blender-archive/source/blender/gpu/shaders/gpu_shader_text_frag.glsl

#if __VERSION__ == 120
  flat varying vec4 color_flat;
  noperspective varying vec2 texCoord_interp;
  #define fragColor gl_FragColor
#else
  flat in vec4 color_flat;
  noperspective in vec2 texCoord_interp;
  out vec4 fragColor;
  #define texture2D texture
#endif

uniform sampler2D glyph;

void main()
{
	// input color replaces texture color
	fragColor.rgb = color_flat.rgb;

	// modulate input alpha & texture alpha
	fragColor.a = color_flat.a * texture2D(glyph, texCoord_interp).a;
}