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

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

/**
* @class WebGLFilterManager
* @constructor
*/
PIXI.WebGLFilterManager = function()
{
    /**
     * @property filterStack
     * @type Array
     */
    this.filterStack = [];

    /**
     * @property offsetX
     * @type Number
     */
    this.offsetX = 0;

    /**
     * @property offsetY
     * @type Number
     */
    this.offsetY = 0;
};

PIXI.WebGLFilterManager.prototype.constructor = PIXI.WebGLFilterManager;

/**
* Initialises the context and the properties.
*
* @method setContext
* @param gl {WebGLContext} the current WebGL drawing context
*/
PIXI.WebGLFilterManager.prototype.setContext = function(gl)
{
    this.gl = gl;
    this.texturePool = [];

    this.initShaderBuffers();
};

/**
* @method begin
* @param renderSession {RenderSession}
* @param buffer {ArrayBuffer}
*/
PIXI.WebGLFilterManager.prototype.begin = function(renderSession, buffer)
{
    this.renderSession = renderSession;
    this.defaultShader = renderSession.shaderManager.defaultShader;

    var projection = this.renderSession.projection;
    this.width = projection.x * 2;
    this.height = -projection.y * 2;
    this.buffer = buffer;
};

/**
* Applies the filter and adds it to the current filter stack.
*
* @method pushFilter
* @param filterBlock {Object} the filter that will be pushed to the current filter stack
*/
PIXI.WebGLFilterManager.prototype.pushFilter = function(filterBlock)
{
    var gl = this.gl;

    var projection = this.renderSession.projection;
    var offset = this.renderSession.offset;

    filterBlock._filterArea = filterBlock.target.filterArea || filterBlock.target.getBounds();

    // >>> modify by nextht
    filterBlock._previous_stencil_mgr = this.renderSession.stencilManager;
    this.renderSession.stencilManager = new PIXI.WebGLStencilManager();
    this.renderSession.stencilManager.setContext(gl);
    gl.disable(gl.STENCIL_TEST);
    // <<<  modify by nextht
    // filter program
    // OPTIMISATION - the first filter is free if its a simple color change?
    this.filterStack.push(filterBlock);

    var filter = filterBlock.filterPasses[0];

    this.offsetX += filterBlock._filterArea.x;
    this.offsetY += filterBlock._filterArea.y;

    var texture = this.texturePool.pop();
    if(!texture)
    {
        texture = new PIXI.FilterTexture(this.gl, this.width * this.renderSession.resolution, this.height * this.renderSession.resolution);
    }
    else
    {
        texture.resize(this.width * this.renderSession.resolution, this.height * this.renderSession.resolution);
    }

    gl.bindTexture(gl.TEXTURE_2D,  texture.texture);

    var filterArea = filterBlock._filterArea;// filterBlock.target.getBounds();///filterBlock.target.filterArea;

    var padding = filter.padding;
    filterArea.x -= padding;
    filterArea.y -= padding;
    filterArea.width += padding * 2;
    filterArea.height += padding * 2;

    // cap filter to screen size..
    if(filterArea.x < 0)filterArea.x = 0;    if(filterArea.width > this.width)filterArea.width = this.width;
    if(filterArea.y < 0)filterArea.y = 0;    if(filterArea.height > this.height)filterArea.height = this.height;

    //gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA,  filterArea.width, filterArea.height, 0, gl.RGBA, gl.UNSIGNED_BYTE, null);
    gl.bindFramebuffer(gl.FRAMEBUFFER, texture.frameBuffer);

    // set view port
    gl.viewport(0, 0, filterArea.width * this.renderSession.resolution, filterArea.height * this.renderSession.resolution);

    projection.x = filterArea.width/2;
    projection.y = -filterArea.height/2;

    offset.x = -filterArea.x;
    offset.y = -filterArea.y;

    // update projection
    // now restore the regular shader..
    // this.renderSession.shaderManager.setShader(this.defaultShader);
    //gl.uniform2f(this.defaultShader.projectionVector, filterArea.width/2, -filterArea.height/2);
    //gl.uniform2f(this.defaultShader.offsetVector, -filterArea.x, -filterArea.y);

    gl.colorMask(true, true, true, true);
    gl.clearColor(0,0,0, 0);
    gl.clear(gl.COLOR_BUFFER_BIT);

    filterBlock._glFilterTexture = texture;

};

/**
* Removes the last filter from the filter stack and doesn't return it.
*
* @method popFilter
*/
PIXI.WebGLFilterManager.prototype.popFilter = function()
{
    var gl = this.gl;
    var filterBlock = this.filterStack.pop();
    var filterArea = filterBlock._filterArea;
    var texture = filterBlock._glFilterTexture;
    var projection = this.renderSession.projection;
    var offset = this.renderSession.offset;

    if(filterBlock.filterPasses.length > 1)
    {
        gl.viewport(0, 0, filterArea.width * this.renderSession.resolution, filterArea.height * this.renderSession.resolution);

        gl.bindBuffer(gl.ARRAY_BUFFER, this.vertexBuffer);

        this.vertexArray[0] = 0;
        this.vertexArray[1] = filterArea.height;

        this.vertexArray[2] = filterArea.width;
        this.vertexArray[3] = filterArea.height;

        this.vertexArray[4] = 0;
        this.vertexArray[5] = 0;

        this.vertexArray[6] = filterArea.width;
        this.vertexArray[7] = 0;

        gl.bufferSubData(gl.ARRAY_BUFFER, 0, this.vertexArray);

        gl.bindBuffer(gl.ARRAY_BUFFER, this.uvBuffer);
        // now set the uvs..
        this.uvArray[2] = filterArea.width/this.width;
        this.uvArray[5] = filterArea.height/this.height;
        this.uvArray[6] = filterArea.width/this.width;
        this.uvArray[7] = filterArea.height/this.height;

        gl.bufferSubData(gl.ARRAY_BUFFER, 0, this.uvArray);

        var inputTexture = texture;
        var outputTexture = this.texturePool.pop();
        if(!outputTexture)outputTexture = new PIXI.FilterTexture(this.gl, this.width * this.renderSession.resolution, this.height * this.renderSession.resolution);
        outputTexture.resize(this.width * this.renderSession.resolution, this.height * this.renderSession.resolution);

        // need to clear this FBO as it may have some left over elements from a previous filter.
        gl.bindFramebuffer(gl.FRAMEBUFFER, outputTexture.frameBuffer );
        gl.clear(gl.COLOR_BUFFER_BIT);

        gl.disable(gl.BLEND);

        for (var i = 0; i < filterBlock.filterPasses.length-1; i++)        {
            var filterPass = filterBlock.filterPasses[i];

            gl.bindFramebuffer(gl.FRAMEBUFFER, outputTexture.frameBuffer );

            // set texture
            gl.activeTexture(gl.TEXTURE0);
            gl.bindTexture(gl.TEXTURE_2D, inputTexture.texture);

            // draw texture..
            //filterPass.applyFilterPass(filterArea.width, filterArea.height);
            this.applyFilterPass(filterPass, filterArea, filterArea.width, filterArea.height);

            // swap the textures..
            var temp = inputTexture;
            inputTexture = outputTexture;
            outputTexture = temp;
        }

        gl.enable(gl.BLEND);

        texture = inputTexture;
        this.texturePool.push(outputTexture);
    }

    var filter = filterBlock.filterPasses[filterBlock.filterPasses.length-1];

    this.offsetX -= filterArea.x;
    this.offsetY -= filterArea.y;

    var sizeX = this.width;
    var sizeY = this.height;

    var offsetX = 0;
    var offsetY = 0;

    var buffer = this.buffer;

    // time to render the filters texture to the previous scene
    if(this.filterStack.length === 0)
    {
        gl.colorMask(true, true, true, true);//this.transparent);
    }
    else
    {
        var currentFilter = this.filterStack[this.filterStack.length-1];
        filterArea = currentFilter._filterArea;

        sizeX = filterArea.width;
        sizeY = filterArea.height;

        offsetX = filterArea.x;
        offsetY = filterArea.y;

        buffer =  currentFilter._glFilterTexture.frameBuffer;
    }

    // TODO need to remove these global elements..
    projection.x = sizeX/2;
    projection.y = -sizeY/2;

    offset.x = offsetX;
    offset.y = offsetY;

    filterArea = filterBlock._filterArea;

    var x = filterArea.x-offsetX;
    var y = filterArea.y-offsetY;

    // update the buffers..
    // make sure to flip the y!
    gl.bindBuffer(gl.ARRAY_BUFFER, this.vertexBuffer);

    this.vertexArray[0] = x;
    this.vertexArray[1] = y + filterArea.height;

    this.vertexArray[2] = x + filterArea.width;
    this.vertexArray[3] = y + filterArea.height;

    this.vertexArray[4] = x;
    this.vertexArray[5] = y;

    this.vertexArray[6] = x + filterArea.width;
    this.vertexArray[7] = y;

    gl.bufferSubData(gl.ARRAY_BUFFER, 0, this.vertexArray);

    gl.bindBuffer(gl.ARRAY_BUFFER, this.uvBuffer);

    this.uvArray[2] = filterArea.width/this.width;
    this.uvArray[5] = filterArea.height/this.height;
    this.uvArray[6] = filterArea.width/this.width;
    this.uvArray[7] = filterArea.height/this.height;

    gl.bufferSubData(gl.ARRAY_BUFFER, 0, this.uvArray);

    gl.viewport(0, 0, sizeX * this.renderSession.resolution, sizeY * this.renderSession.resolution);

    // bind the buffer
    gl.bindFramebuffer(gl.FRAMEBUFFER, buffer );

    // set the blend mode!
    //gl.blendFunc(gl.ONE, gl.ONE_MINUS_SRC_ALPHA)

    // set texture
    gl.activeTexture(gl.TEXTURE0);
    gl.bindTexture(gl.TEXTURE_2D, texture.texture);

    // >>> modify by nextht
    if (this.renderSession.stencilManager) {
        this.renderSession.stencilManager.destroy();
    }
    this.renderSession.stencilManager = filterBlock._previous_stencil_mgr;
    filterBlock._previous_stencil_mgr = null;
    if (this.renderSession.stencilManager.count > 0) {
        gl.enable(gl.STENCIL_TEST);
    }
    else {
        gl.disable(gl.STENCIL_TEST);
    }
    // <<< modify by nextht
    // apply!
    this.applyFilterPass(filter, filterArea, sizeX, sizeY);

    // now restore the regular shader.. should happen automatically now..
    // this.renderSession.shaderManager.setShader(this.defaultShader);
    // gl.uniform2f(this.defaultShader.projectionVector, sizeX/2, -sizeY/2);
    // gl.uniform2f(this.defaultShader.offsetVector, -offsetX, -offsetY);

    // return the texture to the pool
    this.texturePool.push(texture);
    filterBlock._glFilterTexture = null;
};


/**
* Applies the filter to the specified area.
*
* @method applyFilterPass
* @param filter {Phaser.Filter} the filter that needs to be applied
* @param filterArea {Texture} TODO - might need an update
* @param width {Number} the horizontal range of the filter
* @param height {Number} the vertical range of the filter
*/
PIXI.WebGLFilterManager.prototype.applyFilterPass = function(filter, filterArea, width, height)
{
    // use program
    var gl = this.gl;
    var shader = filter.shaders[gl.id];

    if(!shader)
    {
        shader = new PIXI.PixiShader(gl);

        shader.fragmentSrc = filter.fragmentSrc;
        shader.uniforms = filter.uniforms;
        shader.init(true);

        filter.shaders[gl.id] = shader;
    }

    // set the shader
    this.renderSession.shaderManager.setShader(shader);

//    gl.useProgram(shader.program);

    gl.uniform2f(shader.projectionVector, width/2, -height/2);
    gl.uniform2f(shader.offsetVector, 0,0);

    if(filter.uniforms.dimensions)
    {
        filter.uniforms.dimensions.value[0] = this.width;//width;
        filter.uniforms.dimensions.value[1] = this.height;//height;
        filter.uniforms.dimensions.value[2] = this.vertexArray[0];
        filter.uniforms.dimensions.value[3] = this.vertexArray[5];//filterArea.height;
    }

    shader.syncUniforms();

    gl.bindBuffer(gl.ARRAY_BUFFER, this.vertexBuffer);
    gl.vertexAttribPointer(shader.aVertexPosition, 2, gl.FLOAT, false, 0, 0);

    gl.bindBuffer(gl.ARRAY_BUFFER, this.uvBuffer);
    gl.vertexAttribPointer(shader.aTextureCoord, 2, gl.FLOAT, false, 0, 0);

    gl.bindBuffer(gl.ARRAY_BUFFER, this.colorBuffer);
    gl.vertexAttribPointer(shader.colorAttribute, 2, gl.FLOAT, false, 0, 0);

    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, this.indexBuffer);

    // draw the filter...
    gl.drawElements(gl.TRIANGLES, 6, gl.UNSIGNED_SHORT, 0 );

    this.renderSession.drawCount++;
};

/**
* Initialises the shader buffers.
*
* @method initShaderBuffers
*/
PIXI.WebGLFilterManager.prototype.initShaderBuffers = function()
{
    var gl = this.gl;

    // create some buffers
    this.vertexBuffer = gl.createBuffer();
    this.uvBuffer = gl.createBuffer();
    this.colorBuffer = gl.createBuffer();
    this.indexBuffer = gl.createBuffer();

    // bind and upload the vertexs..
    // keep a reference to the vertexFloatData..
    this.vertexArray = new Float32Array([0.0, 0.0,                                         1.0, 0.0,
                                         0.0, 1.0,
                                         1.0, 1.0]);

    gl.bindBuffer(gl.ARRAY_BUFFER, this.vertexBuffer);
    gl.bufferData(gl.ARRAY_BUFFER, this.vertexArray, gl.STATIC_DRAW);

    // bind and upload the uv buffer
    this.uvArray = new Float32Array([0.0, 0.0,                                     1.0, 0.0,
                                     0.0, 1.0,
                                     1.0, 1.0]);

    gl.bindBuffer(gl.ARRAY_BUFFER, this.uvBuffer);
    gl.bufferData(gl.ARRAY_BUFFER, this.uvArray, gl.STATIC_DRAW);

    this.colorArray = new Float32Array([1.0, 0xFFFFFF,                                        1.0, 0xFFFFFF,
                                        1.0, 0xFFFFFF,
                                        1.0, 0xFFFFFF]);

    gl.bindBuffer(gl.ARRAY_BUFFER, this.colorBuffer);
    gl.bufferData(gl.ARRAY_BUFFER, this.colorArray, gl.STATIC_DRAW);

    // bind and upload the index
    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, this.indexBuffer);
    gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, new Uint16Array([0, 1, 2, 1, 3, 2]), gl.STATIC_DRAW);

};

/**
* Destroys the filter and removes it from the filter stack.
*
* @method destroy
*/
PIXI.WebGLFilterManager.prototype.destroy = function()
{
    var gl = this.gl;

    this.filterStack = null;

    this.offsetX = 0;
    this.offsetY = 0;

    // destroy textures
    for (var i = 0; i < this.texturePool.length; i++) {        this.texturePool[i].destroy();
    }

    this.texturePool = null;

    //destroy buffers..
    gl.deleteBuffer(this.vertexBuffer);
    gl.deleteBuffer(this.uvBuffer);
    gl.deleteBuffer(this.colorBuffer);
    gl.deleteBuffer(this.indexBuffer);
};