Media: Use imageflip() in WP_Image_Editor_GD::flip().

src/wp-admin/includes/image-edit.php

@@ -565,6 +565,7 @@ function _rotate_image_resource( $img, $angle ) {
  * Flips an image resource. Internal use only.
  *
  * @since 2.9.0
+ * @since 7.2.0 Switched to using imageflip() instead of imagecopyresampled() with negative dimensions.
  * @deprecated 3.5.0 Use WP_Image_Editor::flip()
  * @see WP_Image_Editor::flip()
  *
@@ -577,25 +578,43 @@ function _rotate_image_resource( $img, $angle ) {
 function _flip_image_resource( $img, $horz, $vert ) {
 	_deprecated_function( __FUNCTION__, '3.5.0', 'WP_Image_Editor::flip()' );
 
-	$w   = imagesx( $img );
-	$h   = imagesy( $img );
-	$dst = wp_imagecreatetruecolor( $w, $h );
+	if ( ! is_gd_image( $img ) ) {
+		return $img;
+	}
 
-	if ( is_gd_image( $dst ) ) {
-		$sx = $vert ? ( $w - 1 ) : 0;
-		$sy = $horz ? ( $h - 1 ) : 0;
-		$sw = $vert ? -$w : $w;
-		$sh = $horz ? -$h : $h;
+	// Keep returning a truecolor image, like the previous implementation did. imagewebp() rejects palette images.
+	if ( ! imageistruecolor( $img ) && ! imagepalettetotruecolor( $img ) ) {
+		return $img;
+	}
 
-		if ( imagecopyresampled( $dst, $img, 0, 0, $sx, $sy, $w, $h, $sw, $sh ) ) {
-			if ( PHP_VERSION_ID < 80000 ) { // imagedestroy() has no effect as of PHP 8.0.
-				imagedestroy( $img );
-			}
+	// Match the alpha settings of the previous implementation's copy, including when no axis is flipped.
+	imagealphablending( $img, false );
+	imagesavealpha( $img, true );
 
-			$img = $dst;
-		}
+	/*
+	 * imageflip() changes the image in place, so the image keeps its current resolution and interlace setting.
+	 * _rotate_image_resource() and _crop_image_resource() copy into a new image, which resets them to GD's defaults:
+	 * 96 DPI and no interlacing. Reset them here too, so a flip gives the same result as the other edits.
+	 *
+	 * @link https://core.trac.wordpress.org/ticket/66113
+	 */
+	imageresolution( $img, 96, 96 );
+	imageinterlace( $img, 0 );
+
+	if ( ! $horz && ! $vert ) {
+		return $img;
 	}
 
+	if ( $horz && $vert ) {
+		$mode = IMG_FLIP_BOTH;
+	} elseif ( $horz ) {
+		$mode = IMG_FLIP_VERTICAL;
+	} else {
+		$mode = IMG_FLIP_HORIZONTAL;
+	}
+
+	imageflip( $img, $mode );
+
 	return $img;
 }
 

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

@@ -432,31 +432,59 @@ public function rotate( $angle ) {
 	 * Flips current image.
 	 *
 	 * @since 3.5.0
+	 * @since 7.2.0 Switched to using imageflip() instead of imagecopyresampled() with negative dimensions.
 	 *
 	 * @param bool $horz Flip along Horizontal Axis.
 	 * @param bool $vert Flip along Vertical Axis.
 	 * @return true|WP_Error
 	 */
 	public function flip( $horz, $vert ) {
-		$w   = $this->size['width'];
-		$h   = $this->size['height'];
-		$dst = wp_imagecreatetruecolor( $w, $h );
+		if ( ! is_gd_image( $this->image ) ) {
+			return new WP_Error( 'image_flip_error', __( 'Image flip failed.' ), $this->file );
+		}
 
-		if ( is_gd_image( $dst ) ) {
-			$sx = $vert ? ( $w - 1 ) : 0;
-			$sy = $horz ? ( $h - 1 ) : 0;
-			$sw = $vert ? -$w : $w;
-			$sh = $horz ? -$h : $h;
+		/*
+		 * imageflip() works in place and keeps palette images as palette images.
+		 * The previous implementation always copied into a new truecolor image,
+		 * and saving or streaming as WebP relies on that, since imagewebp()
+		 * rejects palette images. Convert first to keep that behavior.
+		 */
+		if ( ! imageistruecolor( $this->image ) && ! imagepalettetotruecolor( $this->image ) ) {
+			return new WP_Error( 'image_flip_error', __( 'Image flip failed.' ), $this->file );
+		}
 
-			if ( imagecopyresampled( $dst, $this->image, 0, 0, $sx, $sy, $w, $h, $sw, $sh ) ) {
-				if ( PHP_VERSION_ID < 80000 ) { // imagedestroy() has no effect as of PHP 8.0.
-					imagedestroy( $this->image );
-				}
+		/*
+		 * imageflip() also keeps the current alpha settings, which rotate() leaves
+		 * with blending enabled. Reset them to what the previous implementation
+		 * always produced, including when no axis is flipped.
+		 */
+		imagealphablending( $this->image, false );
+		imagesavealpha( $this->image, true );
 
-				$this->image = $dst;
+		/*
+		 * imageflip() changes the image in place, so the image keeps its current resolution and interlace setting.
+		 * rotate(), crop() and resize() copy into a new image, which resets them to GD's defaults: 96 DPI and no
+		 * interlacing. Reset them here too, so a flip gives the same result as the other edits.
+		 *
+		 * @link https://core.trac.wordpress.org/ticket/66113
+		 */
+		imageresolution( $this->image, 96, 96 );
+		imageinterlace( $this->image, 0 );
 
-				return true;
-			}
+		if ( ! $horz && ! $vert ) {
+			return true;
+		}
+
+		if ( $horz && $vert ) {
+			$mode = IMG_FLIP_BOTH;
+		} elseif ( $horz ) {
+			$mode = IMG_FLIP_VERTICAL;
+		} else {
+			$mode = IMG_FLIP_HORIZONTAL;
+		}
+
+		if ( imageflip( $this->image, $mode ) ) {
+			return true;
 		}
 
 		return new WP_Error( 'image_flip_error', __( 'Image flip failed.' ), $this->file );