In OpenGL Version 4.3 and above, there are six shader stages/types: vertex, geometry, tessellation control, tessellation evaluation, fragment, and compute. In this chapter, we'll focus only on the vertex and fragment stages. In Chapter 7, Using Geometry and Tessellation Shaders, I'll provide some recipes for working with the geometry and tessellation shaders, and in Chapter 11, Using Compute Shaders, I'll focus specifically on compute shaders.

Shaders are fundamental parts of the modern OpenGL pipeline. The following block diagram shows a simplified view of the OpenGL pipeline with only the vertex and fragment shaders installed:

Vertex data is sent down the pipeline and arrives at the vertex shader via shader input variables. The vertex shader's input variables correspond to the vertex attributes (refer to the Sending data to a shader using vertex attributes and vertex buffer objects recipe in Chapter 2, Working with GLSL Programs). In general, a shader receives its input via programmer-defined input variables, and the data for those variables comes either from the main OpenGL application or previous pipeline stages (other shaders). For example, a fragment shader's input variables might be fed from the output variables of the vertex shader. Data can also be provided to any shader stage using uniform variables (refer to the Sending data to a shader using uniform variables recipe in Chapter 2, Working with GLSL Programs). These are used for information that changes less often than vertex attributes (for example, matrices, light position, and other settings). The following diagram shows a simplified view of the relationships between input and output variables when there are two shaders active (vertex and fragment):

The vertex shader is executed once for each vertex, in parallel. The data corresponding to the position of the vertex must be transformed into clip space coordinates and assigned to the output variable gl_Position before the vertex shader finishes execution. The vertex shader can send other information down the pipeline using shader output variables. For example, the vertex shader might also compute the color associated with the vertex. That color would be passed to later stages via an appropriate output variable.

Between the vertex and fragment shader, vertices are assembled into primitives, clipping takes place, and the viewport transformation is applied (among other operations). The rasterization process then takes place and the polygon is filled (if necessary). The fragment shader is executed once for each fragment of the polygon being rendered (typically in parallel). Data provided from the vertex shader is (by default) interpolated in a perspective correct manner, and provided to the fragment shader via shader input variables. The fragment shader determines the appropriate color for the pixel and sends it to the frame buffer using output variables. The depth information is handled automatically, but can be modified by the fragment shader if desired.