Media: Use imageflip() to flip images with GD.

`WP_Image_Editor_GD::flip()` and the deprecated `_flip_image_resource()` mirrored images by copying them with `imagecopyresampled()` and negative source dimensions. With the GD 2.4.0 bundled in PHP 8.6, this produces an all-black image. It affects flips in the image editor, and also JPEG uploads with an EXIF orientation of 2, 3 or 4, which `WP_Image_Editor::maybe_exif_rotate()` flips.

Both functions now flip the image in place with `imageflip()`, and the output stays the same as before. Palette images are still converted to truecolor, alpha blending is turned off and alpha saving on, and the resolution and interlacing are reset to GD's defaults of 96 DPI and off. This matches what `rotate()`, `crop()` and `resize()` produce. Whether every operation should keep the source image's resolution and interlacing will be decided in a follow-up ticket.

When no valid image is loaded, `flip()` now returns a `WP_Error` instead of throwing a `TypeError`.

Props sanjith2023, rumancodes, arshidkv12, dhruvang21, abhishekfdd, desrosj.
Fixes #66113.


git-svn-id: https://develop.svn.wordpress.org/trunk@65367 602fd350-edb4-49c9-b593-d223f7449a82

Author: desrosj

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 );