Source

File: wp-includes/class-wp-image-editor-gd.php

protected function _resize( $max_w, $max_h, $crop = false ) {
		$dims = image_resize_dimensions( $this->size['width'], $this->size['height'], $max_w, $max_h, $crop );

		if ( ! $dims ) {
			return new WP_Error( 'error_getting_dimensions', __( 'Could not calculate resized image dimensions' ), $this->file );
		}

		list( $dst_x, $dst_y, $src_x, $src_y, $dst_w, $dst_h, $src_w, $src_h ) = $dims;

		$resized = wp_imagecreatetruecolor( $dst_w, $dst_h );
		imagecopyresampled( $resized, $this->image, $dst_x, $dst_y, $src_x, $src_y, $dst_w, $dst_h, $src_w, $src_h );

		if ( is_resource( $resized ) ) {
			$this->update_size( $dst_w, $dst_h );
			return $resized;
		}

		return new WP_Error( 'image_resize_error', __( 'Image resize failed.' ), $this->file );
	}