UnrealBloomPassAlpha.js is a modified version of the three.js UnrealBloomPass that preserves alpha channel functionality.

UnrealBloomPassAlpha.js

import {
  AdditiveBlending,
  Color,
  LinearFilter,
  MeshBasicMaterial,
  RGBAFormat,
  ShaderMaterial,
  Texture,
  UniformsUtils,
  Vector2,
  Vector3,
  WebGLRenderer,
  WebGLRenderTarget,
} from "three";
import { Pass, FullScreenQuad } from "three/examples/jsm/postprocessing/Pass";
import { CopyShader } from "three/examples/jsm/shaders/CopyShader.js";
import { LuminosityHighPassShader } from "three/examples/jsm/shaders/LuminosityHighPassShader.js";

/**
 * UnrealBloomPass is inspired by the bloom pass of Unreal Engine. It creates a
 * mip map chain of bloom textures and blurs them with different radii. Because
 * of the weighted combination of mips, and because larger blurs are done on
 * higher mips, this effect provides good quality and performance.
 *
 * Reference:
 * - https://docs.unrealengine.com/latest/INT/Engine/Rendering/PostProcessEffects/Bloom/
 */

class UnrealBloomPassAlpha extends Pass {

  constructor( resolution, strength, radius, threshold ) {

    super();

this.strength = ( strength !== undefined ) ? strength : 1;
    this.radius = radius;
    this.threshold = threshold;
    this.resolution = ( resolution !== undefined ) ? new Vector2( resolution.x, resolution.y ) : new Vector2( 256, 256 );

    // create color only once here, reuse it later inside the render function
    this.clearColor = new Color(0, 0, 0);

    // render targets
    const pars = { minFilter: LinearFilter, magFilter: LinearFilter, format: RGBAFormat };
    this.renderTargetsHorizontal = [];
    this.renderTargetsVertical = [];
    this.nMips = 5;
    let resx = Math.round(this.resolution.x / 2);
    let resy = Math.round(this.resolution.y / 2);

    this.renderTargetBright = new WebGLRenderTarget(resx, resy, pars);
    this.renderTargetBright.texture.name = "UnrealBloomPass.bright";
    this.renderTargetBright.texture.generateMipmaps = false;

    for (let i = 0; i < this.nMips; i++) {

      const renderTargetHorizonal = new WebGLRenderTarget(resx, resy, pars);

      renderTargetHorizonal.texture.name = "UnrealBloomPass.h" + i;
      renderTargetHorizonal.texture.generateMipmaps = false;

      this.renderTargetsHorizontal.push(renderTargetHorizonal);

      const renderTargetVertical = new WebGLRenderTarget(resx, resy, pars);

      renderTargetVertical.texture.name = "UnrealBloomPass.v" + i;
      renderTargetVertical.texture.generateMipmaps = false;

      this.renderTargetsVertical.push(renderTargetVertical);

      resx = Math.round(resx / 2);

      resy = Math.round(resy / 2);
    }

    // luminosity high pass material

    if (LuminosityHighPassShader === undefined)
      console.error("THREE.UnrealBloomPass relies on LuminosityHighPassShader");

    const highPassShader = LuminosityHighPassShader;
    this.highPassUniforms = UniformsUtils.clone(highPassShader.uniforms);

    this.highPassUniforms["luminosityThreshold"].value = threshold;
    this.highPassUniforms["smoothWidth"].value = 0.01;

    this.materialHighPassFilter = new ShaderMaterial({
      uniforms: this.highPassUniforms,
      vertexShader: highPassShader.vertexShader,
      fragmentShader: highPassShader.fragmentShader,
      defines: {},
    });

    // Gaussian Blur Materials
    this.separableBlurMaterials = [];
    const kernelSizeArray = [3, 5, 7, 9, 11];
    resx = Math.round(this.resolution.x / 2);
    resy = Math.round(this.resolution.y / 2);

    for (let i = 0; i < this.nMips; i++) {
      this.separableBlurMaterials.push(
        this.getSeperableBlurMaterial(kernelSizeArray[ i ])
      );

      this.separableBlurMaterials[ i ].uniforms["texSize"].value = new Vector2(
        resx,
        resy
      );

      resx = Math.round(resx / 2);

      resy = Math.round(resy / 2);
    }

    // Composite material
    this.compositeMaterial = this.getCompositeMaterial(this.nMips);
    this.compositeMaterial.uniforms["blurTexture1"].value =
      this.renderTargetsVertical[0].texture;
    this.compositeMaterial.uniforms["blurTexture2"].value =
      this.renderTargetsVertical[1].texture;
    this.compositeMaterial.uniforms["blurTexture3"].value =
      this.renderTargetsVertical[2].texture;
    this.compositeMaterial.uniforms["blurTexture4"].value =
      this.renderTargetsVertical[3].texture;
    this.compositeMaterial.uniforms["blurTexture5"].value =
      this.renderTargetsVertical[4].texture;
    this.compositeMaterial.uniforms["bloomStrength"].value = strength;
    this.compositeMaterial.uniforms["bloomRadius"].value = 0.1;
    this.compositeMaterial.needsUpdate = true;

    const bloomFactors = [1.0, 0.8, 0.6, 0.4, 0.2];
    this.compositeMaterial.uniforms["bloomFactors"].value = bloomFactors;
    this.bloomTintColors = [
      new Vector3(1, 1, 1),
      new Vector3(1, 1, 1),
      new Vector3(1, 1, 1),
      new Vector3(1, 1, 1),
      new Vector3(1, 1, 1),
    ];
    this.compositeMaterial.uniforms["bloomTintColors"].value =
      this.bloomTintColors;

    // copy material
    if (CopyShader === undefined) {
      console.error("THREE.UnrealBloomPass relies on CopyShader");
    }

    const copyShader = CopyShader;

    this.copyUniforms = UniformsUtils.clone(copyShader.uniforms);
    this.copyUniforms["opacity"].value = 1.0;

    this.materialCopy = new ShaderMaterial({
      uniforms: this.copyUniforms,
      vertexShader: copyShader.vertexShader,
      fragmentShader: copyShader.fragmentShader,
      blending: AdditiveBlending,
      depthTest: false,
      depthWrite: false,
      transparent: true,
    });

    this.enabled = true;
    this.needsSwap = false;

    this._oldClearColor = new Color();
    this.oldClearAlpha = 1;

    this.basic = new MeshBasicMaterial();

    this.fsQuad = new FullScreenQuad(null);
  }

  dispose() {
    for (let i = 0; i < this.renderTargetsHorizontal.length; i++) {
      this.renderTargetsHorizontal[ i ].dispose();
    }

    for (let i = 0; i < this.renderTargetsVertical.length; i++) {
      this.renderTargetsVertical[ i ].dispose();
    }

    this.renderTargetBright.dispose();
  }

  setSize(width, height) {
    let resx = Math.round(width / 2);
    let resy = Math.round(height / 2);

    this.renderTargetBright.setSize(resx, resy);

    for (let i = 0; i < this.nMips; i++) {
      this.renderTargetsHorizontal[ i ].setSize(resx, resy);
      this.renderTargetsVertical[ i ].setSize(resx, resy);

      this.separableBlurMaterials[ i ].uniforms["texSize"].value = new Vector2(
        resx,
        resy
      );

      resx = Math.round(resx / 2);
      resy = Math.round(resy / 2);
    }
  }

  render(renderer, writeBuffer, readBuffer, deltaTime, maskActive) {

    renderer.getClearColor(this._oldClearColor);
    this.oldClearAlpha = renderer.getClearAlpha();
    const oldAutoClear = renderer.autoClear;
    renderer.autoClear = false;

    renderer.setClearColor(this.clearColor, 0);

    if (maskActive) renderer.state.buffers.stencil.setTest(false);

    // Render input to screen

    if (this.renderToScreen) {
      this.fsQuad.material = this.basic;
      this.basic.map = readBuffer.texture;

      renderer.setRenderTarget(null);
      renderer.clear();
      this.fsQuad.render(renderer);
    }

    // 1. Extract Bright Areas

    this.highPassUniforms["tDiffuse"].value = readBuffer.texture;
    this.highPassUniforms["luminosityThreshold"].value = this.threshold;
    this.fsQuad.material = this.materialHighPassFilter;

    renderer.setRenderTarget(this.renderTargetBright);
    renderer.clear();
    this.fsQuad.render(renderer);

    // 2. Blur All the mips progressively

    let inputRenderTarget = this.renderTargetBright;

    for (let i = 0; i < this.nMips; i++) {
      this.fsQuad.material = this.separableBlurMaterials[ i ];

      this.separableBlurMaterials[ i ].uniforms["colorTexture"].value = inputRenderTarget.texture;
      this.separableBlurMaterials[ i ].uniforms["direction"].value = UnrealBloomPassAlpha.BlurDirectionX;
      renderer.setRenderTarget(this.renderTargetsHorizontal[ i ] );
      renderer.clear();
      this.fsQuad.render( renderer );

      this.separableBlurMaterials[ i ].uniforms["colorTexture"].value =
        this.renderTargetsHorizontal[ i ].texture;
      this.separableBlurMaterials[ i ].uniforms["direction"].value = UnrealBloomPassAlpha.BlurDirectionY;
      renderer.setRenderTarget(this.renderTargetsVertical[ i ]);
      renderer.clear();
      this.fsQuad.render(renderer);

      inputRenderTarget = this.renderTargetsVertical[ i ];
    }

    // Composite All the mips

    this.fsQuad.material = this.compositeMaterial;
    this.compositeMaterial.uniforms["bloomStrength"].value = this.strength;
    this.compositeMaterial.uniforms["bloomRadius"].value = this.radius;
    this.compositeMaterial.uniforms["bloomTintColors"].value =
      this.bloomTintColors;

    renderer.setRenderTarget(this.renderTargetsHorizontal[0]);
    renderer.clear();
    this.fsQuad.render(renderer);

    // Blend it additively over the input texture

    this.fsQuad.material = this.materialCopy;
    this.copyUniforms["tDiffuse"].value =
      this.renderTargetsHorizontal[0].texture;

    if (maskActive) renderer.state.buffers.stencil.setTest(true);

    if (this.renderToScreen) {
      renderer.setRenderTarget(null);
      this.fsQuad.render(renderer);
    } else {
      renderer.setRenderTarget(readBuffer);
      this.fsQuad.render(renderer);
    }

    // Restore renderer settings

    renderer.setClearColor(this._oldClearColor, this.oldClearAlpha);
    renderer.autoClear = oldAutoClear;
  }

  getSeperableBlurMaterial(kernelRadius) {
    return new ShaderMaterial({
      defines: {
        KERNEL_RADIUS: kernelRadius,
        SIGMA: kernelRadius,
      },

      uniforms: {
        colorTexture: { value: null },
        texSize: { value: new Vector2(0.5, 0.5) },
        direction: { value: new Vector2(0.5, 0.5) },
      },

      vertexShader: `varying vec2 vUv;
				void main() {
					vUv = uv;
					gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );
				}`,

      fragmentShader: `#include <common>
				varying vec2 vUv;
				uniform sampler2D colorTexture;
				uniform vec2 texSize;
				uniform vec2 direction;

				float gaussianPdf(in float x, in float sigma) {
					return 0.39894 * exp( -0.5 * x * x/( sigma * sigma))/sigma;
				}
				void main() {\n\
          vec2 invSize = 1.0 / texSize;\
          float fSigma = float(SIGMA);\
          float weightSum = gaussianPdf(0.0, fSigma);\
          float alphaSum = 0.0;\
          vec3 diffuseSum = texture2D( colorTexture, vUv).rgb * weightSum;\
          for( int i = 1; i < KERNEL_RADIUS; i ++ ) {\
            float x = float(i);\
            float w = gaussianPdf(x, fSigma);\
            vec2 uvOffset = direction * invSize * x;\
            vec4 sample1 = texture2D( colorTexture, vUv + uvOffset);\
            vec4 sample2 = texture2D( colorTexture, vUv - uvOffset);\
            diffuseSum += (sample1.rgb + sample2.rgb) * w;\
            alphaSum += (sample1.a + sample2.a) * w;\
            weightSum += 2.0 * w;\
          }\
          gl_FragColor = vec4(diffuseSum/weightSum, alphaSum/weightSum);\n\
        }`,
    });
  }

  getCompositeMaterial(nMips) {
    return new ShaderMaterial({
      defines: {
        NUM_MIPS: nMips,
      },

      uniforms: {
        blurTexture1: { value: null },
        blurTexture2: { value: null },
        blurTexture3: { value: null },
        blurTexture4: { value: null },
        blurTexture5: { value: null },
        dirtTexture: { value: null },
        bloomStrength: { value: 1.0 },
        bloomFactors: { value: null },
        bloomTintColors: { value: null },
        bloomRadius: { value: 0.0 },
      },

      vertexShader: `varying vec2 vUv;
				void main() {
					vUv = uv;
					gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );
				}`,

      fragmentShader: `varying vec2 vUv;
				uniform sampler2D blurTexture1;
				uniform sampler2D blurTexture2;
				uniform sampler2D blurTexture3;
				uniform sampler2D blurTexture4;
				uniform sampler2D blurTexture5;
				uniform sampler2D dirtTexture;
				uniform float bloomStrength;
				uniform float bloomRadius;
				uniform float bloomFactors[NUM_MIPS];
				uniform vec3 bloomTintColors[NUM_MIPS];

				float lerpBloomFactor(const in float factor) {
					float mirrorFactor = 1.2 - factor;
					return mix(factor, mirrorFactor, bloomRadius);
				}

				void main() {
					gl_FragColor = bloomStrength * ( lerpBloomFactor(bloomFactors[0]) * vec4(bloomTintColors[0], 1.0) * texture2D(blurTexture1, vUv) +
						lerpBloomFactor(bloomFactors[1]) * vec4(bloomTintColors[1], 1.0) * texture2D(blurTexture2, vUv) +
						lerpBloomFactor(bloomFactors[2]) * vec4(bloomTintColors[2], 1.0) * texture2D(blurTexture3, vUv) +
						lerpBloomFactor(bloomFactors[3]) * vec4(bloomTintColors[3], 1.0) * texture2D(blurTexture4, vUv) +
						lerpBloomFactor(bloomFactors[4]) * vec4(bloomTintColors[4], 1.0) * texture2D(blurTexture5, vUv) );
				}`,
    });
  }
}

UnrealBloomPassAlpha.BlurDirectionX = new Vector2(1.0, 0.0);
UnrealBloomPassAlpha.BlurDirectionY = new Vector2(0.0, 1.0);

export { UnrealBloomPassAlpha as UnrealBloomPass };