glBindFragDataLocation

glBindFragDataLocation: man4/glBindFragDataLocation.xml

glBindFragDataLocation explicitly specifies the binding of the user-defined varying out variable name to fragment shader color number colorNumber for program program. If name was bound previously, its assigned binding is replaced with colorNumber. name must be a null-terminated string. colorNumber must be less than GL_MAX_DRAW_BUFFERS. The bindings specified by glBindFragDataLocation have no effect until program is next linked. Bindings may be specified at any time after program has been created. Specifically, they may be specified before shader objects are attached to the program. Therefore, any name may be specified in name, including a name that is never used as a varying out variable in any fragment shader object. Names beginning with gl_ are reserved by the GL. In addition to the errors generated by glBindFragDataLocation, the program program will fail to link if:

1. The number of active outputs is greater than the value GL_MAX_DRAW_BUFFERS.
2. More than one varying out variable is bound to the same color number.

Varying out varyings may have indexed locations assigned explicitly in the shader text using a location layout qualifier. If a shader statically assigns a location to a varying out variable in the shader text, that location is used and any location assigned with glBindFragDataLocation is ignored.