Constant BLUR

const BLUR: &str = r#"
void main() {
    vec2 uv = gl_FragCoord.xy / u_res;
    vec4 src = texture(tex, uv);
    vec4 res = src;
    float r = p0 * u_scale;
    if (r >= 0.5) {
        // ponytail: one-pass Vogel disc instead of a separable Gaussian - if quality matters,
        // add a direction knob and let gpu.rs run this program twice (H then V).
        vec4 acc = vec4(0.0);
        float wsum = 0.0;
        for (int i = 0; i < 24; i++) {
            float fi = float(i) + 0.5;
            float rad = sqrt(fi / 24.0) * r;
            float ang = fi * 2.39996323;
            float w = exp(-2.0 * rad * rad / (r * r));
            acc += texture(tex, uv + vec2(cos(ang), sin(ang)) * rad / u_res) * w;
            wsum += w;
        }
        res = acc / wsum;
    }
    float m = u_has_mask > 0.5 ? texture(u_mask, uv).r : 1.0;
    out_color = mix(src, res, m);
}
"#;

24-tap Vogel disc, Gaussian-weighted; radius p0 project px.