Source: packages/core/src/renderTexture/RenderTextureSystem.js

import { System } from '../System';
import { Rectangle } from '@pixi/math';
const tempRect = new Rectangle();
/**
 * System plugin to the renderer to manage render textures.
 *
 * Should be added after FramebufferSystem
 *
 * @class
 * @extends PIXI.System
 * @memberof PIXI.systems
 */

export class RenderTextureSystem extends System
{
    /**
     * @param {PIXI.Renderer} renderer - The renderer this System works for.
     */
    constructor(renderer)
    {
        super(renderer);
        /**
         * The clear background color as rgba
         * @member {number[]}
         */
        this.clearColor = renderer._backgroundColorRgba;
        // TODO move this property somewhere else!
        /**
         * List of masks for the StencilSystem
         * @member {PIXI.Graphics[]}
         * @readonly
         */
        this.defaultMaskStack = [];
        // empty render texture?
        /**
         * Render texture
         * @member {PIXI.RenderTexture}
         * @readonly
         */
        this.current = null;
        /**
         * Source frame
         * @member {PIXI.Rectangle}
         * @readonly
         */
        this.sourceFrame = new Rectangle();
        /**
         * Destination frame
         * @member {PIXI.Rectangle}
         * @readonly
         */
        this.destinationFrame = new Rectangle();
    }
    /**
     * Bind the current render texture
     * @param {PIXI.RenderTexture} [renderTexture] - RenderTexture to bind, by default its `null`, the screen
     * @param {PIXI.Rectangle} [sourceFrame] - part of screen that is mapped to the renderTexture
     * @param {PIXI.Rectangle} [destinationFrame] - part of renderTexture, by default it has the same size as sourceFrame
     */
    bind(renderTexture = null, sourceFrame, destinationFrame)
    {
        this.current = renderTexture;
        const renderer = this.renderer;
        let resolution;

        if (renderTexture)
        {
            const baseTexture = renderTexture.baseTexture;

            resolution = baseTexture.resolution;
            if (!destinationFrame)
            {
                tempRect.width = baseTexture.realWidth;
                tempRect.height = baseTexture.realHeight;
                destinationFrame = tempRect;
            }
            if (!sourceFrame)
            {
                sourceFrame = destinationFrame;
            }
            this.renderer.framebuffer.bind(baseTexture.framebuffer, destinationFrame);
            this.renderer.projection.update(destinationFrame, sourceFrame, resolution, false);
            this.renderer.mask.setMaskStack(baseTexture.maskStack);
        }
        else
        {
            resolution = this.renderer.resolution;
            // TODO these validation checks happen deeper down..
            // thing they can be avoided..
            if (!destinationFrame)
            {
                tempRect.width = renderer.width;
                tempRect.height = renderer.height;
                destinationFrame = tempRect;
            }
            if (!sourceFrame)
            {
                sourceFrame = destinationFrame;
            }
            renderer.framebuffer.bind(null, destinationFrame);
            // TODO store this..
            this.renderer.projection.update(destinationFrame, sourceFrame, resolution, true);
            this.renderer.mask.setMaskStack(this.defaultMaskStack);
        }
        this.sourceFrame.copyFrom(sourceFrame);
        this.destinationFrame.x = destinationFrame.x / resolution;
        this.destinationFrame.y = destinationFrame.y / resolution;
        this.destinationFrame.width = destinationFrame.width / resolution;
        this.destinationFrame.height = destinationFrame.height / resolution;
        if (sourceFrame === destinationFrame)
        {
            this.sourceFrame.copyFrom(this.destinationFrame);
        }
    }
    /**
     * Erases the render texture and fills the drawing area with a colour
     *
     * @param {number[]} [clearColor] - The color as rgba, default to use the renderer backgroundColor
     * @return {PIXI.Renderer} Returns itself.
     */
    clear(clearColor)
    {
        if (this.current)
        {
            clearColor = clearColor || this.current.baseTexture.clearColor;
        }
        else
        {
            clearColor = clearColor || this.clearColor;
        }
        this.renderer.framebuffer.clear(clearColor[0], clearColor[1], clearColor[2], clearColor[3]);
    }
    resize()
    {
        // resize the root only!
        this.bind(null);
    }
    /**
     * Resets renderTexture state
     */
    reset()
    {
        this.bind(null);
    }
}