#version 400
uniform vec4 color;
uniform sampler2D tex;
in vec2 texCoord;
out vec4 FragColor;
uniform bool modulate;
uniform bool debug;
uniform bool debugColor;
uniform float debugVal;
void main() {
  if (debugColor) {
    FragColor = color;
    return;
  }
  if (debug) {
    FragColor = mix(vec4(1, 1, 1, 1), vec4(1, 0, 0, 1), debugVal);
    return;
  }
	// Cf = color from fragment, Ct = color from texture
	// Cc = color from texture environment -- not set, defaults to (0,0,0,0)
	// Af = alpha from fragment, At = alpha from texture
	// C = output color, A = output alpha
	float f = texture(tex, texCoord).r;
	if (modulate) { 
		// GL_MODULATE: C
		// C = Cf * Ct
		// A = Af * At				
		FragColor = color * f;
	} else {
        // "That is, the colors in the frame buffer are multiplied by the
        // attenuation ratio of the billboard texture and then the colors in
        // the texture are added"
		// GL_BLEND:
		// C = Cf * (1-Ct) + Cc * Ct
		// A = Af * At
		vec3 C = color.rgb * (1 - f);
		float A = color.a * f;
		FragColor = vec4(C, A);
	}
}