OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Sample Shading

If you can’t modify a fragment shader to use the sample keyword (e.g., you’re creating a library that accepts shaders created by another programmer), you can have OpenGL do sample shading by passing GL_SAMPLE_SHADING to glEnable(). This will cause unmodified fragment shader in variables to be interpolated to sample locations automatically.

In order to control the number of samples that receive unique sample-based interpolated values to be evaluated in a fragment shader, you can specify the minimum-sample-shading ratio with glMinSampleShading().

void glMinSampleShading(GLfloat value);

Specifies the fraction of samples per pixels that should be individually shaded. value specifies the ratio of samples to be shaded over total samples, ...