Screen-Space Grass - GPU Pro: Advanced Rendering Techniques - page 224

Figure 1.3. Base smudge grass noise function producing straight vertical lines.
Figure 1.4. Regular terrain rendering on the left. Screen-Space Grass rendering used
in Agricultural Simulator 2013 on the right.
The terrain pixels on the upper edge of the stencil masked area are going to
be smudged by pixels outside of the stencil masked space, creating the effect of
grass blades on the top of terrain ridges, as in Figure 1.5.
1.3.2 Is It Really Grass?
The whole terrain is not necessarily composed of grass. We would also like to
be able to render roads, dirt and sand patches, and other areas that are not
covered in grass. We would also like to support objects that are rendered on top
of the grassy terrain. Such objects are, of course, masked by a stencil; however,
the grass smudge effect can mistake them for grass pixels ( source pixel )evenif
they are just outside the stencil marked area (stencil-marked-area pixels are only
guaranteed not to be the target pixels for the SSG effect). See Figure 1.6.