// Crt: a slot-mask tube's light, with nothing that moves a pixel. There is
// no barrel: every click lands where it is drawn, rails and grips at the
// window's edges included. What is left is light: a beam profile for the
// scanlines, an aperture mask, a soft bloom of the brightest ink, a faint
// vignette and the hum of a live tube. Everything is worked out in linear
// light and dithered back into 8 bits, so the vignette never bands. It is a
// Shadertoy file: pardes puts its prefix before it (shaders/post/prefix.glsl).

const float TAU = 6.28318530718;
const float CHROMA_PX = 0.35;       // red and blue apart, at the corners
const float BLOOM_RADIUS_PX = 1.75;
const float BLOOM_THRESHOLD = 0.60; // linear-light channel peak
const float BLOOM_SOFTNESS = 0.25;
const float BLOOM_STRENGTH = 0.05;
const float SCAN_PERIOD_PX = 3.0;
const float SCAN_STRENGTH = 0.07;
const float MASK_PERIOD_PX = 3.0;
const float MASK_STRENGTH = 0.018;
const float VIGNETTE_STRENGTH = 0.12; // at the far corners only
const float HUM_STRENGTH = 0.004;
const float HUM_HZ = 0.11;
const float EXPOSURE = 1.015;

vec3 toLinear(vec3 c) {
    return mix(pow((c + 0.055) / 1.055, vec3(2.4)), c / 12.92, lessThanEqual(c, vec3(0.04045)));
}

vec3 toSrgb(vec3 c) {
    c = max(c, vec3(0.0));
    return mix(1.055 * pow(c, vec3(1.0 / 2.4)) - 0.055, c * 12.92, lessThanEqual(c, vec3(0.0031308)));
}

vec3 scene(vec2 uv) {
    return toLinear(texture(iChannel0, uv).rgb);
}

vec3 brightPass(vec2 uv) {
    vec3 c = scene(uv);
    float peak = max(c.r, max(c.g, c.b));
    return c * smoothstep(BLOOM_THRESHOLD, BLOOM_THRESHOLD + BLOOM_SOFTNESS, peak);
}

// Interleaved gradient noise: a dither with no visible pattern.
float ign(vec2 p) {
    return fract(52.9829189 * fract(dot(p, vec2(0.06711056, 0.00583715))));
}

void mainImage(out vec4 fragColor, in vec2 fragCoord) {
    vec2 size = iResolution.xy;
    vec2 texel = 1.0 / size;
    vec2 uv = fragCoord / size;
    vec2 cc = uv * 2.0 - 1.0;
    float r2 = dot(cc, cc);

    // The beam converges least at the corners: red and blue drift apart
    // there, by a third of a pixel.
    vec2 chroma = cc * (CHROMA_PX * r2 * 0.5) * texel;
    vec3 col = vec3(scene(uv + chroma).r, scene(uv).g, scene(uv - chroma).b);

    vec2 b = texel * BLOOM_RADIUS_PX;
    vec3 bloom = brightPass(uv + vec2(b.x, 0.0)) + brightPass(uv - vec2(b.x, 0.0)) +
        brightPass(uv + vec2(0.0, b.y)) + brightPass(uv - vec2(0.0, b.y));
    col += bloom * (0.25 * BLOOM_STRENGTH);

    // A beam is brightest on its line and falls off between lines; the
    // mask is three phosphor stripes a triad. Both average to one, so the
    // theme keeps its brightness.
    float line = 0.5 + 0.5 * cos(fragCoord.y * TAU / SCAN_PERIOD_PX);
    col *= 1.0 - SCAN_STRENGTH * (line * line - 0.375) / 0.625;
    col *= 1.0 + MASK_STRENGTH * cos(fragCoord.x * TAU / MASK_PERIOD_PX + vec3(0.0, TAU / 3.0, 2.0 * TAU / 3.0));

    float vignette = 1.0 - VIGNETTE_STRENGTH * pow(clamp(r2 * 0.5, 0.0, 1.0), 1.6);
    float hum = 1.0 + HUM_STRENGTH * sin((uv.y * 1.35 - iTime * HUM_HZ) * TAU);
    col *= vignette * hum * EXPOSURE;

    vec3 srgb = toSrgb(col);
    srgb += (ign(fragCoord + float(iFrame % 64) * vec2(5.588238, 5.588238)) - 0.5) / 255.0;
    fragColor = vec4(clamp(srgb, 0.0, 1.0), texture(iChannel0, uv).a);
}