fix: UI/Additive shader does not support RectMask2D softness.

Shaders/UIAdditive.shader

@@ -40,12 +40,8 @@
         Lighting Off
         ZWrite Off
         ZTest [unity_GUIZTestMode]
-        Fog
+        Fog { Mode Off }
-        {
-            Mode Off
-        }
         Blend One One
-
         ColorMask [_ColorMask]
 
         Pass
@@ -76,6 +72,7 @@
                 fixed4 color : COLOR;
                 float2 texcoord : TEXCOORD0;
                 float4 worldPosition : TEXCOORD1;
+                float4 mask : TEXCOORD2;
                 UNITY_VERTEX_OUTPUT_STEREO
             };
 
@@ -84,16 +81,43 @@
             float4 _MainTex_ST;
             fixed4 _TextureSampleAdd;
             float4 _ClipRect;
+            float _UIMaskSoftnessX;
+            float _UIMaskSoftnessY;
+            int _UIVertexColorAlwaysGammaSpace;
+
+            half3 _UIGammaToLinear(half3 value)
+            {
+                half3 low = 0.0849710 * value - 0.000163029;
+                half3 high = value * (value * (value * 0.265885 + 0.736584) - 0.00980184) + 0.00319697;
+
+                // We should be 0.5 away from any actual gamma value stored in an 8 bit channel
+                const half3 split = (half3)0.0725490; // Equals 18.5 / 255
+                return (value < split) ? low : high;
+            }
 
             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 = UnityObjectToClipPos(OUT.worldPosition);
+                OUT.vertex = vPosition;
 
-                OUT.texcoord = TRANSFORM_TEX(v.texcoord, _MainTex);
+                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)));
+
+                if (_UIVertexColorAlwaysGammaSpace)
+                {
+                    #ifndef UNITY_COLORSPACE_GAMMA
+                    v.color.rgb = _UIGammaToLinear(v.color.rgb);
+                    #endif
+                }
 
                 OUT.color = v.color * _Color;
                 return OUT;
@@ -101,10 +125,18 @@
 
             fixed4 frag(v2f IN) : SV_Target
             {
-                half4 color = (tex2D(_MainTex, IN.texcoord) + _TextureSampleAdd) * IN.color;
+                //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
-                    color.a *= UnityGet2DClipping(IN.worldPosition.xy, _ClipRect);
+                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
@@ -112,6 +144,7 @@
                 #endif
 
                 color.rgb *= color.a;
+
                 return color;
             }
             ENDCG