Media: fix degraded handling for 24 bit PNG uploads.

Add additional logic to correctly identify indexed PNG images so output PNGs correctly  match the uploaded image depth.

Fix a regression introduced in WordPress 6.8 [59589] where uploaded Truecolor PNG images (meaning > 8 bit) were mis-identified as indexed (8 bit) images, causing output images to be indexed instead of Truecolor resulting in a noticeable visual degradation. 

Props elvismdev, wildworks, SirLouen, siliconforks, joemcgill, iamshashank, nosilver4u.
Fixes #63448.




git-svn-id: https://develop.svn.wordpress.org/trunk@60246 602fd350-edb4-49c9-b593-d223f7449a82

Author: adamsilverstein

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

@@ -484,37 +484,28 @@ 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' );
-				// 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' );
-						}
+
+				// Indexed PNG files get some additional handling.
+				// See #63448 for details.
+				if (
+					is_callable( array( $this->image, 'getImageProperty' ) )
+					&& '3' === $this->image->getImageProperty( 'png:IHDR.color-type-orig' )
+				) {
+
+					// 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' );
 					}
+					// Set the image format to Indexed PNG.
+					$this->image->setOption( 'png:format', 'png8' );
+
+				} else {
+					$this->image->setOption( 'png:exclude-chunk', 'all' );
 				}
 			}
 

tests/phpunit/data/images/png-tests/grayscale-test-image.png

@@ -770,6 +770,8 @@ public function __return_eight() {
 	 * @ticket 36477
 	 *
 	 * @dataProvider data_resizes_are_small_for_16bit_images
+	 *
+	 * @param string $file Path to the image file.
 	 */
 	public function test_resizes_are_small_for_16bit_images( $file ) {
 
@@ -783,7 +785,7 @@ public function test_resizes_are_small_for_16bit_images( $file ) {
 
 		$imagick_image_editor->resize( $size['width'] * .5, $size['height'] * .5 );
 
-		$saved = $imagick_image_editor->save( $temp_file );
+		$imagick_image_editor->save( $temp_file );
 
 		$new_filesize = filesize( $temp_file );
 
@@ -793,7 +795,7 @@ public function test_resizes_are_small_for_16bit_images( $file ) {
 	}
 
 	/**
-	 * data_test_resizes_are_small_for_16bit
+	 * Data provider for test_resizes_are_small_for_16bit_images.
 	 *
 	 * @return array[]
 	 */
@@ -816,4 +818,59 @@ public static function data_resizes_are_small_for_16bit_images() {
 			),
 		);
 	}
+
+	/**
+	 * Tests that the 'png:IHDR.color-type-orig' property is preserved after resizing
+	 * Used to identify indexed PNG images, see https://www.w3.org/TR/PNG-Chunks.html#C.IHDR.
+	 *
+	 * @ticket 63448
+	 * @dataProvider data_png_color_type_after_resize
+	 *
+	 * @param string $file_path             Path to the image file.
+	 * @param int    $expected_color_type   The expected original color type.
+	 */
+	public function test_png_color_type_is_preserved_after_resize( $file_path, $expected_color_type ) {
+
+		$temp_file = DIR_TESTDATA . '/images/test-temp.png';
+
+		$imagick_image_editor = new WP_Image_Editor_Imagick( $file_path );
+		$imagick_image_editor->load();
+
+		$size = $imagick_image_editor->get_size();
+		$imagick_image_editor->resize( $size['width'] * 0.5, $size['height'] * 0.5 );
+		$imagick_image_editor->save( $temp_file );
+
+		$imagick           = new Imagick( $temp_file );
+		$actual_color_type = $imagick->getImageProperty( 'png:IHDR.color-type-orig' );
+
+		unlink( $temp_file );
+
+		$this->assertSame( (string) $expected_color_type, $actual_color_type, "The PNG original color type should be preserved after resize for {$file_path}." );
+	}
+
+	/**
+	 * Data provider for test_png_color_type_is_preserved_after_resize.
+	 *
+	 * @return array[]
+	 */
+	public static function data_png_color_type_after_resize() {
+		return array(
+			'vivid-green-bird_color_type_6'         => array(
+				DIR_TESTDATA . '/images/png-tests/vivid-green-bird.png',
+				6, // RGBA.
+			),
+			'grayscale-test-image_color_type_4'     => array(
+				DIR_TESTDATA . '/images/png-tests/grayscale-test-image.png',
+				4, // Grayscale with Alpha.
+			),
+			'rabbit-time-paletted-or8_color_type_3' => array(
+				DIR_TESTDATA . '/images/png-tests/rabbit-time-paletted-or8.png',
+				3, // Paletted.
+			),
+			'test8_color_type_3'                    => array(
+				DIR_TESTDATA . '/images/png-tests/test8.png',
+				3, // Paletted.
+			),
+		);
+	}
 }