Source: src/pixi/renderers/webgl/utils/WebGLStencilManager.js

/**
 * @author Mat Groves http://matgroves.com/ @Doormat23
 */

/**
* @class WebGLStencilManager
* @constructor
* @private
*/
PIXI.WebGLStencilManager = function()
{
    this.stencilStack = [];
    this.reverse = true;
    this.count = 0;
};

/**
* Sets the drawing context to the one given in parameter.
*
* @method setContext
* @param gl {WebGLContext} the current WebGL drawing context
*/
PIXI.WebGLStencilManager.prototype.setContext = function(gl)
{
    this.gl = gl;
};

/**
* Applies the Mask and adds it to the current filter stack.
*
* @method pushMask
* @param graphics {Graphics}
* @param webGLData {Array}
* @param renderSession {Object}
*/
PIXI.WebGLStencilManager.prototype.pushStencil = function(graphics, webGLData, renderSession)
{
    var gl = this.gl;
    this.bindGraphics(graphics, webGLData, renderSession);

    if(this.stencilStack.length === 0)
    {
        gl.enable(gl.STENCIL_TEST);
        gl.clear(gl.STENCIL_BUFFER_BIT);
        this.reverse = true;
        this.count = 0;
    }

    this.stencilStack.push(webGLData);

    var level = this.count;

    gl.colorMask(false, false, false, false);

    gl.stencilFunc(gl.ALWAYS,0,0xFF);
    gl.stencilOp(gl.KEEP,gl.KEEP,gl.INVERT);

    // draw the triangle strip!

    if(webGLData.mode === 1)
    {
        gl.drawElements(gl.TRIANGLE_FAN,  webGLData.indices.length - 4, gl.UNSIGNED_SHORT, 0 );

        if(this.reverse)
        {
            gl.stencilFunc(gl.EQUAL, 0xFF - level, 0xFF);
            gl.stencilOp(gl.KEEP,gl.KEEP,gl.DECR);
        }
        else
        {
            gl.stencilFunc(gl.EQUAL,level, 0xFF);
            gl.stencilOp(gl.KEEP,gl.KEEP,gl.INCR);
        }

        // draw a quad to increment..
        gl.drawElements(gl.TRIANGLE_FAN, 4, gl.UNSIGNED_SHORT, ( webGLData.indices.length - 4 ) * 2 );

        if(this.reverse)
        {
            gl.stencilFunc(gl.EQUAL,0xFF-(level+1), 0xFF);
        }
        else
        {
            gl.stencilFunc(gl.EQUAL,level+1, 0xFF);
        }

        this.reverse = !this.reverse;
    }
    else
    {
        if(!this.reverse)
        {
            gl.stencilFunc(gl.EQUAL, 0xFF - level, 0xFF);
            gl.stencilOp(gl.KEEP,gl.KEEP,gl.DECR);
        }
        else
        {
            gl.stencilFunc(gl.EQUAL,level, 0xFF);
            gl.stencilOp(gl.KEEP,gl.KEEP,gl.INCR);
        }

        gl.drawElements(gl.TRIANGLE_STRIP,  webGLData.indices.length, gl.UNSIGNED_SHORT, 0 );

        if(!this.reverse)
        {
            gl.stencilFunc(gl.EQUAL,0xFF-(level+1), 0xFF);
        }
        else
        {
            gl.stencilFunc(gl.EQUAL,level+1, 0xFF);
        }
    }

    gl.colorMask(true, true, true, true);
    gl.stencilOp(gl.KEEP,gl.KEEP,gl.KEEP);

    this.count++;
};

/**
 * TODO this does not belong here!
 *
 * @method bindGraphics
 * @param graphics {Graphics}
 * @param webGLData {Array}
 * @param renderSession {Object}
 */
PIXI.WebGLStencilManager.prototype.bindGraphics = function(graphics, webGLData, renderSession)
{
    //if(this._currentGraphics === graphics)return;
    this._currentGraphics = graphics;

    var gl = this.gl;

     // bind the graphics object..
    var projection = renderSession.projection,
        offset = renderSession.offset,
        shader;// = renderSession.shaderManager.primitiveShader;

    if(webGLData.mode === 1)
    {
        shader = renderSession.shaderManager.complexPrimitiveShader;

        renderSession.shaderManager.setShader( shader );

        gl.uniform1f(shader.flipY, renderSession.flipY);

        gl.uniformMatrix3fv(shader.translationMatrix, false, graphics.worldTransform.toArray(true));

        gl.uniform2f(shader.projectionVector, projection.x, -projection.y);
        gl.uniform2f(shader.offsetVector, -offset.x, -offset.y);

        gl.uniform3fv(shader.tintColor, Phaser.Color.hexToRGBArray(graphics.tint));
        gl.uniform3fv(shader.color, webGLData.color);

        gl.uniform1f(shader.alpha, graphics.worldAlpha * webGLData.alpha);

        gl.bindBuffer(gl.ARRAY_BUFFER, webGLData.buffer);

        gl.vertexAttribPointer(shader.aVertexPosition, 2, gl.FLOAT, false, 4 * 2, 0);


        // now do the rest..
        // set the index buffer!
        gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, webGLData.indexBuffer);
    }
    else
    {
        //renderSession.shaderManager.activatePrimitiveShader();
        shader = renderSession.shaderManager.primitiveShader;
        renderSession.shaderManager.setShader( shader );

        gl.uniformMatrix3fv(shader.translationMatrix, false, graphics.worldTransform.toArray(true));

        gl.uniform1f(shader.flipY, renderSession.flipY);
        gl.uniform2f(shader.projectionVector, projection.x, -projection.y);
        gl.uniform2f(shader.offsetVector, -offset.x, -offset.y);

        gl.uniform3fv(shader.tintColor, Phaser.Color.hexToRGBArray(graphics.tint));

        gl.uniform1f(shader.alpha, graphics.worldAlpha);

        gl.bindBuffer(gl.ARRAY_BUFFER, webGLData.buffer);

        gl.vertexAttribPointer(shader.aVertexPosition, 2, gl.FLOAT, false, 4 * 6, 0);
        gl.vertexAttribPointer(shader.colorAttribute, 4, gl.FLOAT, false,4 * 6, 2 * 4);

        // set the index buffer!
        gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, webGLData.indexBuffer);
    }
};

/**
 * @method popStencil
 * @param graphics {Graphics}
 * @param webGLData {Array}
 * @param renderSession {Object}
 */
PIXI.WebGLStencilManager.prototype.popStencil = function(graphics, webGLData, renderSession)
{
    var gl = this.gl;
    this.stencilStack.pop();

    this.count--;

    if(this.stencilStack.length === 0)
    {
        // the stack is empty!
        gl.disable(gl.STENCIL_TEST);

    }
    else
    {

        var level = this.count;

        this.bindGraphics(graphics, webGLData, renderSession);

        gl.colorMask(false, false, false, false);

        if(webGLData.mode === 1)
        {
            this.reverse = !this.reverse;

            if(this.reverse)
            {
                gl.stencilFunc(gl.EQUAL, 0xFF - (level+1), 0xFF);
                gl.stencilOp(gl.KEEP,gl.KEEP,gl.INCR);
            }
            else
            {
                gl.stencilFunc(gl.EQUAL,level+1, 0xFF);
                gl.stencilOp(gl.KEEP,gl.KEEP,gl.DECR);
            }

            // draw a quad to increment..
            gl.drawElements(gl.TRIANGLE_FAN, 4, gl.UNSIGNED_SHORT, ( webGLData.indices.length - 4 ) * 2 );

            gl.stencilFunc(gl.ALWAYS,0,0xFF);
            gl.stencilOp(gl.KEEP,gl.KEEP,gl.INVERT);

            // draw the triangle strip!
            gl.drawElements(gl.TRIANGLE_FAN,  webGLData.indices.length - 4, gl.UNSIGNED_SHORT, 0 );

            if(!this.reverse)
            {
                gl.stencilFunc(gl.EQUAL,0xFF-(level), 0xFF);
            }
            else
            {
                gl.stencilFunc(gl.EQUAL,level, 0xFF);
            }

        }
        else
        {
          //  console.log("<<>>")
            if(!this.reverse)
            {
                gl.stencilFunc(gl.EQUAL, 0xFF - (level+1), 0xFF);
                gl.stencilOp(gl.KEEP,gl.KEEP,gl.INCR);
            }
            else
            {
                gl.stencilFunc(gl.EQUAL,level+1, 0xFF);
                gl.stencilOp(gl.KEEP,gl.KEEP,gl.DECR);
            }

            gl.drawElements(gl.TRIANGLE_STRIP,  webGLData.indices.length, gl.UNSIGNED_SHORT, 0 );

            if(!this.reverse)
            {
                gl.stencilFunc(gl.EQUAL,0xFF-(level), 0xFF);
            }
            else
            {
                gl.stencilFunc(gl.EQUAL,level, 0xFF);
            }
        }

        gl.colorMask(true, true, true, true);
        gl.stencilOp(gl.KEEP,gl.KEEP,gl.KEEP);


    }
};

/**
* Destroys the mask stack.
*
* @method destroy
*/
PIXI.WebGLStencilManager.prototype.destroy = function()
{
    this.stencilStack = null;
    this.gl = null;
};