#version 300 es
precision highp float;   // highp: fullscreen UV addressing (mediump fp16 is not enough on mobile)

in vec2 vTexCoord;
uniform mediump sampler2D u_tex;
uniform float u_threshold;
uniform float u_knee;         // soft-knee width below the threshold; 0 = hard cut
uniform float u_chromaBias;   // 0 = Rec.709 luminance, 1 = largest channel (hue-blind)
out vec4 fragColor;

void main() {
    vec3 color = texture(u_tex, vTexCoord).rgb;
    // Rec. 709 luminance is perceptually correct but weights blue at 7% and green at 72%, so a
    // saturated blue needs about ten times a green's value to pass the same threshold. On a palette
    // that assigns each thing its own hue, that silently excludes whole colours from the glow.
    // The largest channel is hue-blind: it asks "how strong is this colour", not "how bright does it
    // look". u_chromaBias picks between them
    float luma = dot(color, vec3(0.2126, 0.7152, 0.0722));
    float peak = max(color.r, max(color.g, color.b));
    float brightness = mix(luma, peak, u_chromaBias);
    // soft knee: ramps in over [threshold - knee, threshold] — a hard cut flickers on moving edges
    float w = smoothstep(u_threshold - max(u_knee, 0.0001), u_threshold, brightness);
    fragColor = vec4(color * w, 1.0);
}