Media: improve Imagick handling of colors and alpha channel for PNG image uploads.

Fix an issue where index color (8 bit) PNG uploads were output as true color (24 bit) PNGs, significantly increasing their size.  When using Imagick, PNG output images will now match the colors of the uploaded image.

Also, correct handling of PNG alpha channel information so it is preserved in output images.

Props adamsilverstein, pbearne, nosilver4u, peterdavehello, joemcgill, azaozz, codex-m, kirasong, justlevine, jokanane, sallyruchman, wpfed, tgsrvrs, antpb, tb1909.
Fixes #36477.



git-svn-id: https://develop.svn.wordpress.org/trunk@59589 602fd350-edb4-49c9-b593-d223f7449a82

Author: adamsilverstein

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/data/images/png-tests/Palette_icon-or8.png

@@ -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',
+			),
+		);
+	}
 }