What can vertex function do except for mapping to clip space?

Question

The Metal Shading Language includes a lot of mathematic functions, but it seems most of the codes inside Metal official documentation just use it to map vertexes from pixel space to clip space like

RasterizerData out;

out.clipSpacePosition = vector_float4(0.0, 0.0, 0.0, 1.0);

float2 pixelSpacePosition = vertices[vertexID].position.xy;

vector_float2 viewportSize = vector_float2(*viewportSizePointer);

out.clipSpacePosition.xy = pixelSpacePosition / (viewportSize / 2.0);

out.color = vertices[vertexID].color;

return out;

Except for GPGPU using kernel functions to do parallel computation, what things that vertex function can do, with some examples? In a game, if all vertices positions are calculated by the CPU, why GPU still matters? What does vertex function do usually?

Answer 1

Vertex shaders compute properties for vertices. That's their point. In addition to vertex positions, they also calculate lighting normals at each vertex. And potentially texture coordinates. And various material properties used by lighting and shading routines. Then, in the fragment processing stage, those values are interpolated and sent to the fragment shader for each fragment.

In general, you don't modify vertices on the CPU. In a game, you'd usually load them from a file into main memory, put them into a buffer and send them to the GPU. Once they're on the GPU you pass them to the vertex shader on each frame along with model, view, and projection matrices. A single buffer containing the vertices of, say, a tree or a car's wheel might be used multiple times. Each time all the CPU sends is the model, view, and projection matrices. The model matrix is used in the vertex shader to reposition and scale the vertice's positions in world space. The view matrix then moves and rotates the world around so that the virtual camera is at the origin and facing the appropriate way. Then the projection matrix modifies the vertices to put them into clip space.

There are other things a vertex shader can do, too. You can pass in vertices that are in a grid in the x-y plane, for example. Then in your vertex shader you can sample a texture and use that to generate the z-value. This gives you a way to change the geometry using a height map.

On older hardware (and some lower-end mobile hardware) it was expensive to do calculations on a texture coordinate before using it to sample from a texture because you lose some cache coherency. For example, if you wanted to sample several pixels in a column, you might loop over them adding an offset to the current texture coordinate and then sampling with the result. One trick was to do the calculation on the texture coordinates in the vertex shader and have them automatically interpolated before being sent to the fragment shader, then doing a normal look-up in the fragment shader. (I don't think this is an optimization on modern hardware, but it was a big win on some older models.)

Answer 2

First, I'll address this statement

In a game, if all vertices positions are calculated by the CPU, why GPU still matters? What does vertex function do usually?

I don't believe I've seen anyone calculating positions for meshes that will be later used to render them on a GPU. It's slow, you would need to get all this data from CPU to a GPU (which means copying it through a bus if you have a dedicated GPU). And it's just not that flexible. There are much more things other than vertex positions that are required to produce any meaningful image and calculating all this stuff on CPU is just wasteful, since CPU doesn't care for this data for the most part.

The sole purpose of vertex shader is to provide rasterizer with primitives that are in clip space. But there are some other uses that are mostly tricks based on different GPU features.

For example, vertex shaders can write out some data to buffers, so, for example, you can stream out transformed geometry if you don't want to transform it again at a later vertex stage if you have multi-pass rendering that uses the same geometry in more than one pass.

You can also use vertex shaders to output just one triangle that covers the whole screen, so that fragment shaders gets called one time per pixel for the whole screen (but, honestly, you are better of using compute (kernel) shaders for this).

You can also write out data from vertex shader and not generate any primitives. You can do that by generating degenerate triangles. You can use this to generate bounding boxes. Using atomic operations you can update min/max positions and read them at a later stage. This is useful for light culling, frustum culling, tile-based processing and many other things.

But, and it's a BIG BUT, you can do most of this stuff in a compute shader without incurring GPU to run all the vertex assembly pipeline. That means, you can do full-screen effects using just a compute shader (instead of vertex and fragment shader and many pipeline stages in between, such as rasterizer, primitive culling, depth testing and output merging). You can calculate bounding boxes and do light culling or frustum culling in compute shader.

There are reasons to fire up the whole rendering pipeline instead of just running a compute shader, for example, if you will still use triangles that are output from vertex shader, or if you aren't sure how primitives are laid out in memory so you need vertex assembler to do the heavy lifting of assembling primitives. But, getting back to your point, almost all of the reasonable uses for vertex shader include outputting primitives in clip space. If you aren't using resulting primitives, it's probably best to stick to compute shaders.