Source

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

public function make_subsize( $size_data ) {
		if ( ! isset( $size_data['width'] ) && ! isset( $size_data['height'] ) ) {
			return new WP_Error( 'image_subsize_create_error', __( 'Cannot resize the image. Both width and height are not set.' ) );
		}

		$orig_size  = $this->size;
		$orig_image = $this->image->getImage();

		if ( ! isset( $size_data['width'] ) ) {
			$size_data['width'] = null;
		}

		if ( ! isset( $size_data['height'] ) ) {
			$size_data['height'] = null;
		}

		if ( ! isset( $size_data['crop'] ) ) {
			$size_data['crop'] = false;
		}

		$resized = $this->resize( $size_data['width'], $size_data['height'], $size_data['crop'] );

		if ( is_wp_error( $resized ) ) {
			$saved = $resized;
		} else {
			$saved = $this->_save( $this->image );

			$this->image->clear();
			$this->image->destroy();
			$this->image = null;
		}

		$this->size  = $orig_size;
		$this->image = $orig_image;

		if ( ! is_wp_error( $saved ) ) {
			unset( $saved['path'] );
		}

		return $saved;
	}