import { D3Selection } from '../types';
/**
 * Inject alpha channel for color.
 * @example
 * mergeAlpha('rgba(0,0,0,0.8)', 0.5) // rgba(0,0,0,0.4)
 * mergeAlpha('rgba(0,0,0,0)', 1) // rgba(0,0,0,0)
 * @returns
 * Return merged color.
 */
export declare function mergeAlpha<T>(color: T, opacity: number): string | T;
/**
 * Extract opacity from color.
 * @example
 * splitAlpha('rgba(0,0,0,0.8)', 0.5) // [0x0, 0.4]
 * splitAlpha('rgba(0,0,0,0)', 1) // [0x0, 0]
 * @returns
 * Color without alpha and merged alpha.
 */
export declare function splitAlpha(color: string, opacity: number): number[];
/**
 * Attribute fetching method for drawing methods.
 * @param index
 * The group index for one attribute.
 * @param defaultValue
 * Return defaultValue when get attribute failed.
 * @returns
 * Return attribute value.
 */
export declare function getAttr<T>(target: MaybeGroup<T>, index: number | undefined, defaultValue: T): T;
/**
 * Batch add styles for d3 selection.
 * @param style
 * The object that represent element styles.
 * @param index
 * The group index for each attributes.
 */
export declare function addStyle(target: D3Selection, style: AnyObject, index?: number): D3Selection<unknown>;