experiment(render): revert sat/gamma/blur to baseline; blue-channel ×0.95

Experiment #3 (sat 115, gamma 1.2, blur 0.6) was a net loss — greens
desaturated, no help on sky→face blue bleed.

Reverting those three to baseline (130, 1.0, 0.0). New experiment #4:
multiply the source's blue channel by 0.95 before dither. Real sky
stays well above the BLUE-vs-WHITE boundary, but borderline-bluish
skin (sky cast on outdoor faces) drops below it and maps to YELLOW /
WHITE / RED instead of feeding the dither's BLUE attractor.

Adds public BLUE_CHANNEL_MUL constant; render-compare's baseline
struct gets blue_mul=1.0 so half-A is still frozen at the original.

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

src/Command/RenderCompareCommand.php

@@ -58,6 +58,7 @@ final class RenderCompareCommand extends Command
         'gamma'      => 1.0,
         'sharpen'    => 0.8,
         'blur'       => 0.0,
+        'blue_mul'   => 1.0,
         'dither'     => \Imagick::DITHERMETHOD_FLOYDSTEINBERG,
     ];
 
@@ -94,6 +95,7 @@ final class RenderCompareCommand extends Command
             'gamma'      => RenderImageMessageHandler::GAMMA,
             'sharpen'    => RenderImageMessageHandler::SHARPEN_SIGMA,
             'blur'       => RenderImageMessageHandler::BLUR_SIGMA,
+            'blue_mul'   => RenderImageMessageHandler::BLUE_CHANNEL_MUL,
             'dither'     => RenderImageMessageHandler::DITHER_METHOD,
         ];
 
@@ -140,8 +142,8 @@ final class RenderCompareCommand extends Command
             ? 'FloydSteinberg'
             : ($p['dither'] === \Imagick::DITHERMETHOD_RIEMERSMA ? 'Riemersma' : 'No');
         return sprintf(
-            'sat=%d gamma=%.2f sharpen=%.2f blur=%.2f dither=%s',
-            $p['saturation'], $p['gamma'], $p['sharpen'], $p['blur'], $dither,
+            'sat=%d gamma=%.2f sharpen=%.2f blur=%.2f blue_mul=%.2f dither=%s',
+            $p['saturation'], $p['gamma'], $p['sharpen'], $p['blur'], $p['blue_mul'], $dither,
         );
     }
 
@@ -164,6 +166,9 @@ final class RenderCompareCommand extends Command
         if ($params['blur'] > 0) {
             $im->blurImage(0, $params['blur']);
         }
+        if ($params['blue_mul'] !== 1.0) {
+            $im->evaluateImage(\Imagick::EVALUATE_MULTIPLY, $params['blue_mul'], \Imagick::CHANNEL_BLUE);
+        }
 
         if ($im->getImageWidth() !== self::W || $im->getImageHeight() !== self::FULL_H) {
             $canvas = new \Imagick();

src/MessageHandler/RenderImageMessageHandler.php

@@ -37,29 +37,28 @@ final class RenderImageMessageHandler
 
     /** modulateImage(brightness=100, SATURATION, hue=100). 100 = no change.
      *  Baseline: 130 (+30%, prevents desaturated photos collapsing to dither
-     *  noise). Experiment #1: 115 (less risk of garish faces). */
-    public const SATURATION_PCT = 115;
+     *  noise; greens stay vibrant). */
+    public const SATURATION_PCT = 130;
 
-    /** gammaImage(GAMMA). 1.0 = no change; >1 lifts midtones, <1 crushes them.
-     *  Baseline: 1.0 (no gamma pass). Experiment #1: 1.2 (gentle midtone lift). */
-    public const GAMMA = 1.2;
+    /** gammaImage(GAMMA). 1.0 = no change; >1 lifts midtones, <1 crushes. */
+    public const GAMMA = 1.0;
 
     /** sharpenImage(radius=0, SHARPEN_SIGMA). Baseline: 0.8 (light). */
     public const SHARPEN_SIGMA = 0.8;
 
-    /** blurImage(radius=0, BLUR_SIGMA) applied *before* dither. 0 = no blur.
-     *  Baseline: 0.0. Experiment #3: 0.6 — softens sharp blue-sky → skin
-     *  transitions so Floyd-Steinberg has less per-pixel error to carry
-     *  forward, reducing the blue "bleed" into faces below the sky. */
-    public const BLUR_SIGMA = 0.6;
+    /** blurImage(radius=0, BLUR_SIGMA) applied *before* dither. 0 = no blur. */
+    public const BLUR_SIGMA = 0.0;
 
-    /** Imagick dither method.
-     *  FLOYDSTEINBERG (baseline) — error diffuses down-right in row order,
-     *                              risks blue bleed into faces below sky.
-     *  RIEMERSMA — Hilbert-curve scan, error stays local but produces
-     *              visible "ink-spill" streaks in low-contrast regions.
-     *              Rejected experiment #2 — much worse than the bleed.
-     *  Sticking with Floyd-Steinberg, attacking the bleed via blur. */
+    /** evaluateImage(MULTIPLY, BLUE_CHANNEL_MUL, CHANNEL_BLUE) — scales the
+     *  source's blue channel before dither. 1.0 = no change.
+     *  Experiment #4: 0.95 (knock 5% off blue). Real sky stays well above
+     *  the BLUE-vs-WHITE boundary, but borderline-bluish skin pixels (sky
+     *  cast on faces in outdoor photos) drop below it and map to YELLOW /
+     *  WHITE / RED instead of contaminating the face with BLUE dither. */
+    public const BLUE_CHANNEL_MUL = 0.95;
+
+    /** Imagick dither method. Sticking with Floyd-Steinberg — Riemersma
+     *  produced visible Hilbert-curve "ink-spill" streaks in skin/sky. */
     public const DITHER_METHOD = \Imagick::DITHERMETHOD_FLOYDSTEINBERG;
 
     public function __construct(
@@ -166,15 +165,20 @@ final class RenderImageMessageHandler
         // Light sharpen so edges survive the dithering scatter.
         $imagick->sharpenImage(0, self::SHARPEN_SIGMA);
 
-        // Optional pre-dither blur to soften sharp colour transitions —
-        // Floyd-Steinberg propagates each pixel's error to its neighbours,
-        // and a hard sky→skin boundary leaks blue into faces below. Even
-        // a sub-1px Gaussian smooths the boundary enough that the carried
-        // error is small. Sharpening above keeps edge crispness.
+        // Optional pre-dither blur to soften sharp colour transitions.
         if (self::BLUR_SIGMA > 0) {
             $imagick->blurImage(0, self::BLUR_SIGMA);
         }
 
+        // Optional blue-channel knock-down. Spectra-6 has only 6 inks; skin
+        // tones in outdoor photos pick up a sky cast that the dither would
+        // otherwise map to BLUE. Scaling the blue channel down a few % pulls
+        // borderline-bluish-skin pixels off the BLUE attractor while leaving
+        // real sky safely above it.
+        if (self::BLUE_CHANNEL_MUL !== 1.0) {
+            $imagick->evaluateImage(\Imagick::EVALUATE_MULTIPLY, self::BLUE_CHANNEL_MUL, \Imagick::CHANNEL_BLUE);
+        }
+
         // Now composite onto a white canvas of exact target dims so that any
         // aspect mismatch shows up as letterbox bars, not as a sliced crop.
         if ($imagick->getImageWidth() !== $width || $imagick->getImageHeight() !== $height) {