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

Order-Independent Transparency

Order-independent transparency is a technique where blending operations are carried out in a manner such that rasterization order is not important. The fixed function blending provided by OpenGL through functions such as glBlendEquation() and glBlendFunc() are fairly restrictive. They provide a small, fixed set of operations that may be performed, most of which are not commutative. That is, the order of their operations matters—blend(a, blend(b, c)) does not produce the same result as blend(blend(a, b), c). This means that geometry must be sorted into a fixed, known order before being rendered. This can be very time-consuming, especially for complex geometry and in some cases, such as where geometry may self-intersect, ...