A short implementation of a shader pass based on (mostly stolen from) https://github.com/stackgl/gl-particles

shader-pass.js

import drawTriangle from "a-big-triangle";
import createShader from "gl-shader";
import createFBO from "gl-fbo";

const vertex = [
    "precision mediump float;",
    "attribute vec2 position;",
    "void main() {",
    "    gl_Position = vec4( position, 1, 1 );",
    "}"
].join('\n')

export class ShaderPass {

    constructor( gl, size, source ) {
        this.gl = gl;
        this.size = size;

        this.logic = createShader( gl, vertex, source );

        this.previous = createFBO( gl, size, size, { float: true } );
        this.current = createFBO( gl, size, size, { float: true } );
    }

    step( update ) {
        this.gl.disable( this.gl.BLEND );

        this.current.bind();
        this.gl.viewport( 0, 0, this.size, this.size );

        this.logic.bind();
        this.logic.uniforms.resolution = [ this.size, this.size ];
        this.logic.uniforms.data = this.previous.color[ 0 ].bind( 0 );

        if ( update ) update( this.logic.uniforms );

        drawTriangle( this.gl );

        this.gl.bindFramebuffer( this.gl.FRAMEBUFFER, null );

        const temp = this.previous;

        this.previous = this.current;
        this.current = temp;

        this.gl.enable( this.gl.BLEND );
    }

    getTexture() {
        return this.previous.color[ 0 ];
    }

    dispose() {
        this.logic.dispose();

        this.previous.dispose();
        this.current.dispose();

        this.logic = this.previous = this.current = undefined;
    }
}