3D Blending with the Depth Buffer

You also need to consider the effect of the depth buffer when determining the correct order. The depth buffer (sometimes called the z-buffer) is usually used for hidden-surface elimination. It keeps track of the distance between the viewpoint and the portion of the object occupying a given pixel in a window on the screen; when another candidate color arrives for that pixel, it is drawn only if its object is closer to the viewpoint, in which case its depth value is stored in the depth buffer. With this method, obscured (or hidden) portions of surfaces aren't necessarily drawn and therefore aren't used for blending.

Typically, you want to render both opaque and translucent objects in the same scene, and you want to use the depth buffer to perform hidden-surface removal for objects that lie behind the opaque objects. If an opaque object hides either a translucent object or another opaque object, you want the depth buffer to eliminate the more distant object. If the translucent object is closer, however, you want to blend it with the opaque object. You can generally figure out the correct order to draw the polygons if everything in the scene is stationary, but the problem can easily become too hard if either the viewpoint or the object is moving.

The solution is to enable depth-buffering but make the depth buffer read-only while drawing the translucent objects. First you draw all the opaque objects, with the depth buffer in normal operation. Then, you preserve these depth values by making the depth buffer read-only. When the translucent objects are drawn, their depth values are still compared to the values established by the opaque objects, so they aren't drawn if they're behind the opaque ones. If they're closer to the viewpoint, however, they don't eliminate the opaque objects, since the depth-buffer values can't change. Instead, they're blended with the opaque objects.
To control whether the depth buffer is writable, use glDepthMask(); if you pass GL_FALSE as the argument, the buffer becomes read-only, whereas GL_TRUE restores the normal, writable operation.