Pin-Hole like effect

pin-hole.fs

// fragment shader
uniform sampler2D tex;
uniform vec2 viewport;
uniform vec2 origin;
uniform float radius; 
uniform float attenuation;
uniform vec3 background_color;

void main()
{
	vec2 uv = gl_FragCoord.xy / viewport;
	uv.y = 1.0 - uv.y;

	vec2 point = 2.0*uv - 1.0;
	point.y *= viewport.y / viewport.x;
	
	vec3 tex_color = texture2D(tex, uv).xyz;
	vec3 color = background_color;

	if (radius > 0.0) {
		float d = distance(point, origin);
		if (d <= radius) {
			color = tex_color;
		} else {
			float k = (d - radius);
			color = mix(tex_color, background_color, attenuation*k*k);
		}
	}

	gl_FragColor = vec4(color, 1.0);
}