OpenGL Programming Guide

Chapter 7Blending, Antialiasing, and Fog

Blending

You've already seen alpha values (alpha is the A in RGBA), but they've always been 1.0, and they haven't been discussed. Alpha values are specified with glColor*(), when using glClearColor() to specify a clearing color, and when specifying certain lighting parameters such as a material property or light-source intensity. As you learned in Chapter 5 , the pixels on a monitor screen emit red, green, and blue light, which is controlled by the red, green, and blue color values. So how does an alpha value affect what gets drawn in a window on the screen? When blending is enabled, the alpha value is used to combine the color value of the fragment being processed with that of the pixel already stored in the framebuffer. Blending occurs after your scene has been rasterized and converted to fragments, but just before the final pixels are drawn in the framebuffer. Alpha values can also be used in the alpha test to accept or reject a fragment based on its alpha value. See Chapter 10 for more information about this process.

Without blending, each new fragment overwrites any existing color values in the framebuffer, as though the fragment is opaque. With blending, you can control how much of the existing color value should be combined with the new fragment's value. Thus, you can use alpha blending to create a translucent fragment, one that lets some of the previously stored color value "show through." Color blending lies at the heart of techniques such as transparency, digital compositing, and painting.

Alpha values aren't specified in color-index mode. Thus, blending operations aren't performed in color-index mode

The most natural way for you to think of blending operations is to view the RGB components of a fragment as representing its color, and the alpha component as representing opacity. Thus, transparent or translucent surfaces have lower opacity than opaque ones. For example, if you're viewing an object through green glass, the color you see is partly green from the glass and partly the color of the object. The percentage varies depending on the transmission properties of the glass: If the glass transmits 80 percent of the light that strikes it (that is, has an opacity of 20 percent), the color you see is a combination of 20 percent glass color and 80 percent of the color of the object behind it. You can easily imagine situations with multiple translucent surfaces. If you look at an automobile, for instance, its interior has one piece of glass between it and your viewpoint; some objects behind the automobile are visible through two pieces of glass.

The Source and Destination Factors
Sample Uses of Blending
A Blending Example
Three-Dimensional Blending with the Depth Buffer

OpenGL Programming Guide