OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Blending

Once an incoming fragment has passed all of the enabled fragment tests, it can be combined with the current contents of the color buffer in one of several ways. The simplest way, which is also the default, is to overwrite the existing values, which admittedly isn’t much of a combination. Alternatively, you might want to combine the color present in the framebuffer with the incoming fragment color—a process called blending. Most often, blending is associated with the fragment’s alpha value (or commonly just alpha), but that’s not a strict requirement. We’ve mentioned alpha several times but haven’t given it a proper description. Alpha is the fourth color component, and all colors in OpenGL have an alpha value (even if you don’t explicitly ...