Check images for true alpha transparency

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

@@ -444,6 +444,39 @@ protected function thumbnail_image( $dst_w, $dst_h, $filter_name = 'FILTER_TRIAN
 		}
 
 		try {
+			$is_png                                      = false;
+			$is_indexed_png                              = false;
+			$is_indexed_png_with_alpha_channel           = false;
+			$is_indexed_png_with_true_alpha_transparency = false;
+
+			if ( 'image/png' === $this->mime_type ) {
+				$is_png = true;
+
+				if (
+					is_callable( array( $this->image, 'getImageProperty' ) )
+					&& '3' === $this->image->getImageProperty( 'png:IHDR.color-type-orig' )
+				) {
+					$is_indexed_png = true;
+
+					if (
+						is_callable( array( $this->image, 'getImageAlphaChannel' ) )
+						&& $this->image->getImageAlphaChannel()
+					) {
+						$is_indexed_png_with_alpha_channel = true;
+
+						if (
+							is_callable( array( $this->image, 'getImageChannelDepth' ) )
+							&& defined( 'Imagick::CHANNEL_ALPHA' )
+						) {
+							$alpha_channel_depth = $this->image->getImageChannelDepth( Imagick::CHANNEL_ALPHA );
+							if ( $alpha_channel_depth > 1 ) {
+								$is_indexed_png_with_true_alpha_transparency = true;
+							}
+						}
+					}
+				}
+			}
+
 			/*
 			 * To be more efficient, resample large images to 5x the destination size before resizing
 			 * whenever the output size is less that 1/3 of the original image size (1/3^2 ~= .111),
@@ -480,30 +513,42 @@ protected function thumbnail_image( $dst_w, $dst_h, $filter_name = 'FILTER_TRIAN
 				$this->image->setOption( 'jpeg:fancy-upsampling', 'off' );
 			}
 
-			if ( 'image/png' === $this->mime_type ) {
+			if ( $is_png ) {
 				$this->image->setOption( 'png:compression-filter', '5' );
 				$this->image->setOption( 'png:compression-level', '9' );
 				$this->image->setOption( 'png:compression-strategy', '1' );
 
 				// 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' )
-				) {
+				if ( $is_indexed_png ) {
 
 					// Check for an alpha channel.
-					if (
-						is_callable( array( $this->image, 'getImageAlphaChannel' ) )
-						&& $this->image->getImageAlphaChannel()
-					) {
+					if ( $is_indexed_png_with_alpha_channel ) {
 						$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' );
 
+					$this->image->quantizeImage( 256, $this->image->getColorspace(), 0, false, false );
+
+					/*
+					 * If the colorspace is 'gray', use the png8 format to ensure it stays indexed.
+					 * ImageMagick tends to save grayscale images as grayscale PNGs rather than indexed PNGs,
+					 * even though grayscale PNGs usually have considerably larger file sizes.
+					 * But we can force ImageMagick to save the image as an indexed PNG instead,
+					 * by telling it to use png8 format.
+					 *
+					 * Note that we need to first call quantizeImage() before checking getImageColorspace(),
+					 * because only after calling quantizeImage() will the colorspace be COLORSPACE_GRAY for grayscale images
+					 * (and we have not found any other way to identify grayscale images).
+					 *
+					 * We need to avoid forcing indexed format for images with true alpha transparency,
+					 * because ImageMagick does not support saving an image with true alpha transparency as an indexed PNG.
+					 */
+					if ( Imagick::COLORSPACE_GRAY === $this->image->getImageColorspace() && ! $is_indexed_png_with_true_alpha_transparency ) {
+						// Set the image format to Indexed PNG.
+						$this->image->setOption( 'png:format', 'png8' );
+					}
 				} else {
 					$this->image->setOption( 'png:exclude-chunk', 'all' );
 				}

tests/phpunit/tests/image/editorImagick.php

@@ -873,4 +873,42 @@ public static function data_png_color_type_after_resize() {
 			),
 		);
 	}
+
+	/**
+	 * Tests that alpha transparency is preserved after resizing.
+	 *
+	 * @ticket 63448
+	 * @dataProvider data_alpha_transparency_is_preserved_after_resize
+	 *
+	 * @param string $file_path Path to the image file.
+	 */
+	public function test_alpha_transparency_is_preserved_after_resize( $file_path ) {
+
+		$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 );
+		$alpha_channel_depth = $imagick->getImageChannelDepth( Imagick::CHANNEL_ALPHA );
+
+		unlink( $temp_file );
+
+		$this->assertGreaterThan( 1, $alpha_channel_depth, "Alpha transparency should be preserved after resize for {$file_path}." );
+	}
+
+	public static function data_alpha_transparency_is_preserved_after_resize() {
+		return array(
+			'oval-or8'                   => array(
+				DIR_TESTDATA . '/images/png-tests/oval-or8.png',
+			),
+			'oval-or8-grayscale-indexed' => array(
+				DIR_TESTDATA . '/images/png-tests/oval-or8-grayscale-indexed.png',
+			),
+		);
+	}
 }