消除“顶点着色器”中的“顶点”歧义

Question

https://www.khronos.org/opengl/wiki/Vertex_Shader says that "The vertex shader will be executed roughly once for every vertex in the stream."

If we are rendering a cube, vertex could refer to the 8 vertexes of the entire shape (meaning One). Or, it could refer to the 24 vertexes of the 6 squares with 4 corners each (meaning Two).

As I understand it, if a cube is being rendered, the 8 corners of the cube have to be converted into the coordinate system of the viewer. But also there are texture coordinates that have to be calculated based on the individual textures associate with each face of the cube.

So if "vertex" is intended by meaning One, then why are textures being supplied to a shader which is a per face concept? Or if "vertexes" are being fed to the shader by meaning two, does that mean that the coordinate transforms and projections are all being done redundantly? Or is something else going on? These guides seem to have an allergy to actually saying what is going on.

Answer 1

The page on Vertex Specification could be a bit more clear on this, but a vertex is just a single index in the set of vertex arrays as requested by a rendering operation.

That is, you render some number of vertices from your vertex arrays. You could be specifying these vertices as a range of indices from a given start index to an end index ( glDrawArrays ), or you could specify a set of indices in order to use ( glDrawElements ). Regardless of how you do it, you get one vertex for each datum specified by your rendering command. If you render 10 indices with indexed rendering, you get 10 vertices.

Each vertex is composed of data fetched from the active vertex arrays in the currently bound VAO. Given the index for that vertex, a value is fetched from each active array at that index. Each individual array feeds a vertex attribute, which is passed to the vertex shader.

A vertex shader operates on the attributes of a vertex (passed as in qualified variables).

The relationship between a "vertex" and your geometry's vertices is entirely up to you. Vertices usually include positions as part of their vertex attributes, but they usually also include other stuff. The only limitation is that the value fetched for each attribute of a particular vertex always uses the same vertex index.

How you live within those rules is up to you and your data.