Optimize color use in PNG writer

Author: endroid

LICENSE

@@ -1,4 +1,4 @@
-Copyright 2024 (c) Jeroen van den Enden
+Copyright 2025 (c) Jeroen van den Enden
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal

README.md

@@ -137,6 +137,7 @@ found as a constant prefixed with WRITER_OPTION_ in the writer class.
   * `link`: a URL or an identifier returned by `AddLink()`.
 * `PngWriter`
   * `compression_level`: compression level (0-9, default: -1 = zlib default)
+  * `number_of_colors`: number of colors (1-256, null for true color)
 * `SvgWriter`
   * `block_id`: id of the block element for external reference (default: block)
   * `exclude_xml_declaration`: exclude XML declaration (default: false)

src/Writer/PngWriter.php

@@ -14,16 +14,27 @@
 final readonly class PngWriter extends AbstractGdWriter
 {
     public const WRITER_OPTION_COMPRESSION_LEVEL = 'compression_level';
+    public const WRITER_OPTION_NUMBER_OF_COLORS = 'number_of_colors';
 
     public function write(QrCodeInterface $qrCode, ?LogoInterface $logo = null, ?LabelInterface $label = null, array $options = []): ResultInterface
     {
         if (!isset($options[self::WRITER_OPTION_COMPRESSION_LEVEL])) {
             $options[self::WRITER_OPTION_COMPRESSION_LEVEL] = -1;
         }
 
+        if (!isset($options[self::WRITER_OPTION_NUMBER_OF_COLORS])) {
+            // When a logo is present use true color, otherwise use a palette of 16 colors
+            $options[self::WRITER_OPTION_NUMBER_OF_COLORS] = $logo instanceof LogoInterface ? null : 16;
+        }
+
         /** @var GdResult $gdResult */
         $gdResult = parent::write($qrCode, $logo, $label, $options);
 
-        return new PngResult($gdResult->getMatrix(), $gdResult->getImage(), $options[self::WRITER_OPTION_COMPRESSION_LEVEL]);
+        return new PngResult(
+            $gdResult->getMatrix(),
+            $gdResult->getImage(),
+            $options[self::WRITER_OPTION_COMPRESSION_LEVEL],
+            $options[self::WRITER_OPTION_NUMBER_OF_COLORS]
+        );
     }
 }

src/Writer/Result/PngResult.php

@@ -12,13 +12,17 @@ public function __construct(
         MatrixInterface $matrix,
         \GdImage $image,
         private readonly int $quality = -1,
+        private readonly ?int $numberOfColors = null,
     ) {
         parent::__construct($matrix, $image);
     }
 
     public function getString(): string
     {
         ob_start();
+        if (null !== $this->numberOfColors) {
+            imagetruecolortopalette($this->image, false, $this->numberOfColors);
+        }
         imagepng($this->image, quality: $this->quality);
 
         return strval(ob_get_clean());