Source: packages/filters/filter-displacement/src/DisplacementFilter.ts

import { Filter } from '@pixi/core';
import { Matrix, Point } from '@pixi/math';
import fragment from './displacement.frag';
import vertex from './displacement.vert';

import type { CLEAR_MODES } from '@pixi/constants';
import type { FilterSystem, RenderTexture, Texture, ISpriteMaskTarget } from '@pixi/core';

/**
 * The DisplacementFilter class uses the pixel values from the specified texture
 * (called the displacement map) to perform a displacement of an object.
 *
 * You can use this filter to apply all manor of crazy warping effects.
 * Currently the `r` property of the texture is used to offset the `x`
 * and the `g` property of the texture is used to offset the `y`.
 *
 * The way it works is it uses the values of the displacement map to look up the
 * correct pixels to output. This means it's not technically moving the original.
 * Instead, it's starting at the output and asking "which pixel from the original goes here".
 * For example, if a displacement map pixel has `red = 1` and the filter scale is `20`,
 * this filter will output the pixel approximately 20 pixels to the right of the original.
 *
 * @class
 * @extends PIXI.Filter
 * @memberof PIXI.filters
 */
export class DisplacementFilter extends Filter
{
    public maskSprite: ISpriteMaskTarget;
    public maskMatrix: Matrix;
    public scale: Point;

    /**
     * @param {PIXI.Sprite} sprite - The sprite used for the displacement map. (make sure its added to the scene!)
     * @param {number} [scale] - The scale of the displacement
     */
    constructor(sprite: ISpriteMaskTarget, scale: number)
    {
        const maskMatrix = new Matrix();

        sprite.renderable = false;

        super(vertex, fragment, {
            mapSampler: sprite._texture,
            filterMatrix: maskMatrix,
            scale: { x: 1, y: 1 },
            rotation: new Float32Array([1, 0, 0, 1]),
        });

        this.maskSprite = sprite;
        this.maskMatrix = maskMatrix;

        if (scale === null || scale === undefined)
        {
            scale = 20;
        }

        /**
         * scaleX, scaleY for displacements
         * @member {PIXI.Point}
         */
        this.scale = new Point(scale, scale);
    }

    /**
     * Applies the filter.
     *
     * @param {PIXI.FilterSystem} filterManager - The manager.
     * @param {PIXI.RenderTexture} input - The input target.
     * @param {PIXI.RenderTexture} output - The output target.
     * @param {PIXI.CLEAR_MODES} clearMode - clearMode.
     */
    public apply(
        filterManager: FilterSystem, input: RenderTexture, output: RenderTexture, clearMode: CLEAR_MODES
    ): void
    {
        // fill maskMatrix with _normalized sprite texture coords_
        this.uniforms.filterMatrix = filterManager.calculateSpriteMatrix(this.maskMatrix, this.maskSprite);
        this.uniforms.scale.x = this.scale.x;
        this.uniforms.scale.y = this.scale.y;

        // Extract rotation from world transform
        const wt = this.maskSprite.worldTransform;
        const lenX = Math.sqrt((wt.a * wt.a) + (wt.b * wt.b));
        const lenY = Math.sqrt((wt.c * wt.c) + (wt.d * wt.d));

        if (lenX !== 0 && lenY !== 0)
        {
            this.uniforms.rotation[0] = wt.a / lenX;
            this.uniforms.rotation[1] = wt.b / lenX;
            this.uniforms.rotation[2] = wt.c / lenY;
            this.uniforms.rotation[3] = wt.d / lenY;
        }

        // draw the filter...
        filterManager.applyFilter(this, input, output, clearMode);
    }

    /**
     * The texture used for the displacement map. Must be power of 2 sized texture.
     *
     * @member {PIXI.Texture}
     */
    get map(): Texture
    {
        return this.uniforms.mapSampler;
    }

    set map(value: Texture)
    {
        this.uniforms.mapSampler = value;
    }
}