16.2.4.1 Over

The over operator [79] is the most common way to blend for volume visualization. Volumes blended with the over operator approximate the flow of light through a colored, transparent material. The transparency of each point in the material is determined by the value of the texel's alpha channel. Texels with higher alpha values tend to obscure texels behind them, and stand out through the obscuring texels in front of them.

The over operator can be implemented in OpenGL by setting the blend function to perform the over operation:

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)