adjust opacity for normal blur...

src/Avalonia.Visuals/Media/ExperimentalAcrylicBrush.cs

@@ -139,12 +139,18 @@ namespace Avalonia.Media
             }
             else
             {
-                tintColor = new Color((byte)(Math.Round(tintColor.A * tintOpacity * tintOpacityModifier)), tintColor.R, tintColor.G, tintColor.B);
+                tintColor = new Color((byte)(255 * ((255.0 / tintColor.A) * tintOpacity) * tintOpacityModifier), tintColor.R, tintColor.G, tintColor.B);
             }
 
             return tintColor;
         }
 
+        private static double AdjustOpacity(double opacity)
+        {
+            var result = Math.Max((1.0 - Math.Pow((1.0 - opacity), 3.85)), 0.92);
+            return result;
+        }
+
         private static double GetTintOpacityModifier(Color tintColor)
         {
             // This method supresses the maximum allowable tint opacity depending on the luminosity and saturation of a color by 
@@ -155,9 +161,9 @@ namespace Avalonia.Media
 
             const double midPoint = 0.5; // Mid point of HsvV range that these calculations are based on. This is here for easy tuning.
 
-            const double whiteMaxOpacity = 0.40; // 100% luminosity
-            const double midPointMaxOpacity = 0.50; // 50% luminosity
-            const double blackMaxOpacity = 0.80; // 0% luminosity
+            double whiteMaxOpacity = AdjustOpacity(0.45); // 100% luminosity
+            double midPointMaxOpacity = AdjustOpacity(0.40); // 50% luminosity
+            double blackMaxOpacity = AdjustOpacity(0.60); // 0% luminosity
             
             var hsv = RgbToHsv(tintColor);
 
@@ -189,7 +195,7 @@ namespace Avalonia.Media
                 if (hsv.Saturation > 0)
                 {
                     // Dampen opacity suppression based on how much saturation there is
-                    maxOpacitySuppression *= Math.Max(1 - (hsv.Saturation * 2), 0.0);
+                    //maxOpacitySuppression *= Math.Max(1 - (hsv.Saturation * 2), 0.0);
                 }
 
                 double opacitySuppression = maxOpacitySuppression * normalizedDeviation;

src/Skia/Avalonia.Skia/DrawingContextImpl.cs

@@ -668,6 +668,27 @@ namespace Avalonia.Skia
             return SKColorFilter.CreateTable(a, c, c, c);
         }
 
+        static byte Blend(byte leftColor, byte leftAlpha, byte rightColor, byte rightAlpha)
+        {
+            var ca = leftColor / 255d;
+            var aa = leftAlpha / 255d;
+            var cb = rightColor / 255d;
+            var ab = rightAlpha / 255d;
+            var r = (ca * aa + cb * ab * (1 - aa)) / (aa + ab * (1 - aa));
+            return (byte)(r * 255);
+        }
+        static SKColor Blend(SKColor left, SKColor right)
+        {
+            var aa = left.Alpha / 255d;
+            var ab = right.Alpha / 255d;
+            return new SKColor(
+                Blend(left.Red, left.Alpha, right.Red, right.Alpha),
+                Blend(left.Green, left.Alpha, right.Green, right.Alpha),
+                Blend(left.Blue, left.Alpha, right.Blue, right.Alpha),
+                (byte)((aa + ab * (1 - aa)) * 255)
+            );
+        }        
+
         /// <summary>
         /// Creates paint wrapper for given brush.
         /// </summary>
@@ -700,9 +721,14 @@ namespace Avalonia.Skia
                 const double noiseOpcity = 0.0225;
 
                 var tintColor = acrylicBrush.TintColor;                
-                var tint = new SKColor(tintColor.R, tintColor.G, tintColor.B, (byte)(255 * ((tintColor.A / 255.0) * acrylicBrush.Opacity * tintOpacity)));                
+                var tint = new SKColor(tintColor.R, tintColor.G, tintColor.B, tintColor.A);
+                var tracingPaper = new SKColor(0xaf, 0x7f, 0x7f, 0x7f);
+
+                //tint = Blend(tracingPaper, tint);
+
                 
-                if(s_acrylicNoiseShader == null)
+
+                if (s_acrylicNoiseShader == null)
                 {
                     using(var stream = typeof(DrawingContextImpl).Assembly.GetManifestResourceStream("Avalonia.Skia.Assets.NoiseAsset_256X256_PNG.png"))
                     using (var bitmap = SKBitmap.Decode(stream))