add support for semi-transparency when overlaying images

1 files changed, 36 insertions, 3 deletions

diff --git a/src/SMAPI/Framework/Content/AssetDataForImage.cs b/src/SMAPI/Framework/Content/AssetDataForImage.cs
index 5c7b87de..cd372948 100644
--- a/src/SMAPI/Framework/Content/AssetDataForImage.cs
+++ b/src/SMAPI/Framework/Content/AssetDataForImage.cs
@@ -8,6 +8,14 @@ namespace StardewModdingAPI.Framework.Content
     internal class AssetDataForImage : AssetData<Texture2D>, IAssetDataForImage
     {
         /*********
+        ** Properties
+        *********/
+        /// <summary>The minimum value to consider non-transparent.</summary>
+        /// <remarks>On Linux/Mac, fully transparent pixels may have an alpha up to 4 for some reason.</remarks>
+        private const byte MinOpacity = 5;
+
+
+        /*********
         ** Public methods
         *********/
         /// <summary>Construct an instance.</summary>
@@ -53,13 +61,38 @@ namespace StardewModdingAPI.Framework.Content
             // merge data in overlay mode
             if (patchMode == PatchMode.Overlay)
             {
+                // get target data
+                Color[] targetData = new Color[pixelCount];
+                target.GetData(0, targetArea, targetData, 0, pixelCount);
+
+                // merge pixels
                 Color[] newData = new Color[targetArea.Value.Width * targetArea.Value.Height];
                 target.GetData(0, targetArea, newData, 0, newData.Length);
                 for (int i = 0; i < sourceData.Length; i++)
                 {
-                    Color pixel = sourceData[i];
-                    if (pixel.A > 4) // not transparent (note: on Linux/Mac, fully transparent pixels may have an alpha up to 4 for some reason)
-                        newData[i] = pixel;
+                    Color above = sourceData[i];
+                    Color below = targetData[i];
+
+                    // shortcut transparency
+                    if (above.A < AssetDataForImage.MinOpacity)
+                        continue;
+                    if (below.A < AssetDataForImage.MinOpacity)
+                    {
+                        newData[i] = above;
+                        continue;
+                    }
+
+                    // merge pixels
+                    // This performs a conventional alpha blend for the pixels, which are already
+                    // premultiplied by the content pipeline.
+                    float alphaAbove = above.A / 255f;
+                    float alphaBelow = (255 - above.A) / 255f;
+                    newData[i] = new Color(
+                        r: (int)((above.R * alphaAbove) + (below.R * alphaBelow)),
+                        g: (int)((above.G * alphaAbove) + (below.G * alphaBelow)),
+                        b: (int)((above.B * alphaAbove) + (below.B * alphaBelow)),
+                        a: Math.Max(above.A, below.A)
+                    );
                 }
                 sourceData = newData;
             }