experiment(render): revert blue_mul; shift BLUE palette target to (8,32,220)

Experiment #4 (blue×0.95) made shadow regions appear MORE blue, not
less — reducing source blue still leaves positive blue error after a
BLACK mapping, and the dither spends that error on neighbours,
creating blue dither dots in dark regions.

Reverting blue_mul to 1.0. Experiment #5 takes a different attack on
the same problem: shift the BLUE palette mapping target from the
muted (24, 64, 192) to a more saturated (8, 32, 220). Doesn't change
what the panel displays (the blue ink is fixed); it just makes
Euclidean distance from skin tones to "BLUE" larger in the algorithm's
view, so the dither prefers RED/WHITE/YELLOW for borderline pixels.

Render-compare's BASELINE struct now carries its own frozen palette,
so half-A keeps the original (24,64,192) BLUE target while half-B
pulls the shifted palette from the live pipeline.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Command/RenderCompareCommand.php

@@ -39,7 +39,8 @@ use Symfony\Component\DependencyInjection\Attribute\Autowire;
 )]
 final class RenderCompareCommand extends Command
 {
-    private const PALETTE = [
+    /** Frozen baseline palette — render-baseline-2026-05-14. NEVER touch. */
+    private const BASELINE_PALETTE = [
         0x0 => [26,  26,  26 ],
         0x1 => [245, 245, 240],
         0x2 => [240, 208, 0  ],
@@ -52,13 +53,14 @@ final class RenderCompareCommand extends Command
     private const HALF_H = 800;
     private const FULL_H = 1600;
 
-    /** Baseline tunables — must NOT track the live pipeline. Frozen reference. */
+    /** Baseline tunables — frozen reference, never tracks the live pipeline. */
     private const BASELINE = [
         'saturation' => 130,
         'gamma'      => 1.0,
         'sharpen'    => 0.8,
         'blur'       => 0.0,
         'blue_mul'   => 1.0,
+        'palette'    => self::BASELINE_PALETTE,
         'dither'     => \Imagick::DITHERMETHOD_FLOYDSTEINBERG,
     ];
 
@@ -96,6 +98,7 @@ final class RenderCompareCommand extends Command
             'sharpen'    => RenderImageMessageHandler::SHARPEN_SIGMA,
             'blur'       => RenderImageMessageHandler::BLUR_SIGMA,
             'blue_mul'   => RenderImageMessageHandler::BLUE_CHANNEL_MUL,
+            'palette'    => RenderImageMessageHandler::PALETTE,
             'dither'     => RenderImageMessageHandler::DITHER_METHOD,
         ];
 
@@ -186,7 +189,7 @@ final class RenderCompareCommand extends Command
         $im->cropImage(self::W, self::HALF_H, 0, 0);
         $im->setImagePage(self::W, self::HALF_H, 0, 0);
 
-        $palStrip = $this->buildPaletteStrip();
+        $palStrip = $this->buildPaletteStrip($params['palette']);
         $im->remapImage($palStrip, $params['dither']);
         $palStrip->destroy();
 
@@ -200,17 +203,17 @@ final class RenderCompareCommand extends Command
         for ($i = 0; $i < $total; $i += 2) {
             $base0 = $i * 3;
             $base1 = $base0 + 3;
-            $n0    = $this->nearestPalette(ord($blob[$base0]),     ord($blob[$base0 + 1]), ord($blob[$base0 + 2]));
+            $n0    = $this->nearestPalette($params['palette'], ord($blob[$base0]),     ord($blob[$base0 + 1]), ord($blob[$base0 + 2]));
-            $n1    = $this->nearestPalette(ord($blob[$base1]),     ord($blob[$base1 + 1]), ord($blob[$base1 + 2]));
+            $n1    = $this->nearestPalette($params['palette'], ord($blob[$base1]),     ord($blob[$base1 + 1]), ord($blob[$base1 + 2]));
             $output .= chr(($n0 << 4) | $n1);
         }
         return $output;
     }
 
-    private function buildPaletteStrip(): \Imagick
+    private function buildPaletteStrip(array $palette): \Imagick
     {
         $palImagick = new \Imagick();
-        foreach (self::PALETTE as $rgb) {
+        foreach ($palette as $rgb) {
             $hex = sprintf('#%02x%02x%02x', $rgb[0], $rgb[1], $rgb[2]);
             $tmp = new \Imagick();
             $tmp->newImage(1, 1, new \ImagickPixel($hex));
@@ -224,11 +227,11 @@ final class RenderCompareCommand extends Command
         return $strip;
     }
 
-    private function nearestPalette(int $r, int $g, int $b): int
+    private function nearestPalette(array $palette, int $r, int $g, int $b): int
     {
         $best     = 0x1;
         $bestDist = PHP_INT_MAX;
-        foreach (self::PALETTE as $index => $rgb) {
+        foreach ($palette as $index => $rgb) {
             $dist = ($r - $rgb[0]) ** 2 + ($g - $rgb[1]) ** 2 + ($b - $rgb[2]) ** 2;
             if ($dist < $bestDist) {
                 $bestDist = $dist;

src/MessageHandler/RenderImageMessageHandler.php

@@ -18,13 +18,23 @@ use Symfony\Component\Messenger\Attribute\AsMessageHandler;
 #[AsMessageHandler]
 final class RenderImageMessageHandler
 {
-    // Waveshare Spectra 6 palette — must match gen_screens.py
+    /**
-    private const PALETTE = [
+     * Waveshare Spectra 6 palette. Values are the dither's *mapping targets* —
+     * panel still displays each nibble as its actual ink colour regardless of
+     * what RGB we tell the algorithm. Shifting BLUE further from neutral pulls
+     * borderline-bluish pixels (sky cast on skin) toward RED/WHITE/YELLOW.
+     *
+     * Baseline (frozen at render-baseline-2026-05-14):
+     *   0x5 BLUE  = [24, 64, 192]
+     * Experiment #5:
+     *   0x5 BLUE  = [ 8, 32, 220]  — more saturated mapping target.
+     */
+    public const PALETTE = [
         0x0 => [26,  26,  26 ],  // BLACK
         0x1 => [245, 245, 240],  // WHITE
         0x2 => [240, 208, 0  ],  // YELLOW
         0x3 => [192, 48,  32 ],  // RED
-        0x5 => [24,  64,  192],  // BLUE
+        0x5 => [8,   32,  220],  // BLUE (shifted in experiment #5)
         0x6 => [16,  160, 64 ],  // GREEN
     ];
 
@@ -50,12 +60,12 @@ final class RenderImageMessageHandler
     public const BLUR_SIGMA = 0.0;
 
     /** evaluateImage(MULTIPLY, BLUE_CHANNEL_MUL, CHANNEL_BLUE) — scales the
-     *  source's blue channel before dither. 1.0 = no change.
+     *  source's blue channel before dither. 1.0 = no change. Reverted from
-     *  Experiment #4: 0.95 (knock 5% off blue). Real sky stays well above
+     *  experiment #4 (0.95) which made shadow regions appear MORE blue, not
-     *  the BLUE-vs-WHITE boundary, but borderline-bluish skin pixels (sky
+     *  less — likely because reducing source blue still left positive blue
-     *  cast on faces in outdoor photos) drop below it and map to YELLOW /
+     *  error after a BLACK mapping, which the dither then spent on
-     *  WHITE / RED instead of contaminating the face with BLUE dither. */
+     *  neighbours, creating blue dither dots in dark regions. */
-    public const BLUE_CHANNEL_MUL = 0.95;
+    public const BLUE_CHANNEL_MUL = 1.0;
 
     /** Imagick dither method. Sticking with Floyd-Steinberg — Riemersma
      *  produced visible Hilbert-curve "ink-spill" streaks in skin/sky. */