Source: tiny/core/renderers/webgl/managers/StencilManager.js

import WebGLManager from './WebGLManager';

/**
 * @class
 * @extends Tiny.WebGLManager
 * @memberof Tiny
 */
export default class StencilManager extends WebGLManager {
  /**
   * @param {Tiny.WebGLRenderer} renderer - The renderer this manager works for.
   */
  constructor(renderer) {
    super(renderer);
    this.stencilMaskStack = null;
  }

  /**
   * Changes the mask stack that is used by this manager.
   *
   * @param {Tiny.Graphics[]} stencilMaskStack - The mask stack
   */
  setMaskStack(stencilMaskStack) {
    this.stencilMaskStack = stencilMaskStack;

    const gl = this.renderer.gl;

    if (stencilMaskStack.length === 0) {
      gl.disable(gl.STENCIL_TEST);
    } else {
      gl.enable(gl.STENCIL_TEST);
    }
  }

  /**
   * Applies the Mask and adds it to the current filter stack. @alvin
   *
   * @param {Tiny.Graphics} graphics - The mask
   */
  pushStencil(graphics) {
    this.renderer.setObjectRenderer(this.renderer.plugins.graphics);

    this.renderer._activeRenderTarget.attachStencilBuffer();

    const gl = this.renderer.gl;
    const sms = this.stencilMaskStack;

    if (sms.length === 0) {
      gl.enable(gl.STENCIL_TEST);
      gl.clear(gl.STENCIL_BUFFER_BIT);
      gl.stencilFunc(gl.ALWAYS, 1, 1);
    }

    sms.push(graphics);

    gl.colorMask(false, false, false, false);
    gl.stencilOp(gl.KEEP, gl.KEEP, gl.INCR);

    this.renderer.plugins.graphics.render(graphics);

    gl.colorMask(true, true, true, true);
    gl.stencilFunc(gl.NOTEQUAL, 0, sms.length);
    gl.stencilOp(gl.KEEP, gl.KEEP, gl.KEEP);
  }

  /**
   * TODO @alvin
   */
  popStencil() {
    this.renderer.setObjectRenderer(this.renderer.plugins.graphics);

    const gl = this.renderer.gl;
    const sms = this.stencilMaskStack;

    const graphics = sms.pop();

    if (sms.length === 0) {
      // the stack is empty!
      gl.disable(gl.STENCIL_TEST);
    } else {
      gl.colorMask(false, false, false, false);
      gl.stencilOp(gl.KEEP, gl.KEEP, gl.DECR);

      this.renderer.plugins.graphics.render(graphics);

      gl.colorMask(true, true, true, true);
      gl.stencilFunc(gl.NOTEQUAL, 0, sms.length);
      gl.stencilOp(gl.KEEP, gl.KEEP, gl.KEEP);
    }
  }

  /**
   * Destroys the mask stack.
   *
   */
  destroy() {
    WebGLManager.prototype.destroy.call(this);

    this.stencilMaskStack.stencilStack = null;
  }
}