In Chapter 1, Getting Started with GLSL, we covered the basics of compiling, linking, and exporting shader programs. In this chapter, we'll cover techniques for communication between shader programs and the host OpenGL program. To be more specific, we'll focus primarily on input. The input to shader programs is generally accomplished via attributes and uniform variables. In this chapter, we'll see several examples of the use of these types of variables. We'll also cover a recipe for mixing and matching shader program stages, and a recipe for creating a C++ shader program object.

We won't cover shader output in this chapter. Obviously, shader programs send their output to the default framebuffer, but there are several other techniques for receiving shader output. For example, the use of custom framebuffer objects allow us to store shader...

The vertex shader is invoked once per vertex. Its main job is to process the data associated with the vertex, and pass it (and possibly other information) along to the next stage of the pipeline. In order to give our vertex shader something to work with, we must have some way of providing (per-vertex) input to the shader. Typically, this includes the vertex position, normal vector, and texture coordinates (among other things). In earlier versions of OpenGL (prior to 3.0), each piece of vertex information had a specific channel in the pipeline. It was provided to the shaders using functions such as glVertex, glTexCoord, and glNormal (or within client vertex arrays using glVertexPointer, glTexCoordPointer, or glNormalPointer). The shader would then access these values via...

As covered in the previous recipe, the input variables within a vertex shader are linked to generic vertex attribute indices at the time the program is linked. If we need to specify the relationship, we can either use layout qualifiers within the shader, or we could call glBindAttribLocation before linking.

However, it may be preferable to let the linker create the mappings automatically and query for them after program linking is complete. In this recipe, we'll see a simple example that prints all the active attributes and their indices.

Start with an OpenGL program that compiles and links a shader pair. You could use the shaders...

Vertex attributes offer one avenue for providing input to shaders; a second technique is uniform variables. Uniform variables are intended to be used for data that may change relatively infrequently compared to per-vertex attributes. In fact, it is simply not possible to set per-vertex attributes with uniform variables. For example, uniform variables are well-suited to the matrices used for modeling, viewing, and projective transformations.

Within a shader, uniform variables are read-only. Their values can only be changed from outside the shader, via the OpenGL API. However, they can be initialized within the shader by assigning them to a constant value along with the declaration.

Uniform variables can appear in any shader within a shader program, and are always used as input variables. They can be declared in one or more shaders...

While it is a simple process to query for the location of an individual uniform variable, there may be instances where it can be useful to generate a list of all active uniform variables. For example, one might choose to create a set of variables to store the location of each uniform and assign their values after the program is linked. This would avoid the need to query for uniform locations when setting the value of the uniform variables, creating slightly more efficient code.

The process for listing uniform variables is very similar to the process for listing attributes (see the Getting a list of active vertex input attributes and locations recipe), so this recipe will refer the reader back to the previous recipe for a detailed explanation.

If your program involves multiple shader programs that use the same uniform variables, one has to manage the variables separately for each program. Uniform locations are generated when a program is linked, so the locations of the uniforms may change from one program to the next. The data for those uniforms may have to be regenerated and applied to the new locations.

Uniform blocks were designed to ease the sharing of uniform data between programs. With uniform blocks, one can create a buffer object for storing the values of all the uniform variables, and bind the buffer to the uniform block. When changing programs, the same buffer object need only be rebound to the corresponding block in the new program.

A uniform block is simply a group of uniform variables defined within a syntactical structure known as a uniform block. For example...

Program pipeline objects were introduced as part of the separable shader objects extension, and moved into core OpenGL with version 4.1. They allow programmers to mix and match shader stages from multiple separable shader programs. To understand how this works and why it may be useful, let's go through a hypothetical example. 

Suppose we have one vertex shader and two fragment shaders. Suppose that the code in the vertex shader will function correctly with both fragment shaders. I could simply create two different shader programs, reusing the OpenGL shader object containing the vertex shader. However, if the vertex shader has a lot of uniform variables, then every time that I switch between the two shader programs, I would (potentially) need to reset some or all of those uniform variables. This is because the uniform variables...