Blending and Transparency

　　Blending in OpenGL provides pixel-level control of RGBA color storage in the color buffer. To enable blending we must first call glEnable(GL_BLEND). We have to set up the blending function glBlendFunc with two arguments: the source and the destination colors. By default these are GL_ONE and GL_ZERO respectively, which is equivalent to glDisable(GL_BLEND). The blend functions are applied to the source color set by glColor and destination color in the color buffer. The results of the blending functions are added together to generate the new color value which is put onscreen.

　　Transparency is perhaps the most typical use of blending. In order to produce transparency we should set up the blending function as follows - glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA). This combination takes the source color, scales it based on the alpha component and then adds the destination pixel color scaled by 1 minus the alpha value. It basically takes a fraction of the current drawing color and overlays it on the pixel on the screen. The alpha component can be from 0 (completely transparent) to 1 (completely opaque).

　　Anti-Aliasing

　　You might have noticed in some of your OpenGL pictures that lines, especially nearly horizontal or nearly vertical ones, appear jagged. These jaggies appear because the ideal line is approximated by a series of pixels that must lie on the pixel grid. The jaggedness is called aliasing. Antialiasing can be enabled by calling the function - glEnable(GL_POLYGON_SMOOTH) for polygons.