gistfile1.cpp

// input that you pass in, however unity does it
uniform vec2 uvTouchPos;
uniform float circleRadius;

// the fragment shader basically tells the GPU what to color that exact pixel on screen
// the UV is what texture coordinate you are looking at
vec4 FragColor( vec2 uv )
{
	// find out how far you are from touch
	float distanceToTouch = length( uv - uvTouchPos );
	
	// is inside, color black
	if ( distanceToTouch < circleRadius )
		return vec4(0,0,0,1);
		
	// else color white
	return vec4(1,1,1,1);
}