13. The smooth type qualifier

• GLSL smooth type qualifier is one of the interpolation qualifiers in GLSL. The value is interpolated in a perspective-correct fashion. This is the default if no qualifier is present.

  
      // Declaration in vertex shader
      smooth out vec4 interpolated_color;
  
      // Declaration in fragment shader
      smooth in vec4 interpolated_color;
  
  

• Other choices are flat and noperspective. See http://www.opengl.org/wiki/Type_Qualifier_(GLSL) article for details.