URP 渲染实战 / 第 26 章 自定义后处理 / 案例 26.6
案例 26.6色差与暗角：两个效果的叠加入门
目标：实现两种最常见的“镜头感”效果，并通过它们演示多个自定义后处理的组合。
色差（Chromatic Aberration）来自真实镜头对不同波长折射率不同：红光和蓝光在画面边缘的放大倍率略有差异，于是边缘出现彩色镶边。实现上，沿径向对不同颜色通道在略有不同的位置采样。只取 R、G、B 三次时，强度大了会出现三道分离的重影；这里在径向上取多个样本，每个样本按“光谱位置”给 RGB 三个权重（红→绿→蓝的三段帐篷函数），最后按通道归一化，色散就变成连续的彩虹过渡。偏移量与半径的平方成正比，中心几乎没有色差：
Assets/Cookbook/Ch26/MyChromaticAberration.shader（节选：frag）
half4 Frag(Varyings input) : SV_Target { UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX(input); float2 uv = input.texcoord; float2 d = uv - 0.5; // 偏移随半径平方增长：中心几乎无色差，边缘最强（和真实镜头的横向色差一致） float2 offset = d * dot(d, d) * _CAParams.x * 0.15; int n = (int)_CAParams.y; half3 sum = 0, wsum = 0; [loop] for (int i = 0; i < n; i++) { float t = i / (float)(n - 1); // 0 = 红（向外）… 1 = 蓝（向内） half3 w = SpectrumWeight(t); float2 suv = uv + offset * (0.5 - t) * 2.0; sum += SAMPLE_TEXTURE2D_X(_BlitTexture, sampler_LinearClamp, suv).rgb * w; wsum += w; } return half4(sum / max(wsum, 1e-4), 1.0); }
vec4 Frag() { vec2 uv = texcoord; vec2 d = uv - 0.5; // 偏移随半径平方增长：中心几乎无色差，边缘最强（和真实镜头的横向色差一致） vec2 offset = d * dot(d, d) * _CAParams.x * 0.15; int n = int(_CAParams.y); vec3 sum = vec3(0.0), wsum = vec3(0.0); for (int i = 0; i < n; i++) { float t = float(i) / float(n - 1); // 0 = 红（向外）… 1 = 蓝（向内） vec3 w = SpectrumWeight(t); vec2 suv = uv + offset * (0.5 - t) * 2.0; sum += texture(_BlitTexture, suv).rgb * w; wsum += w; } return vec4(sum / max(wsum, vec3(1e-4)), 1.0); }
暗角（Vignette）：画面向边缘逐渐变暗。以 center 为圆心计算半径，smoothstep 控制从哪里开始变暗、过渡带多宽；rounded 打开时按屏幕宽高比修正，使暗角是正圆而不是随屏幕拉伸的椭圆。颜色可以不是黑色——深红的暗角常用于表现紧张或受伤：
Assets/Cookbook/Ch26/MyVignette.shader（节选：frag）
half4 Frag(Varyings input) : SV_Target { UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX(input); float2 uv = input.texcoord; half4 color = SAMPLE_TEXTURE2D_X(_BlitTexture, sampler_LinearClamp, uv); float2 d = (uv - _VignetteParams.xy) * 2.0; // 屏幕边缘中点处 |d| = 1 d.x *= lerp(1.0, _ScreenParams.x / _ScreenParams.y, _VignetteRounded); float r = length(d); // 强度决定暗角从哪里开始，柔和度决定过渡带宽度 float inner = 1.3 - _VignetteParams.z; half mask = smoothstep(inner, inner + _VignetteParams.w, r); color.rgb = lerp(color.rgb, _VignetteColor.rgb, mask); return color; }
vec4 Frag() { vec2 uv = texcoord; vec4 color = texture(_BlitTexture, uv); vec2 d = (uv - _VignetteParams.xy) * 2.0; // 屏幕边缘中点处 |d| = 1 d.x *= mix(1.0, _ScreenParams.x / _ScreenParams.y, _VignetteRounded); float r = length(d); // 强度决定暗角从哪里开始，柔和度决定过渡带宽度 float inner = 1.3 - _VignetteParams.z; float mask = smoothstep(inner, inner + _VignetteParams.w, r); color.rgb = mix(color.rgb, _VignetteColor.rgb, mask); return color; }
Assets/Cookbook/Ch26/LensEffects.cs（节选：volumes）
[Serializable, VolumeComponentMenuForRenderPipeline("Cookbook/My Chromatic Aberration", typeof(UniversalRenderPipeline))] public sealed class MyChromaticAberration : VolumeComponent, IPostProcessComponent { public ClampedFloatParameter intensity = new ClampedFloatParameter(0f, 0f, 1f); [Tooltip("沿径向分几段采样；3 = 只取 RGB 三次，越多色散越平滑")] public ClampedIntParameter samples = new ClampedIntParameter(6, 3, 16); public bool IsActive() => intensity.value > 0f; public bool IsTileCompatible() => false; } [Serializable, VolumeComponentMenuForRenderPipeline("Cookbook/My Vignette", typeof(UniversalRenderPipeline))] public sealed class MyVignette : VolumeComponent, IPostProcessComponent { public ColorParameter color = new ColorParameter(Color.black, false, false, true); public Vector2Parameter center = new Vector2Parameter(new Vector2(0.5f, 0.5f)); public ClampedFloatParameter intensity = new ClampedFloatParameter(0f, 0f, 1f); public ClampedFloatParameter smoothness = new ClampedFloatParameter(0.5f, 0.05f, 1f); [Tooltip("勾选后按屏幕宽高比修正成正圆")] public BoolParameter rounded = new BoolParameter(false); public bool IsActive() => intensity.value > 0f; public bool IsTileCompatible() => false; }
Assets/Cookbook/Ch26/Editor/Ch26Shots.cs（节选：shot-lens）
[BookShot("ch26-lens", W = 640, H = 400, Panels = 3)] static void Lens(Shot s) { Stage(s); // 两个 Feature 都在 BeforeRenderingPostProcessing：入队顺序 = Renderer 列表顺序 s.Feature<MyChromaticAberrationFeature>(); s.Feature<MyVignetteFeature>(); var prof = s.Post(); var ca = prof.Add<MyChromaticAberration>(); var vg = prof.Add<MyVignette>(); if (s.Panel != 1) ca.intensity.Override(1f); if (s.Panel != 0) { vg.intensity.Override(0.6f); vg.smoothness.Override(0.7f); vg.rounded.Override(true); } if (s.Panel == 2) vg.color.Override(new Color(0.12f, 0.02f, 0.05f)); }
两个 Feature 都在 BeforeRenderingPostProcessing，执行顺序由 Renderer 上的列表顺序决定：先色差后暗角。顺序有时会影响结果：如果先暗角后色差，暗角的边界本身也会被色散成彩色。实际项目中，这两个效果更适合合并进一个 Pass（都是逐像素、都只读一次颜色），这正是 URP 把它们放进 Uber Post 的原因。