Alpha blending combines the colour of the current primitive with the colour already in the frame buffer.

Terminology:

• The colour value already stored in the frame buffer is the destination colour (Cd)
• The colour value that is coming into the frame buffer is the source colour (Cs)
• By default, the blending equation is

In the blending equation, s and d are blending factors, that describe how much of the source and destination colours to use when producing the blended result.

In OpenGL, the blending factors are enumerants, not direct values.  The blending factors are set with

glBlendFunc (GLenum s, GLenum d);

There are many possible values s and d can take.  Some of the common ones include

• GL_ONE
• GL_ZERO
• GL_SRC_ALPHA
• GL_ONE_MINUS_SRC_ALPHA

Perhaps the most common blending function is

glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

This tells OpenGL to take the source colour and multiply it by the source alpha value.  The destination colour is multiplied by one minus the source alpha.  The result is the sum of these two terms.

This blending function is often used to render a transparent object in front of an opaque object.  To get the desired effect,

• Draw the opaque objects first before drawing the transparent ones
• If you’re rendering back-facing polygons, it may be necessary to draw the transparent polygons far to near after depth sorting the polygons (based on distance to the camera).  This is called the painter’s algorithm.
• Otherwise, use glEnable(GL_CULL_FACE) to eliminate back-facing triangles.

High