Blending, Antialiasing, and Fog, and Polygon Offset A mixed bag of topics: Blending of colors to achieve translucency Smoothing jagged edges of lines and polygons Producing realistic atmospheric effects Rendering co-planar geometry    We've covered the basics of geometry - let's get a little fancy

Blending Up to this point the alpha component (A of RGBA) has been ignored Alpha values are specified with glColor() and glClearColor()    When blending is enabled alpha value determines the fraction that a pixel's color contributes to the value already in the framebuffer Without blending polygons are considered opaque With blending objects can appear transparent NOTE!  No alpha blending in color-index mode

	Source and Destination Factors      During blending, the  incoming fragment (source) is added to the existing framebuffer value (the destination)  Final Pixel =      (R s S r +R d D r , G s S g +G d D g , B s S b +B d D b , A s S a +A d D a ) All values are clamped to 1.0

Generating blending factors Blending must be enabled with glEnable(GL_BLEND)   glBlendFunc() defines two constraints: How the source factor should be computed How the destination factor should be computed void glBlendFunc(GLenum sfactor, GLenum dfactor);  Controls how color values in the fragment being processed (the source) are combined with those already stored in the framebuffer (the destination).  The argument sfactor indicates how to compute a source blending factor; dfactor indicates how to compute a destination blending factor.  The blend factors are assumed to lie in the range [0,1];  Values are clamped to the range [0,1].

	Parameters for glBlend()

	Sample uses of Blending Not all combinations make sense Sometimes there's more than one way to do something    Mixing polygons of two different transparencies Source = GL_ONE  Dest = GL_ZERO Set alpha = 0.25 Draw first object Source = GL_SOURCE_ALPHA Dest = GL_ONE_MINUS_SRC_ALPHA Set alpha = 0.75 Draw second object

	Sample uses of Blending (cont) Two blend 3 objects equally: Set the destination = GL_ONE Set the source = GL_ALPHA Draw each of the objects with alpha = 0.33333

	Sample uses of Blending (cont) Billboarding

	Three-Dimensional Blending  The ORDER in which polygons are drawn greatly affects the final outcome F->B vs B->F can greatly affect the final image      This is especially important for volume rendering

	Use of the Depth Buffer for Translucency For a static viewpoint, just sort the objects B->F Lots of ways (calc distance, then quicksort) For a dynamic viewpoint: Draw all opaque objects w/Z-buffer Make the Z-buffer read only Draw the translucent objects Objects behind opaque objects (won't be drawn) Objects in front of opaque objects will be blended