varying vec2 v_uv;

varying float v_color;

void main(void) {

    const float sinPi4 = 0.7071;

    float feather = 0.3;

    // display a diamond shape

    float fade = pow(abs(v_uv.x) + abs(v_uv.y), feather);

    // rotate the texture coordinates by 45 degrees to add a smaller offset diamond

    vec2 uv45 = vec2(v_uv.x * sinPi4 - v_uv.y * sinPi4,

                    v_uv.y * sinPi4 + v_uv.x * sinPi4);

    fade *= pow(abs(uv45.x) + abs(uv45.y), feather * 0.4);

    float col = v_color * max(1. - fade, 0.);

    gl_FragColor = vec4(col, col, col, col);

}