Fix incorrect color results after color reduction (#1410)

* Add tests to verify issue 1409
* Enable GD's ColorProcessor to resolve array color format
* Fix bug when reading colors from palette GDImage
* Add detailed type hint

Author: olivervogel

src/Drivers/Gd/Analyzers/PixelColorAnalyzer.php

@@ -36,6 +36,10 @@ protected function colorAt(ColorspaceInterface $colorspace, GdImage $gd): ColorI
     {
         $index = @imagecolorat($gd, $this->x, $this->y);
 
+        if (!imageistruecolor($gd)) {
+            $index = imagecolorsforindex($gd, $index);
+        }
+
         if ($index === false) {
             throw new GeometryException(
                 'The specified position is not in the valid image area.'

src/Drivers/Gd/ColorProcessor.php

@@ -25,6 +25,7 @@ class ColorProcessor implements ColorProcessorInterface
      */
     public function __construct(protected ColorspaceInterface $colorspace = new Colorspace())
     {
+        //
     }
 
     /**
@@ -57,14 +58,29 @@ public function colorToNative(ColorInterface $color): int
      */
     public function nativeToColor(mixed $value): ColorInterface
     {
-        if (!is_int($value)) {
-            throw new ColorException('GD driver can only decode colors in integer format.');
+        if (!is_int($value) && !is_array($value)) {
+            throw new ColorException('GD driver can only decode colors in integer and array format.');
         }
 
-        $a = ($value >> 24) & 0xFF;
-        $r = ($value >> 16) & 0xFF;
-        $g = ($value >> 8) & 0xFF;
-        $b = $value & 0xFF;
+        if (is_array($value)) {
+            // array conversion
+            if (!$this->isValidArrayColor($value)) {
+                throw new ColorException(
+                    'GD driver can only decode array color format array{red: int, green: int, blue: int, alpha: int}.',
+                );
+            }
+
+            $r = $value['red'];
+            $g = $value['green'];
+            $b = $value['blue'];
+            $a = $value['alpha'];
+        } else {
+            // integer conversion
+            $a = ($value >> 24) & 0xFF;
+            $r = ($value >> 16) & 0xFF;
+            $g = ($value >> 8) & 0xFF;
+            $b = $value & 0xFF;
+        }
 
         // convert gd apha integer to intervention alpha integer
         // ([opaque]0-127[transparent]) to ([opaque]255-0[transparent])
@@ -93,4 +109,49 @@ protected function convertRange(
     ): float|int {
         return ceil(((($input - $min) * ($targetMax - $targetMin)) / ($max - $min)) + $targetMin);
     }
+
+    /**
+     * Check if given array is valid color format
+     * array{red: int, green: int, blue: int, alpha: int}
+     * i.e. result of imagecolorsforindex()
+     *
+     * @param array<mixed> $color
+     * @return bool
+     */
+    private function isValidArrayColor(array $color): bool
+    {
+        if (!array_key_exists('red', $color)) {
+            return false;
+        }
+
+        if (!array_key_exists('green', $color)) {
+            return false;
+        }
+
+        if (!array_key_exists('blue', $color)) {
+            return false;
+        }
+
+        if (!array_key_exists('alpha', $color)) {
+            return false;
+        }
+
+        if (!is_int($color['red'])) {
+            return false;
+        }
+
+        if (!is_int($color['green'])) {
+            return false;
+        }
+
+        if (!is_int($color['blue'])) {
+            return false;
+        }
+
+        if (!is_int($color['alpha'])) {
+            return false;
+        }
+
+        return true;
+    }
 }

tests/Unit/Drivers/Gd/ColorProcessorTest.php

@@ -26,7 +26,7 @@ public function testColorToNative(): void
         $this->assertEquals(16725760, $result);
     }
 
-    public function testNativeToColor(): void
+    public function testNativeToColorInteger(): void
     {
         $processor = new ColorProcessor();
         $result = $processor->nativeToColor(16725760);
@@ -37,6 +37,17 @@ public function testNativeToColor(): void
         $this->assertEquals(255, $result->channel(Alpha::class)->value());
     }
 
+    public function testNativeToColorArray(): void
+    {
+        $processor = new ColorProcessor();
+        $result = $processor->nativeToColor(['red' => 255, 'green' => 55, 'blue' => 0, 'alpha' => 0]);
+        $this->assertInstanceOf(Color::class, $result);
+        $this->assertEquals(255, $result->channel(Red::class)->value());
+        $this->assertEquals(55, $result->channel(Green::class)->value());
+        $this->assertEquals(0, $result->channel(Blue::class)->value());
+        $this->assertEquals(255, $result->channel(Alpha::class)->value());
+    }
+
     public function testNativeToColorInvalid(): void
     {
         $processor = new ColorProcessor();

tests/Unit/Drivers/Gd/Modifiers/QuantizeColorsModifierTest.php

@@ -55,4 +55,11 @@ private function assertColorCount(int $count, ImageInterface $image): void
 
         $this->assertEquals(count($colors), $count);
     }
+
+    public function testVerifyColorValueAfterQuantization(): void
+    {
+        $image = $this->createTestImage(3, 2)->fill('f00');
+        $image->modify(new QuantizeColorsModifier(1));
+        $this->assertColor(255, 0, 0, 255, $image->pickColor(1, 1), 4);
+    }
 }

tests/Unit/Drivers/Imagick/Modifiers/QuantizeColorsModifierTest.php

@@ -37,4 +37,11 @@ public function testInvalidColorInput(): void
         $this->expectException(InputException::class);
         $image->modify(new QuantizeColorsModifier(0));
     }
+
+    public function testVerifyColorValueAfterQuantization(): void
+    {
+        $image = $this->createTestImage(3, 2)->fill('f00');
+        $image->modify(new QuantizeColorsModifier(1));
+        $this->assertColor(255, 0, 0, 255, $image->pickColor(1, 1));
+    }
 }