_factor, destination_factor)
  • Need to override the Z-buffer algorithm to enable this blending
    • glDepthMask(GL_FALSE)
    • This function makes the depth buffer read-only for any polygon that is translucent
    • When the depth buffer is read-only, a translucent polygon that lies behind any opaque polygon already rendered is discarded
    • A translucent polygon that lies in front of any polygon that has already been rendered is blended with the color of the polygons it is in front of
    • However, because the Z-buffer is read-only for this polygon, the depth values in the buffer are unchanged