Output WebP by default for core sizes

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

@@ -356,26 +356,7 @@ protected function get_output_format( $filename = null, $mime_type = null ) {
 			$new_ext   = $file_ext;
 		}
 
-		/**
-		 * Filters the image editor output format mapping.
-		 *
-		 * Enables filtering the mime type used to save images. By default,
-		 * the mapping array is empty, so the mime type matches the source image.
-		 *
-		 * @see WP_Image_Editor::get_output_format()
-		 *
-		 * @since 5.8.0
-		 *
-		 * @param string[] $output_format {
-		 *     An array of mime type mappings. Maps a source mime type to a new
-		 *     destination mime type. Default empty array.
-		 *
-		 *     @type string ...$0 The new mime type.
-		 * }
-		 * @param string $filename  Path to the image.
-		 * @param string $mime_type The source image mime type.
-		 */
-		$output_format = apply_filters( 'image_editor_output_format', array(), $filename, $mime_type );
+		get_default_image_editor_output_format( $filename, $mime_type );
 
 		if ( isset( $output_format[ $mime_type ] )
 			&& $this->supports_mime_type( $output_format[ $mime_type ] )

src/wp-includes/functions.php

@@ -2692,7 +2692,7 @@ function wp_unique_filename( $dir, $filename, $unique_filename_callback = null )
 		 */
 		if ( $is_image ) {
 			/** This filter is documented in wp-includes/class-wp-image-editor.php */
-			$output_formats = apply_filters( 'image_editor_output_format', array(), $_dir . $filename, $mime_type );
+			$output_formats = get_default_image_editor_output_format( $_dir . $filename, $mime_type );
 			$alt_types      = array();
 
 			if ( ! empty( $output_formats[ $mime_type ] ) ) {
@@ -2760,6 +2760,47 @@ function wp_unique_filename( $dir, $filename, $unique_filename_callback = null )
 		}
 	}
 
+	/**
+	 * Get the default output format. Return is filtered by `image_editor_output_format`.
+	 *
+	 * @since 6.4.0
+	 *
+	 * @param string $filename  Path to the image.
+	 * @param string $mime_type The source image mime type.
+	 *
+	 * @return string[] $output_format {
+	 *     An array of mime type mappings. Maps a source mime type to a new
+	 *     destination mime type. Default maps jpeg to webp.
+	 *
+	 *     @type string ...$0 The new mime type.
+	 * }
+	 */
+	function get_default_image_editor_output_format( $filename, $mime_type ) {
+		$default = array (
+			'image/jpeg' => 'image/webp',
+		);
+		/**
+		 * Filters the image editor output format mapping.
+		 *
+		 * Enables filtering the mime type used to save images. By default,
+		 * the mapping array is empty, so the mime type matches the source image.
+		 *
+		 * @see WP_Image_Editor::get_output_format()
+		 *
+		 * @since 5.8.0
+		 *
+		 * @param string[] $output_format {
+		 *     An array of mime type mappings. Maps a source mime type to a new
+		 *     destination mime type. Default maps jpeg to webp.
+		 *
+		 *     @type string ...$0 The new mime type.
+		 * }
+		 * @param string $filename  Path to the image.
+		 * @param string $mime_type The source image mime type.
+		 */
+		return apply_filters( 'image_editor_output_format', $default(), $filename, $mime_type );
+	}
+
 	/**
 	 * Filters the result when generating a unique file name.
 	 *

src/wp-includes/media.php

@@ -3919,8 +3919,7 @@ function wp_get_image_editor( $path, $args = array() ) {
 
 	// Check and set the output mime type mapped to the input type.
 	if ( isset( $args['mime_type'] ) ) {
-		/** This filter is documented in wp-includes/class-wp-image-editor.php */
-		$output_format = apply_filters( 'image_editor_output_format', array(), $path, $args['mime_type'] );
+		$output_format = get_default_image_editor_output_format( $path, $args['mime_type'] );
 		if ( isset( $output_format[ $args['mime_type'] ] ) ) {
 			$args['output_mime_type'] = $output_format[ $args['mime_type'] ];
 		}