uniform float opacity;
uniform bool invert_color;
uniform sampler2D tex;
void main() {
vec4 c = texture2D(tex, gl_TexCoord[0]);
if (invert_color)
c = vec4(vec3(c.a, c.a, c.a) - vec3(c), c.a);
c *= opacity;
gl_FragColor = c;
}