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, in this recipe, we'll use the following uniform block:

uniform BlobSettings { 
  vec4 InnerColor; 
  vec4 OuterColor; 
  float RadiusInner; 
  float RadiusOuter; 
}; 

This defines a block with the name BlobSettings that contains four uniform variables. With this type of block definition, the variables within the block are still part of the global scope and do not need to be qualified with the block name.

The buffer object used to store the data for the uniforms is often referred to as a uniform buffer object. We'll see that a uniform buffer object is simply a buffer object that is bound to a certain location.

For this recipe, we'll use a simple example to demonstrate the use of uniform buffer objects and uniform blocks. We'll draw a quad (two triangles) with texture coordinates, and use our fragment shader to fill the quad with a fuzzy circle. The circle is a solid color in the center, but at its edge, it gradually fades to the background color, as shown in the following image: