Source

File: wp-admin/includes/image.php

function _wp_make_subsizes( $new_sizes, $file, $image_meta, $attachment_id ) {
	if ( empty( $image_meta ) || ! is_array( $image_meta ) ) {
		// Not an image attachment.
		return array();
	}

	// Check if any of the new sizes already exist.
	if ( isset( $image_meta['sizes'] ) && is_array( $image_meta['sizes'] ) ) {
		foreach ( $image_meta['sizes'] as $size_name => $size_meta ) {
			/*
			 * Only checks "size name" so we don't override existing images even if the dimensions
			 * don't match the currently defined size with the same name.
			 * To change the behavior, unset changed/mismatched sizes in the `sizes` array in image meta.
			 */
			if ( array_key_exists( $size_name, $new_sizes ) ) {
				unset( $new_sizes[ $size_name ] );
			}
		}
	} else {
		$image_meta['sizes'] = array();
	}

	if ( empty( $new_sizes ) ) {
		// Nothing to do...
		return $image_meta;
	}

	/*
	 * Sort the image sub-sizes in order of priority when creating them.
	 * This ensures there is an appropriate sub-size the user can access immediately
	 * even when there was an error and not all sub-sizes were created.
	 */
	$priority = array(
		'medium'       => null,
		'large'        => null,
		'thumbnail'    => null,
		'medium_large' => null,
	);

	$new_sizes = array_filter( array_merge( $priority, $new_sizes ) );

	$editor = wp_get_image_editor( $file );

	if ( is_wp_error( $editor ) ) {
		// The image cannot be edited.
		return $image_meta;
	}

	// If stored EXIF data exists, rotate the source image before creating sub-sizes.
	if ( ! empty( $image_meta['image_meta'] ) ) {
		$rotated = $editor->maybe_exif_rotate();

		if ( is_wp_error( $rotated ) ) {
			// TODO: Log errors.
		}
	}

	if ( method_exists( $editor, 'make_subsize' ) ) {
		foreach ( $new_sizes as $new_size_name => $new_size_data ) {
			$new_size_meta = $editor->make_subsize( $new_size_data );

			if ( is_wp_error( $new_size_meta ) ) {
				// TODO: Log errors.
			} else {
				// Save the size meta value.
				$image_meta['sizes'][ $new_size_name ] = $new_size_meta;
				wp_update_attachment_metadata( $attachment_id, $image_meta );
			}
		}
	} else {
		// Fall back to `$editor->multi_resize()`.
		$created_sizes = $editor->multi_resize( $new_sizes );

		if ( ! empty( $created_sizes ) ) {
			$image_meta['sizes'] = array_merge( $image_meta['sizes'], $created_sizes );
			wp_update_attachment_metadata( $attachment_id, $image_meta );
		}
	}

	return $image_meta;
}