Vertex shader vs Fragment Shader

I've read some tutorials regarding Cg, yet one thing is not quite clear to me. What exactly is the difference between vertex and fragment shaders? And for what situations is one better suited than the other?

A fragment shader is the same as pixel shader.

One main difference is that a vertex shader can manipulate the attributes of vertices. which are the corner points of your polygons.

The fragment shader on the other hand takes care of how the pixels between the vertices look. They are interpolated between the defined vertices following specific rules.

For example: if you want your polygon to be completely red, you would define all vertices red. If you want for specific effects like a gradient between the vertices, you have to do that in the fragment shader.

Put another way:

The vertex shader is part of the early steps in the graphic pipeline, somewhere between model coordinate transformation and polygon clipping I think. At that point, nothing is really done yet.

However, the fragment/pixel shader is part of the rasterization step, where the image is calculated and the pixels between the vertices are filled in or "coloured".

Just read about the graphics pipeline here and everything will reveal itself: http://en.wikipedia.org/wiki/Graphics_pipeline

Vertex shader is done on every vertex, while fragment shader is done on every pixel. The fragment shader is applied after vertex shader. More about the shaders GPU pipeline link text

Nvidia Cg Tutorial:

> Vertex transformation is the first processing stage in the graphics hardware pipeline. Vertex transformation performs a sequence of math operations on each vertex. These operations include transforming the vertex position into a screen position for use by the rasterizer, generating texture coordinates for texturing, and lighting the vertex to determine its color. > > The results of rasterization are a set of pixel locations as well as a set of fragments. There is no relationship between the number of vertices a primitive has and the number of fragments that are generated when it is rasterized. For example, a triangle made up of just three vertices could take up the entire screen, and therefore generate millions of fragments! > > Earlier, we told you to think of a fragment as a pixel if you did not know precisely what a fragment was. At this point, however, the distinction between a fragment and a pixel becomes important. The term pixel is short for "picture element." A pixel represents the contents of the frame buffer at a specific location, such as the color, depth, and any other values associated with that location. A fragment is the state required potentially to update a particular pixel. > > The term "fragment" is used because rasterization breaks up each geometric primitive, such as a triangle, into pixel-sized fragments for each pixel that the primitive covers. A fragment has an associated pixel location, a depth value, and a set of interpolated parameters such as a color, a secondary (specular) color, and one or more texture coordinate sets. These various interpolated parameters are derived from the transformed vertices that make up the particular geometric primitive used to generate the fragments. You can think of a fragment as a "potential pixel." If a fragment passes the various rasterization tests (in the raster operations stage, which is described shortly), the fragment updates a pixel in the frame buffer.

Vertex Shaders and Fragment Shaders are both feature of 3-D implementation that does not uses fixed-pipeline rendering. In any 3-D rendering vertex shaders are applied before fragment/pixel shaders.

Vertex shader operates on each vertex. If you have a fixed polygon mesh and you want to deform it in a shader, you have to implement it in vertex shader. I.e. any physical change in vertex appearances can be done in vertex shaders.

Fragment shader takes the output from the vertex shader and associates colors, depth value of a pixel, etc. After these operations the fragment is send to Framebuffer for display on the screen.

Some operation, as for example lighting calculation, you can perform in vertex shader as well as fragment shader. But fragment shader provides better result than the vertex shader.

In rendering images via 3D hardware you typically have a mesh (point, polygons, lines) these are defined by vertices. To manipulate vertices individually typically for motions in a model or waves in an ocean you can use vertex shaders. These vertices can have static colour or colour assigned by textures, to manipulate vertex colours you use fragment shaders. At the end of the pipeline when the view goes to screen you can also use fragment shaders.