Playcanvas后处理-辉光bloom

（一）Bloom介绍

Bloom（辉光、光晕、泛光）是一种常见的摄像机后处理（PostProcessing）效果，用于再现真实世界相机的成像伪影。这种效果会产生从图像中明亮区域边界延伸的光条纹（或羽毛），从而产生一种极其明亮的光线压倒捕捉场景的相机的错觉。

辉光简单的说法，就是有些地方足够亮，看上去是溢出到周围的区域，下面是playcanvas官方提供的辉光效果对比图

图1 未使用辉光效果

图2 使用辉光效果

（二）Bloom实现原理

bloom的实现原理很简单：就是取camera获取图像的高亮部分，进行高斯模糊，并和原图进行合并就可以实现。

1. 提取较亮区域

通过阈值提取较亮区域的像素点

varying vec2 vUv0;uniform sampler2D uBaseTexture
uniform float uBloomThreshold;float luminance(vec4 color)
{return  0.2125 * color[0] + 0.7154 * color[1] + 0.0721 * color[2]; 
}void main(void)
{vec4 color = texture2D(uBaseTexture, vUv0);// 只保留亮度超过阈值亮度的像素点float val = clamp(luminance(color) - uBloomThreshold, 0.0, 1.0);gl_FragColor = color * val;//这是playcanvas官网提供提取亮度高于某个阈值的算法，但还不知道它的原理//gl_FragColor = clamp((color - uBloomThreshold) / (1.0 - uBloomThreshold), 0.0, 1.0);
}

2. 高斯模糊

接下来，就是如何得到模糊图uBloomTexture

一般模糊图像的算法，我们可以选择常见的高斯模糊，它可以减少图像噪声、降低细节层次

高斯模糊的实现原理，这里不做多赘述，这里提供2篇文章供参考。

原理参考：2D Shader学习——高斯模糊

shader实现参考：基于线性采样的高效高斯模糊实现（译）

参考第二篇文章，我们在JavaScript中，计算我们高斯卷积核的权重和位移

var SAMPLE_COUNT = 15;//高斯曲线
function computeGaussian(n, theta) {return ((1.0 / Math.sqrt(2 * Math.PI * theta)) * Math.exp(-(n * n) / (2 * theta * theta)));
}function calculateBlurValues(sampleWeights, sampleOffsets, dx, dy, blurAmount) {// Create temporary arrays for computing our filter settings.// The first sample always has a zero offset.sampleWeights[0] = computeGaussian(0, blurAmount);sampleOffsets[0] = 0;sampleOffsets[1] = 0;// Maintain a sum of all the weighting values.var totalWeights = sampleWeights[0];// Add pairs of additional sample taps, positioned// along a line in both directions from the center.var i, len;for (i = 0, len = Math.floor(SAMPLE_COUNT / 2); i < len; i++) {// Store weights for the positive and negative taps.var weight = computeGaussian(i + 1, blurAmount);sampleWeights[i * 2] = weight;sampleWeights[i * 2 + 1] = weight;totalWeights += weight * 2;var sampleOffset = i * 2 + 1.5;sampleOffsets[i * 4] = dx * sampleOffset;sampleOffsets[i * 4 + 1] = dy * sampleOffset;sampleOffsets[i * 4 + 2] = -dx * sampleOffset;sampleOffsets[i * 4 + 3] = -dy * sampleOffset;}// Normalize the list of sample weightings, so they will always sum to one.for (i = 0, len = sampleWeights.length; i < len; i++) {sampleWeights[i] /= totalWeights;}
}

在fragment shader中，对图像进行卷积模糊（注意：这里的shader只对水平或垂直一个方向卷积）

#define SAMPLE_COUNT 15varying vec2 vUv0;uniform sampler2D uBloomTexture;
uniform vec2 uBlurOffsets[15];
uniform float uBlurWeights[15];void main(void)
{vec4 color = vec4(0.0);for (int i = 0; i < SAMPLE_COUNT; i++){color += texture2D(uBloomTexture, vUv0 + uBlurOffsets[i]) * uBlurWeights[i];}gl_FragColor = color;
}    

最后，我们需要进行2次方向的滤波处理

原理具体参考文章二中，如何将高斯滤波器分为水平方向和垂直方向的滤波器的原理

// Pass 2: draw from rendertarget 1 into rendertarget 2（垂直方向）
calculateBlurValues(this.sampleWeights, this.sampleOffsets, 1.0 / this.targets[1].width, 0, this.blurAmount);
scope.resolve("uBlurWeights[0]").setValue(this.sampleWeights);
scope.resolve("uBlurOffsets[0]").setValue(this.sampleOffsets);
scope.resolve("uBloomTexture").setValue(this.targets[0].colorBuffer);
this.drawQuad(this.targets[1], this.blurShader);// Pass 3: draw from rendertarget 2 back into rendertarget 1（水平方向）
calculateBlurValues(this.sampleWeights, this.sampleOffsets, 0, 1.0 / this.targets[0].height, this.blurAmount);
scope.resolve("uBlurWeights[0]").setValue(this.sampleWeights);
scope.resolve("uBlurOffsets[0]").setValue(this.sampleOffsets);
scope.resolve("uBloomTexture").setValue(this.targets[1].colorBuffer);
this.drawQuad(this.targets[0], this.blurShader);

3. 混合原图和模糊图

那我们最后的fragment shader就可以这样实现，原图+模糊图进行混合

varying vec2 vUv0;//bloom 强度
uniform float uBloomEffectIntensity;uniform sampler2D uBaseTexture;
uniform sampler2D uBloomTexture;void main(void)
{vec4 bloom = texture2D(uBloomTexture, vUv0) * uBloomEffectIntensity;vec4 base = texture2D(uBaseTexture, vUv0);//将原图变暗，防止两图叠加后，像素溢出1base *= (1.0 - clamp(bloom, 0.0, 1.0));//合并原图和模糊图，得到最终的bloom效果gl_FragColor = base + bloom;
}

（三）在playcanvas编辑器中使用

1. 创建脚本bloom.js

// --------------- POST EFFECT DEFINITION --------------- //
var SAMPLE_COUNT = 15;function computeGaussian(n, theta) {return ((1.0 / Math.sqrt(2 * Math.PI * theta)) * Math.exp(-(n * n) / (2 * theta * theta)));
}function calculateBlurValues(sampleWeights, sampleOffsets, dx, dy, blurAmount) {// Look up how many samples our gaussian blur effect supports.// Create temporary arrays for computing our filter settings.// The first sample always has a zero offset.sampleWeights[0] = computeGaussian(0, blurAmount);sampleOffsets[0] = 0;sampleOffsets[1] = 0;// Maintain a sum of all the weighting values.var totalWeights = sampleWeights[0];// Add pairs of additional sample taps, positioned// along a line in both directions from the center.var i, len;for (i = 0, len = Math.floor(SAMPLE_COUNT / 2); i < len; i++) {// Store weights for the positive and negative taps.var weight = computeGaussian(i + 1, blurAmount);sampleWeights[i * 2] = weight;sampleWeights[i * 2 + 1] = weight;totalWeights += weight * 2;// To get the maximum amount of blurring from a limited number of// pixel shader samples, we take advantage of the bilinear filtering// hardware inside the texture fetch unit. If we position our texture// coordinates exactly halfway between two texels, the filtering unit// will average them for us, giving two samples for the price of one.// This allows us to step in units of two texels per sample, rather// than just one at a time. The 1.5 offset kicks things off by// positioning us nicely in between two texels.var sampleOffset = i * 2 + 1.5;// Store texture coordinate offsets for the positive and negative taps.sampleOffsets[i * 4] = dx * sampleOffset;sampleOffsets[i * 4 + 1] = dy * sampleOffset;sampleOffsets[i * 4 + 2] = -dx * sampleOffset;sampleOffsets[i * 4 + 3] = -dy * sampleOffset;}// Normalize the list of sample weightings, so they will always sum to one.for (i = 0, len = sampleWeights.length; i < len; i++) {sampleWeights[i] /= totalWeights;}
}/*** @class* @name BloomEffect* @classdesc Implements the BloomEffect post processing effect.* @description Creates new instance of the post effect.* @augments PostEffect* @param {GraphicsDevice} graphicsDevice - The graphics device of the application.* @property {number} bloomThreshold Only pixels brighter then this threshold will be processed. Ranges from 0 to 1.* @property {number} blurAmount Controls the amount of blurring.* @property {number} bloomIntensity The intensity of the effect.*/
function BloomEffect(graphicsDevice) {pc.PostEffect.call(this, graphicsDevice);// Shadersvar attributes = {aPosition: pc.SEMANTIC_POSITION};// Pixel shader extracts the brighter areas of an image.// This is the first step in applying a bloom postprocess.var extractFrag = ["varying vec2 vUv0;","","uniform sampler2D uBaseTexture;","uniform float uBloomThreshold;","","float luminance(vec4 color)","{","    return  0.2125 * color[0] + 0.7154 * color[1] + 0.0721 * color[2]; ","}","","void main(void)","{",// Look up the original image color."    vec4 color = texture2D(uBaseTexture, vUv0);","",// Adjust it to keep only values brighter than the specified threshold."    float val = clamp(luminance(color) - uBloomThreshold, 0.0, 1.0);","    gl_FragColor = color * val;","}"].join("\n");// Pixel shader applies a one dimensional gaussian blur filter.// This is used twice by the bloom postprocess, first to// blur horizontally, and then again to blur vertically.var gaussianBlurFrag = ["#define SAMPLE_COUNT " + SAMPLE_COUNT,"","varying vec2 vUv0;","","uniform sampler2D uBloomTexture;","uniform vec2 uBlurOffsets[" + SAMPLE_COUNT + "];","uniform float uBlurWeights[" + SAMPLE_COUNT + "];","","void main(void)","{","    vec4 color = vec4(0.0);",// Combine a number of weighted image filter taps."    for (int i = 0; i < SAMPLE_COUNT; i++)","    {","        color += texture2D(uBloomTexture, vUv0 + uBlurOffsets[i]) * uBlurWeights[i];","    }","","    gl_FragColor = color;","}"].join("\n");// Pixel shader combines the bloom image with the original// scene, using tweakable intensity levels.// This is the final step in applying a bloom postprocess.var combineFrag = ["varying vec2 vUv0;","","uniform float uBloomEffectIntensity;","uniform sampler2D uBaseTexture;","uniform sampler2D uBloomTexture;","","void main(void)","{",// Look up the bloom and original base image colors."    vec4 bloom = texture2D(uBloomTexture, vUv0) * uBloomEffectIntensity;","    vec4 base = texture2D(uBaseTexture, vUv0);","",// Darken down the base image in areas where there is a lot of bloom,// to prevent things looking excessively burned-out."    base *= (1.0 - clamp(bloom, 0.0, 1.0));","",// Combine the two images."    gl_FragColor = base + bloom;","}"].join("\n");this.extractShader = pc.createShaderFromCode(graphicsDevice, pc.PostEffect.quadVertexShader, extractFrag, 'BloomExtractShader', attributes);this.blurShader = pc.createShaderFromCode(graphicsDevice, pc.PostEffect.quadVertexShader, gaussianBlurFrag, 'BloomBlurShader', attributes);this.combineShader = pc.createShaderFromCode(graphicsDevice, pc.PostEffect.quadVertexShader, combineFrag, 'BloomCombineShader', attributes);this.targets = [];// Effect defaultsthis.bloomThreshold = 0.25;this.blurAmount = 4;this.bloomIntensity = 1.25;// Uniformsthis.sampleWeights = new Float32Array(SAMPLE_COUNT);this.sampleOffsets = new Float32Array(SAMPLE_COUNT * 2);
}BloomEffect.prototype = Object.create(pc.PostEffect.prototype);
BloomEffect.prototype.constructor = BloomEffect;BloomEffect.prototype._destroy = function () {if (this.targets) {var i;for (i = 0; i < this.targets.length; i++) {this.targets[i].destroyTextureBuffers();this.targets[i].destroy();}}this.targets.length = 0;
};BloomEffect.prototype._resize = function (target) {var width = target.colorBuffer.width;var height = target.colorBuffer.height;if (width === this.width && height === this.height)return;this.width = width;this.height = height;this._destroy();// Render targetsvar i;for (i = 0; i < 2; i++) {var colorBuffer = new pc.Texture(this.device, {name: "Bloom Texture" + i,format: pc.PIXELFORMAT_RGBA8,width: width >> 1,height: height >> 1,mipmaps: false});colorBuffer.minFilter = pc.FILTER_LINEAR;colorBuffer.magFilter = pc.FILTER_LINEAR;colorBuffer.addressU = pc.ADDRESS_CLAMP_TO_EDGE;colorBuffer.addressV = pc.ADDRESS_CLAMP_TO_EDGE;colorBuffer.name = 'pe-bloom-' + i;var bloomTarget = new pc.RenderTarget({name: "Bloom Render Target " + i,colorBuffer: colorBuffer,depth: false});this.targets.push(bloomTarget);}
};Object.assign(BloomEffect.prototype, {render: function (inputTarget, outputTarget, rect) {this._resize(inputTarget);var device = this.device;var scope = device.scope;// Pass 1: draw the scene into rendertarget 1, using a// shader that extracts only the brightest parts of the image.scope.resolve("uBloomThreshold").setValue(this.bloomThreshold);scope.resolve("uBaseTexture").setValue(inputTarget.colorBuffer);this.drawQuad(this.targets[0], this.extractShader);// Pass 2: draw from rendertarget 1 into rendertarget 2,// using a shader to apply a horizontal gaussian blur filter.calculateBlurValues(this.sampleWeights, this.sampleOffsets, 1.0 / this.targets[1].width, 0, this.blurAmount);scope.resolve("uBlurWeights[0]").setValue(this.sampleWeights);scope.resolve("uBlurOffsets[0]").setValue(this.sampleOffsets);scope.resolve("uBloomTexture").setValue(this.targets[0].colorBuffer);this.drawQuad(this.targets[1], this.blurShader);// Pass 3: draw from rendertarget 2 back into rendertarget 1,// using a shader to apply a vertical gaussian blur filter.calculateBlurValues(this.sampleWeights, this.sampleOffsets, 0, 1.0 / this.targets[0].height, this.blurAmount);scope.resolve("uBlurWeights[0]").setValue(this.sampleWeights);scope.resolve("uBlurOffsets[0]").setValue(this.sampleOffsets);scope.resolve("uBloomTexture").setValue(this.targets[1].colorBuffer);this.drawQuad(this.targets[0], this.blurShader);// Pass 4: draw both rendertarget 1 and the original scene// image back into the main backbuffer, using a shader that// combines them to produce the final bloomed result.scope.resolve("uBloomEffectIntensity").setValue(this.bloomIntensity);scope.resolve("uBloomTexture").setValue(this.targets[0].colorBuffer);scope.resolve("uBaseTexture").setValue(inputTarget.colorBuffer);this.drawQuad(outputTarget, this.combineShader, rect);}
});// ----------------- SCRIPT DEFINITION ------------------ //
var Bloom = pc.createScript('bloom');Bloom.attributes.add('bloomIntensity', {type: 'number',default: 1,min: 0,title: 'Intensity'
});Bloom.attributes.add('bloomThreshold', {type: 'number',default: 0.25,min: 0,max: 1,title: 'Threshold'
});Bloom.attributes.add('blurAmount', {type: 'number',default: 4,min: 1,'title': 'Blur amount'
});Bloom.prototype.initialize = function () {this.effect = new BloomEffect(this.app.graphicsDevice);this.effect.bloomThreshold = this.bloomThreshold;this.effect.blurAmount = this.blurAmount;this.effect.bloomIntensity = this.bloomIntensity;var queue = this.entity.camera.postEffects;queue.addEffect(this.effect);this.on('attr', function (name, value) {this.effect[name] = value;}, this);this.on('state', function (enabled) {if (enabled) {queue.addEffect(this.effect);} else {queue.removeEffect(this.effect);}});this.on('destroy', function () {queue.removeEffect(this.effect);this.effect._destroy();});
};

2. 将脚本挂载在相机