uniform float opacity;
uniform bool invert_color;
uniform sampler2D tex;

void main() {
vec4 c = texture2D(tex, gl_TexCoord[0].xy);
float y = dot(c.rgb, vec3(0.299, 0.587, 0.114));
// y = 1.0 -y;
gl_FragColor = vec4(y, y, y, 1.0);
}