I have a texture 2048x2048.
I want to process the texture line by line in parallel. But I dun know how to program my shader program.
Shader can be used to process each vertex in parallel. But I dun understand how it works? Can anyone explain further?

The basis for understanding how shaders work is to imagine that your graphics card has *hundreds* of teeny-tiny processors in it. When you hand a list of verticies off to the vertex shader, each processor works on ONE of the verticies. On their own.

Likewise, each pixel processor running your pixel shader gets to work on ONE of the output pixels -- thousands of them in each triangle.

So those programs need to written as if they're the only thing happening. In other words; you don't need to worry about the "line-by-line" thing. You only need to worry about what happens for each individual pixel.

Could you perhaps elaborate a little more on what it is you're trying to achieve?