Add handling for indexed-color image type in WP Image Editor

src/wp-includes/class-wp-image-editor-imagick.php

@@ -484,7 +484,38 @@ protected function thumbnail_image( $dst_w, $dst_h, $filter_name = 'FILTER_TRIAN
 				$this->image->setOption( 'png:compression-filter', '5' );
 				$this->image->setOption( 'png:compression-level', '9' );
 				$this->image->setOption( 'png:compression-strategy', '1' );
-				$this->image->setOption( 'png:exclude-chunk', 'all' );
+				// Check to see if a PNG is indexed, and find the pixel depth.
+				if ( is_callable( array( $this->image, 'getImageDepth' ) ) ) {
+					$indexed_pixel_depth = $this->image->getImageDepth();
+
+					// Indexed PNG files get some additional handling.
+					if ( 0 < $indexed_pixel_depth && 8 >= $indexed_pixel_depth ) {
+						// Check for an alpha channel.
+						if (
+							is_callable( array( $this->image, 'getImageAlphaChannel' ) )
+							&& $this->image->getImageAlphaChannel()
+						) {
+							$this->image->setOption( 'png:include-chunk', 'tRNS' );
+						} else {
+							$this->image->setOption( 'png:exclude-chunk', 'all' );
+						}
+
+						// Reduce colors in the images to maximum needed, using the global colorspace.
+						$max_colors = pow( 2, $indexed_pixel_depth );
+						if ( is_callable( array( $this->image, 'getImageColors' ) ) ) {
+							$current_colors = $this->image->getImageColors();
+							$max_colors = min( $max_colors, $current_colors );
+						}
+						$this->image->quantizeImage( $max_colors, $this->image->getColorspace(), 0, false, false );
+
+						/**
+						 * If the colorspace is 'gray', use the png8 format to ensure it stays indexed.
+						 */
+						if ( Imagick::COLORSPACE_GRAY === $this->image->getImageColorspace() ) {
+							$this->image->setOption( 'png:format', 'png8' );
+						}
+					}
+				}
 			}
 
 			/*

tests/phpunit/tests/image/editorImagick.php

@@ -758,4 +758,57 @@ public function test_image_max_bit_depth() {
 	public function __return_eight() {
 		return 8;
 	}
+
+	/**
+	 * Test that resizes are smaller for 16 bit PNG images.
+	 *
+	 * @ticket 36477
+	 *
+	 * @dataProvider data_resizes_are_small_for_16bit_images
+	 */
+	public function test_resizes_are_small_for_16bit_images( $file ) {
+
+		$temp_file = DIR_TESTDATA . '/images/test-temp.png';
+
+		$imagick_image_editor = new WP_Image_Editor_Imagick( $file );
+		$imagick_image_editor->load();
+		$size = $imagick_image_editor->get_size();
+
+		$org_filesize = filesize( $file );
+
+		$imagick_image_editor->resize( $size['width'] * .5, $size['height'] * .5 );
+
+		$saved = $imagick_image_editor->save( $temp_file );
+
+		$new_filesize = filesize( $temp_file );
+
+		unlink( $temp_file );
+
+		$this->assertLessThan( $org_filesize, $new_filesize, 'The resized image file size is not smaller than the original file size.' );
+	}
+
+	/**
+	 * data_test_resizes_are_small_for_16bit
+	 *
+	 * @return array[]
+	 */
+	public static function data_resizes_are_small_for_16bit_images() {
+		return array(
+			'cloudflare-status'       => array(
+				DIR_TESTDATA . '/images/png-tests/cloudflare-status.png',
+			),
+			'deskcat8'                => array(
+				DIR_TESTDATA . '/images/png-tests/deskcat8.png',
+			),
+			'17-c3-duplicate-entries' => array(
+				DIR_TESTDATA . '/images/png-tests/Palette_icon-or8.png',
+			),
+			'rabbit-time-paletted'    => array(
+				DIR_TESTDATA . '/images/png-tests/rabbit-time-paletted-or8.png',
+			),
+			'test8'                   => array(
+				DIR_TESTDATA . '/images/png-tests/test8.png',
+			),
+		);
+	}
 }