precision mediump float;

varying mediump vec2 v_texcoord;
uniform sampler2D tex;

uniform float radius;
uniform vec2 halfpixel;

void main() {
    vec2 uv = v_texcoord * 2.0;

    vec4 sum = texture2D(tex, uv) * 4.0;
    sum += texture2D(tex, uv - halfpixel.xy * radius);
    sum += texture2D(tex, uv + halfpixel.xy * radius);
    sum += texture2D(tex, uv + vec2(halfpixel.x, -halfpixel.y) * radius);
    sum += texture2D(tex, uv - vec2(halfpixel.x, -halfpixel.y) * radius);

    gl_FragColor = sum / 8.0;
}
