float luminance(float3 color)
{
    return dot(color, float3(0.2126, 0.7152, 0.0722));
}

float4 buildHistory(ShaderContext context)
{
    float4 previousFeedback = sampleFeedback(context.uv);
    float4 previousHistory = context.feedbackAvailable > 0
        ? previousFeedback
        : float4(0.0, 0.0, 0.0, 0.0);

    // Highlight selection is always based on the live source image only.
    // The stored feedback never feeds back into the threshold test.
    float currentLuma = luminance(context.sourceColor.rgb);
    float thresholdWidth = max(softness, 0.0001);
    float brightMask = smoothstep(highlightThreshold - thresholdWidth, highlightThreshold + thresholdWidth, currentLuma);

    // Treat RGBA as a 4-slot rolling scalar history:
    // R = current frame, G = 1 frame ago, B = 2 frames ago, A = 3 frames ago.
    // Each frame shifts the history forward and drops the oldest sample.
    float currentContribution = currentLuma * brightMask * max(accumulateAmount, 0.0);
    float4 nextHistory = float4(
        currentContribution,
        previousHistory.r,
        previousHistory.g,
        previousHistory.b
    );

    return saturate(nextHistory);
}

float4 displayHistory(ShaderContext context)
{
    // In the display pass, context.sourceColor is the same-frame historyBuffer
    // produced by buildHistory().
    float4 history = context.sourceColor;
    float recentEnergy = history.r + history.g + history.b;
    float3 originalColor = sampleLayerInput(context.uv).rgb;
    float highlightBoost = recentEnergy * max(displayGain, 0.0);
    float3 sourceHue = originalColor / max(max(originalColor.r, originalColor.g), max(originalColor.b, 0.0001));
    float3 displayColor =
        originalColor +
        sourceHue * highlightBoost * 1.5;

    return float4(saturate(displayColor), 1.0);
}