GLSL Programming/GLUT/Order-Independent Transparency

This tutorial covers order-independent blending.

It continues the discussion of the tutorial on transparency and solves some problems of standard transparency. If you haven't read that tutorial, you should read it first.

Order-Independent Blending
As noted in the tutorial on transparency, the result of blending often (in particular for standard alpha blending) depends on the order in which triangles are rendered and therefore results in rendering artifacts if the triangles are not sorted from back to front (which they usually aren't). The term “order-independent transparency” describes various techniques to avoid this problem. One of these techniques is order-independent blending, i.e. the use of a blend equation that does not depend on the order in which triangles are rasterized. There two basic possibilities: additive blending and multiplicative blending.

Additive Blending
The standard example for additive blending are double exposures as in the images on this page: colors are added such that it is impossible (or at least very hard) to say in which order the photos were taken. Additive blending can be characterized in terms of the blend equation introduced in the tutorial on transparency:

where  and   are set by a function of OpenGL:

{code for },{code for  }

For additive blending, the code for  has to be   and the code for   must not depend on the pixel color in the framebuffer; i.e., it can be ,  ,  ,  , or.

An example is: Remember that you have to set Viewport Shading to Texture in the menu of a 3D View in order to activate blending. Also, you should activate Z Transparency and deactivate Backface Culling as described in the tutorial on transparency.

Multiplicative Blending
An example for multiplicative blending in photography is the use of multiple uniform grey filters: the order in which the filters are put onto a camera doesn't matter for the resulting attenuation of the image. In terms of the rasterization of triangles, the image corresponds to the contents of the framebuffer before the triangles are rasterized, while the filters correspond to the triangles.

When specifying mutiplicative blending in OpenGL with

{code for },{code for  }

the code for  has to be   and the code for   must depend on the fragment color; i.e., it can be ,  ,  , or. A typical example for attenuating the background with the opacity specified by the alpha component of fragments would use  for the code for  :

Summary
Congratulations, you have reached the end of this tutorial. We have looked at:
 * What order-independent transparency and order-independent blending is.
 * What the two most important kinds of order-independent blending are (additive and multiplicative).
 * How to implement additive and multiplicative blending.