Varying Variables

As mentioned before we have two types of shaders: vertex and fragment shaders. In order to compute values per fragment it is often required to access vertex interpolated data. For instance, when performing lighting computation per fragment, we need to access the normal at the fragment. However in OpenGL, the normals are only specified per vertex. These normals are accessible to the vertex shader, but not to the fragment shader since they come from the OpenGL application as an attribute variable.

After the vertices, including all the vertex data, are processed they move on to the next stage of the pipeline (which still remains fixed functionality) where connectivity information is available. It is in this stage that the primitives are assembled and fragments computed. For each fragment there is a set of variables that are interpolated automatically and provided to the fragment shader. An example is the color of the fragment. The color that arrives at the fragment shader is the result of the interpolation of the colors of the vertices that make up the primitive.

This type of variables, where the fragment receives interpolated, data are “varying variables”. GLSL has some predefined varying variables, such as the above mentioned color. GLSL also allows user defined varying variables. These must be declared in both the vertex and fragment shaders, for instance:

varying float intensity;

A varying variable must be written on a vertex shader, where we compute the value of the variable for each vertex. In the fragment shader the variable, whose value results from an interpolation of the vertex values computed previously, can only be read.