AttributeBinder

Binder is the interface definition for the strategies for constructing, uploading, and binding paint property data as GLSL attributes. Most style- spec properties have a 1:1 relationship to shader attribute/uniforms, but some require multiple values per feature to be passed to the GPU, and in those cases we bind multiple attributes/uniforms.

It has three implementations, one for each of the three strategies we use:

• For constant properties -- those whose value is a constant, or the constant result of evaluating a camera expression at a particular camera position -- we don't need a vertex attribute buffer, and instead use a uniform.
• For data expressions, we use a vertex buffer with a single attribute value, the evaluated result of the source function for the given feature.
• For composite expressions, we use a vertex buffer with two attributes: min and max values covering the range of zooms at which we expect the tile to be displayed. These values are calculated by evaluating the composite expression for the given feature at strategically chosen zoom levels. In addition to this attribute data, we also use a uniform value which the shader uses to interpolate between the min and max value at the final displayed zoom level. The use of a uniform allows us to cheaply update the value on every frame.

Note that the shader source varies depending on whether we're using a uniform or attribute. We dynamically compile shaders at runtime to accommodate this.