Constant JPEG_COMPRESS

const JPEG_COMPRESS: &str = r#"
vec3 fx_ycc(vec3 c) {
    return vec3(dot(c, vec3(0.299, 0.587, 0.114)),
                dot(c, vec3(-0.168736, -0.331264, 0.5)) + 0.5,
                dot(c, vec3(0.5, -0.418688, -0.081312)) + 0.5);
}
vec3 fx_rgb(vec3 y) {
    float cb = y.y - 0.5;
    float cr = y.z - 0.5;
    return vec3(y.x + 1.402 * cr, y.x - 0.344136 * cb - 0.714136 * cr, y.x + 1.772 * cb);
}
void main() {
    vec2 uv = gl_FragCoord.xy / u_res;
    vec4 src = texture(tex, uv);
    float sc = max(u_scale, 0.05);
    float b = max(p1, 2.0) * sc;
    vec2 org = floor(gl_FragCoord.xy / b) * b;
    vec3 dc = texture(tex, (org + b * vec2(0.25, 0.25)) / u_res).rgb
            + texture(tex, (org + b * vec2(0.75, 0.25)) / u_res).rgb
            + texture(tex, (org + b * vec2(0.25, 0.75)) / u_res).rgb
            + texture(tex, (org + b * vec2(0.75, 0.75)) / u_res).rgb;
    dc *= 0.25;
    float q = clamp(p0, 1.0, 100.0);
    float qs = q < 50.0 ? 50.0 / q : 2.0 - q / 50.0;
    float step_ = clamp(qs * 0.05, 0.004, 0.6);
    vec3 ydc = fx_ycc(dc);
    vec3 ypx = fx_ycc(src.rgb);
    float dcs = step_ * 0.5;
    float luma = floor(ydc.x / dcs + 0.5) * dcs + floor((ypx.x - ydc.x) / step_ + 0.5) * step_;
    vec2 ch = ypx.yz;
    if (p2 >= 0.5) {
        vec2 sub = (floor(gl_FragCoord.xy / (2.0 * sc)) + 0.5) * (2.0 * sc);
        ch = fx_ycc(texture(tex, sub / u_res).rgb).yz;
    }
    float cstep = step_ * 2.0;
    ch = floor((ch - 0.5) / cstep + 0.5) * cstep + 0.5;
    vec4 res = vec4(clamp(fx_rgb(vec3(luma, ch)), 0.0, 1.0), src.a);
    float m = u_has_mask > 0.5 ? texture(u_mask, uv).r : 1.0;
    out_color = mix(src, res, m);
}
"#;

JPEG-ish: the block DC (4 taps) is quantised coarsely, the AC residual more coarsely still, and the chroma optionally read from a 2x2 grid (4:2:0). This is an approximation - there is no DCT here, but the visible artefacts (blocking, colour smear, banding) track Quality the same way.