Native bindings to the EXT_depth_bounds_test extension.

This extension adds a new per-fragment test that is, logically, after the scissor test and before the alpha test. The depth bounds test compares the depth value stored at the location given by the incoming fragment's (xw,yw) coordinates to a user-defined minimum and maximum depth value. If the stored depth value is outside the user-defined range (exclusive), the incoming fragment is discarded.

Unlike the depth test, the depth bounds test has NO dependency on the fragment's window-space depth value.

This functionality is useful in the context of attenuated stenciled shadow volume rendering. To motivate the functionality's utility in this context, we first describe how conventional scissor testing can be used to optimize shadow volume rendering.

If an attenuated light source's illumination can be bounded to a rectangle in XY window-space, the conventional scissor test can be used to discard shadow volume fragments that are guaranteed to be outside the light source's window-space XY rectangle. The stencil increments and decrements that would otherwise be generated by these scissored fragments are inconsequential because the light source's illumination can pre-determined to be fully attenuated outside the scissored region. In other words, the scissor test can be used to discard shadow volume fragments rendered outside the scissor, thereby improving performance, without affecting the ultimate illumination of these pixels with respect to the attenuated light source.

This scissoring optimization can be used both when rendering the stenciled shadow volumes to update stencil (incrementing and decrementing the stencil buffer) AND when adding the illumination contribution of attenuated light source's.

In a similar fashion, we can compute the attenuated light source's window-space Z bounds (zmin,zmax) of consequential illumination. Unless a depth value (in the depth buffer) at a pixel is within the range [zmin,zmax], the light source's illumination can be pre-determined to be inconsequential for the pixel. Said another way, the pixel being illuminated is either far enough in front of or behind the attenuated light source so that the light source's illumination for the pixel is fully attenuated. The depth bounds test can perform this test.