OpenGL Programming Guide

Chapter 7Blending, Antialiasing, and Fog

Blending

Sample Uses of Blending

Not all of the combinations of source and destination factors make sense. The majority of applications use a small number of combinations. The following paragraphs describe typical uses for particular combinations of the source and destination factors. Some of these examples use only the incoming alpha value, so they work even when alpha values aren't stored in the framebuffer. Also, note that often there's more than one way to achieve some of these effects.

1. One way to draw a picture composed half of one image and half of another, equally blended, is to set the source factor to GL_ONE, draw the first image, then set the source and destination factors to GL_SRC_ALPHA, and draw the second image with alpha equal to 0.5. If the picture is supposed to be blended with 0.75 of the first image and 0.25 of the second, draw the first image as before, and draw the second with an alpha of 0.25, but with GL_SRC_ALPHA (source) and GL_ONE_MINUS_SRC_ALPHA (destination). This pair of factors probably represents the most commonly used blending operation.
   
   

2. To blend three different images equally, set the destination factor to GL_ONE and the source factor to GL_SRC_ALPHA. Draw each of the images with an alpha equal to 0.3333333. With this technique, each image is only one-third of its original brightness, which is noticeable where the images don't overlap.
   
   

3. Suppose you're writing a paint program, and you want to have a brush that gradually adds color so that each brush stroke blends in a little more color with whatever is currently in the image (say 10 percent color with 90 percent image on each pass). To do this, draw the image of the brush with alpha of 10 percent and use GL_SRC_ALPHA (source) and GL_ONE_MINUS_SRC_ALPHA (destination). (Note that you can vary the alphas across the brush to make the brush add more of its color in the middle and less on the edges, for an antialiased brush shape. See "Antialiasing." ) Similarly, erasers can be implemented by setting the eraser color to the background color.
   
   

4. The blending functions that use the source or destination colors - GL_DST_COLOR or GL_ONE_MINUS_DST_COLOR for the source factor and GL_SRC_COLOR or GL_ONE_MINUS_SRC_COLOR for the destination factor - effectively allow you to modulate each color component individually. This operation is equivalent to applying a simple filter - for example, multiplying the red component by 80 percent, the green component by 40 percent, and the blue component by 72 percent would simulate viewing the scene through a photographic filter that blocks 20 percent of red light, 60 percent of green, and 28 percent of blue.
   
   

5. Suppose you want to draw a picture composed of three translucent surfaces, some obscuring others and all over a solid background. Assume the farthest surface transmits 80 percent of the color behind it, the next transmits 40 percent, and the closest transmits 90 percent. To compose this picture, draw the background first with the default source and destination factors, and then change the blending factors to GL_SRC_ALPHA (source) and GL_ONE_MINUS_SRC_ALPHA (destination). Next, draw the farthest surface with an alpha of 0.2, then the middle surface with an alpha of 0.6, and finally the closest surface with an alpha of 0.1.

   Advanced
   
   

6. If your system has alpha planes, you can render objects one at a time (including their alpha values), read them back, and then perform interesting matting or compositing operations with the fully rendered objects. See "Compositing 3D Rendered Images" by Tom Duff, SIGGRAPH 1985 Proceedings, p. 41-44, for examples of this technique. Note that objects used for picture composition can come from any source - they can be rendered using OpenGL commands, rendered using techniques such as ray-tracing or radiosity that are implemented in another graphics library, or obtained by scanning in existing images.

   Advanced
   
   

7. You can create the effect of a nonrectangular raster image by assigning different alpha values to individual fragments in the image. Assign an alpha of 0 to each "invisible" fragment, and an alpha of 1.0 to each opaque fragment. For example, you can draw a polygon in the shape of a tree and apply a texture map of foliage; the viewer can see through parts of the rectangular texture that aren't part of the tree if you've assigned them alpha values of 0. This method, sometimes called billboarding, is much faster than creating the tree out of three-dimensional polygons. An example of this technique is shown in Figure 7-1 : The tree is a single rectangular polygon that can be rotated about the center of the trunk, as shown by the outlines, so that it's always facing the viewer. See "Modulating and Blending" for more information about blending textures.

Figure 7-1 : Creating a Nonrectangular Raster Image

OpenGL Programming Guide