 <\/td><\/tr><\/table><\/p>\nNot to scale. Red is the sparse mesh, blue is the medium one, and green is the final one to fill in the gaps. The default grass mesh is just a cluster of white polygon X’sThat is, two upright panels, like a pair of intersecting billboards.<\/span>, all resting at the same height, all exactly one unit tall. I add this texture:<\/p>\n <\/td><\/tr><\/table><\/p>\nNow we make another little shader. Every frame, it grabs this mesh and shifts it so that it’s always under you. In one of my surface textures I’ve packed some data for this shader to use. One field says “draw your grass texture from this region of the texture”. Another says, “Adjust the panel height to be height N.” It also draws from the color map I made earlier in the project to color the grass, and uses the heightmap to move the grass up to match the terrain.<\/p>\n The result?<\/p>\n  <\/td><\/tr><\/table><\/p>\nYeah. The colors are kind of random. I was trying to make sure it was drawing the proper colors from the right parts of the texture, and the best way to do that was to make the color variations drastic. <\/p>\n Note the variations in height and texture. We’re just drawing the same three grass meshes over and over again every frame. So no sorting. No alpha-blending. No worrying about draw order. We don’t need to edit the mesh when the player moves around, or add or remove tufts of grass if they go to someplace that has more or less foliage. We’ve offloaded everything onto the GPU. The CPU is just throwing static models at the graphics card and then taking a nap, because there’s nothing to do.<\/p>\n So what happens when you’re someplace that doesn’t need grass? As it’s rendering, it looks up in the texture and sees that there’s not supposed to be any grass here. So it scales the grass down so that it’s exactly zero tall. Then it renders it anyway, because vertex shaders are stupid like that. If they were smarter, they’d be slower. I guess it’s sort of like a fly having reflexes an order of magnitude better than a human. Simplicity makes for speed. <\/p>\n Rendering a zero-tall tuft of grass isn’t a big deal. It ends up drawing a couple of lines, which are usually tucked under the terrain anyway. <\/p>\n  <\/td><\/tr><\/table><\/p>\nI’ve mentioned before that there are two major kinds of shaders: Vertex shaders and fragment shadersThe third type is a GeoMetry Shader. I’ve been looking for an excuse to do something with them. More on this later.<\/span>. (I would love to know what they’re called “fragment” shaders in technical documents and not just “pixel shaders”. The latter is much more descriptive and easier to grasp. This is probably some bit of nomenclature that makes more sense if you’re spend the last decade fooling around on the driver and hardware level, and not on the software level where I usually tinker.) A verttex takes the corners of a polygon and puts them into place for rendering. The fragment<\/del> pixel shader then fills in the pixels.<\/p>\nLet’s say I’ve got a couple of shaders. The vertex shader figures out where each vertex ends up on screen. Then the pixel shader comes along and fills the space defined by that polygon. And because I’m not feeling really ambitious, let’s say all my pixel shader does is output blue pixels. <\/p>\n  <\/td><\/tr><\/table><\/p>\nNitpick shield: Yes, this would actually be two triangles and not a quad, but I’m trying to keep this visually simple.<\/p>\n The vertex shader would run four times, once for each vertex. The pixel shader would run… I dunno. I’m not counting those blue pixels. Let’s say about plentylots<\/sup>. Obviously the GPU spends a lot more time chewing on pixels than shuffling vertices. In the long run, processing those zero-height grass tufts just isn’t a big deal. They come down to a few vertex transforms and (essentially) a single line of pixels. It really is faster to mindlessly attempt to draw everything than to spend time avoiding drawing stuff we don’t want. <\/p>\nMaybe I should add another bunch of polygons for flowers:<\/p>\n |
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