问题描述：美术使用PS在Gamma空间下设计的UI图，导入到Unity，因为Unity使用的是线性空间，导致半透明的UI效果和美术设计的不一致。

解决方案：

（一）让美术在线性空间下工作

（二）在Unity里使用自定义Shader处理半透明UI

PS中Gamma空间计算公式：color = A.rgb*A.alpha + B.rgb*(1-A.alpha)

Unity为线性空间，图片不勾选sRGB，图片是Gamma空间的，思路就是转到Gamma空间下进行alpha混合，然后再转回线性空间返回结果。（shader可能有问题，先记录下来，后续再改）

// Unity built-in shader source. Copyright (c) 2016 Unity Technologies. MIT license (see license.txt)

Shader "UI/DefaultExt"
{
    Properties
    {
        [Toggle(_True)] _IsGamma ("IsGamma", Float) = 1

        [PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
        _Color ("Tint", Color) = (1,1,1,1)

        _StencilComp ("Stencil Comparison", Float) = 8
        _Stencil ("Stencil ID", Float) = 0
        _StencilOp ("Stencil Operation", Float) = 0
        _StencilWriteMask ("Stencil Write Mask", Float) = 255
        _StencilReadMask ("Stencil Read Mask", Float) = 255

        _ColorMask ("Color Mask", Float) = 15

        [Toggle(UNITY_UI_ALPHACLIP)] _UseUIAlphaClip ("Use Alpha Clip", Float) = 0
    }

    SubShader
    {
        Tags
        {
            "Queue"="Transparent"
            "IgnoreProjector"="True"
            "RenderType"="Transparent"
            "PreviewType"="Plane"
            "CanUseSpriteAtlas"="True"
        }

        Stencil
        {
            Ref [_Stencil]
            Comp [_StencilComp]
            Pass [_StencilOp]
            ReadMask [_StencilReadMask]
            WriteMask [_StencilWriteMask]
        }

        Cull Off
        Lighting Off
        ZWrite Off
        ZTest [unity_GUIZTestMode]
        Blend One OneMinusSrcAlpha
        ColorMask [_ColorMask]

        Pass
        {
            Name "Default"
        CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #pragma target 2.0

            #include "UnityCG.cginc"
            #include "UnityUI.cginc"

            #pragma multi_compile_local _ UNITY_UI_CLIP_RECT
            #pragma multi_compile_local _ UNITY_UI_ALPHACLIP

            struct appdata_t
            {
                float4 vertex   : POSITION;
                float4 color    : COLOR;
                float2 texcoord : TEXCOORD0;
                UNITY_VERTEX_INPUT_INSTANCE_ID
            };

            struct v2f
            {
                float4 vertex   : SV_POSITION;
                fixed4 color    : COLOR;
                float2 texcoord  : TEXCOORD0;
                float4 worldPosition : TEXCOORD1;
                float4  mask : TEXCOORD2;
                UNITY_VERTEX_OUTPUT_STEREO
            };

            sampler2D _MainTex;
            fixed4 _Color;
            fixed4 _TextureSampleAdd;
            float4 _ClipRect;
            float4 _MainTex_ST;
            float _UIMaskSoftnessX;
            float _UIMaskSoftnessY;

            v2f vert(appdata_t v)
            {
                v2f OUT;
                UNITY_SETUP_INSTANCE_ID(v);
                UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(OUT);
                float4 vPosition = UnityObjectToClipPos(v.vertex);
                OUT.worldPosition = v.vertex;
                OUT.vertex = vPosition;

                float2 pixelSize = vPosition.w;
                pixelSize /= float2(1, 1) * abs(mul((float2x2)UNITY_MATRIX_P, _ScreenParams.xy));

                float4 clampedRect = clamp(_ClipRect, -2e10, 2e10);
                float2 maskUV = (v.vertex.xy - clampedRect.xy) / (clampedRect.zw - clampedRect.xy);
                OUT.texcoord = TRANSFORM_TEX(v.texcoord.xy, _MainTex);
                OUT.mask = float4(v.vertex.xy * 2 - clampedRect.xy - clampedRect.zw, 0.25 / (0.25 * half2(_UIMaskSoftnessX, _UIMaskSoftnessY) + abs(pixelSize.xy)));

                OUT.color = v.color * _Color;
                return OUT;
            }
            // inline half3 GammaToLinearSpace (half3 sRGB) 
            // {
            //     // Approximate version from http://chilliant.blogspot.com.au/2012/08/srgb-approximations-for-hlsl.html?m=1     
            //     return sRGB * (sRGB * (sRGB * 0.305306011h + 0.682171111h) + 0.012522878h);     // Precise version, useful for debugging.     
            //     //return half3(GammaToLinearSpaceExact(sRGB.r), GammaToLinearSpaceExact(sRGB.g), GammaToLinearSpaceExact(sRGB.b)); 
            // }
            half3 LinearToGammaSpace3(half3 col)
            {
                col.r = LinearToGammaSpaceExact(col.r);
                col.g = LinearToGammaSpaceExact(col.g);
                col.b = LinearToGammaSpaceExact(col.b);
                return col;
            }
 
            half3 GammaToLinearSpace3(half3 col)
            {
                col.r = GammaToLinearSpaceExact(col.r);
                col.g = GammaToLinearSpaceExact(col.g);
                col.b = GammaToLinearSpaceExact(col.b);
                return col;
            }
            float _IsGamma;
            fixed4 frag(v2f IN) : SV_Target
            {
                //Round up the alpha color coming from the interpolator (to 1.0/256.0 steps)
                //The incoming alpha could have numerical instability, which makes it very sensible to
                //HDR color transparency blend, when it blends with the world's texture.
                const half alphaPrecision = half(0xff);
                const half invAlphaPrecision = half(1.0/alphaPrecision);
                IN.color.a = round(IN.color.a * alphaPrecision)*invAlphaPrecision;

                half4 color = IN.color * (tex2D(_MainTex, IN.texcoord) + _TextureSampleAdd);

                #ifdef UNITY_UI_CLIP_RECT
                half2 m = saturate((_ClipRect.zw - _ClipRect.xy - abs(IN.mask.xy)) * IN.mask.zw);
                color.a *= m.x * m.y;
                #endif

                #ifdef UNITY_UI_ALPHACLIP
                clip (color.a - 0.001);
                #endif

                color.rgb *= color.a;

                if (_IsGamma != 1)
                {
                    color.rgb = GammaToLinearSpace(color.rgb);
                    // color.rgb = GammaToLinearSpace3(color.rgb);
                    // color.a = pow(color.a, 0.45);
                    color.a = LinearToGammaSpaceExact(color.a);
                }

                return color;
            }
        ENDCG
        }
    }
}

参考：

Gamma、Linear、sRGB 和Unity Color Space，你真懂了吗？ - 知乎

Unity的颜色空间管理与转换 - 知乎

unity gamma(伽马) linear(线性) 互转代码及问题处理_unity gamma转linear_Dawn·张的博客-CSDN博客

Unity线性空间UI的问题_unity 线性空间_zhjzhjxzhl的博客-CSDN博客