fix: update blend functions to respect alpha channel (fixes #82) (#83)

* fix: update linear_blend to respect alpha channel

* fix: update masked_blend to respect alpha channel

Unclear if this is the right implementation.

* misc: remove unnecessary fixed declaration

* misc: update mask_blend

* update changes.md

* misc: fix typo in changes.md

CHANGES.md

@@ -2,21 +2,23 @@ Magic Set Editor changelog, for the details see `git log`
 ==============================================================================
 
 ------------------------------------------------------------------------------
-version 2.5.0 (Unofficial), 2024-04-28
+version 2.5.0 (Unofficial), 2024-08-23
 ------------------------------------------------------------------------------
 
 Features:
  * Add Russian and Korean translations. Update locale files from Full-Magic-Pack (haganbmj#69)
+ * Increase stats column width by 50% (haganbmj#72)
 
 Bug fixes:
  * Fixed scrolled widgets not scrolling fully (or at all) on Linux (@llemoi, haganbmj#54)
  * Fixed crash when 0 keywords match filter string in keyword panel (@llemoi, haganbmj#57)
- * Correct "all images" filter when selecting files. (haganbmj#70)
+ * Correct "all images" filter when selecting files. (@BaconCatBug, haganbmj#70)
  * Correct localization of strings on Print window.
- * Fixed issue where "All Images" filter on the art picker was not showing .jpeg files (BaconCatBug)
+ * Correct how blend functions handle alpha channels. (haganbmj#83)
 
 Internal:
  * Add Mac OS build resources (@halian, haganbmj#67, haganbmj#68)
+ * CMakeLists updates for static compilation. (@BackCatBug, haganbmj#74) 
 
 ------------------------------------------------------------------------------
 version 2.4.0 (Unofficial), 2022-11-19

src/gfx/blend_image.cpp

@@ -55,23 +55,53 @@ void linear_blend(Image& img1, const Image& img2, double x1,double y1, double x2
       data1 += 3;
       data2 += 3;
     }
+  }
+
+  // Blend Alpha for the two images.
+  if (img1.HasAlpha() && img2.HasAlpha()) {
+    Byte *alpha1 = img1.GetAlpha(), *alpha2 = img2.GetAlpha();
+    for (int y = 0; y < height; ++y) {
+      for (int x = 0; x < width; ++x) {
+        int mult = x * xm + y * ym + d;
+        if (mult < 0)      mult = 0;
+        if (mult > fixed)  mult = fixed;
+        alpha1[0] = alpha1[0] + mult * (alpha2[0] - alpha1[0]) / fixed;
+        alpha1 += 1;
+        alpha2 += 1;
+      }
+    }
   }
 }
 
 // ----------------------------------------------------------------------------- : Mask Blend
 
 void mask_blend(Image& img1, const Image& img2, const Image& mask) {
-  if (img2.GetWidth() != img1.GetWidth() || img2.GetHeight() != img1.GetHeight()
-   || mask.GetWidth() != img1.GetWidth() || mask.GetHeight() != img1.GetHeight()) {
-    throw Error(_("Images used for blending must have the same size"));
+  int width = img1.GetWidth(), height = img1.GetHeight();
+  if (img2.GetWidth() != width || img2.GetHeight() != height) {
+    throw Error(_("Images used for blending in masked_blend function must have the same size"));
   }
-
-  UInt size = img1.GetWidth() * img1.GetHeight() * 3;
+  if (mask.GetWidth() != width || mask.GetHeight() != height) {
+    throw Error(_("Mask used for blending in masked_blend function must have the same size as the images"));
+  }
+
+  UInt size = width * height;
+  // these have the following structure:
+  // [pixel1red, pixel1green, pixel1blue, pixel2red, pixel2green, pixel2blue, pixel3red, etc...]
   Byte *data1 = img1.GetData(), *data2 = img2.GetData(), *dataM = mask.GetData();
   // for each subpixel...
-  for (UInt i = 0 ; i < size ; ++i) {
+  for (UInt i = 0; i < (size * 3); ++i) {
     data1[i] = (data1[i] * dataM[i] + data2[i] * (255 - dataM[i])) / 255;
   }
+
+  if (img1.HasAlpha() && img2.HasAlpha()) {
+    // these have the following structure:
+    // [pixel1alpha, pixel2alpha, pixel3alpha, etc...]
+    Byte *alpha1 = img1.GetAlpha(), *alpha2 = img2.GetAlpha();
+    for (UInt i = 0; i < size; ++i) {
+      // use mask's red channel to blend alpha (all mask channels should be identical since it's grey scale)
+      alpha1[i] = (alpha1[i] * dataM[i * 3] + alpha2[i] * (255 - dataM[i * 3])) / 255;
+    }
+  }
 }
 
 // ----------------------------------------------------------------------------- : Alpha