From OpenGL Wiki
Interpolation could refer to:
- The interpolation done to the per-vertex outputs of the final Vertex Processing Shader stage during Rasterization. The particular form of interpolation performed on each per-vertex value is controlled by the interpolation qualifiers assigned to the Fragment Shader input variables. Note that before GLSL 4.30, interpolation qualifiers were required to match between the Fragment Shader and the previous shader stage. In 4.30 and beyond, they are not required to match.
- Blending: interpolation between the Fragment color outputs and Framebuffer colors when writing.
- mix: The GLSL built-in function for performing linear interpolation.