Check that -sheet keeps the transparent color index of the source

cli/sheet.sh

@@ -119,3 +119,17 @@ end
 assert(expected:isEqual(sheet.cels[1].image))
 EOF
 $ASEPRITE -b -script "$d/compare.lua" || exit 1
+
+# Test that the transparent color persists in the output sheet
+
+d=$t/sheet-custom-transparent-index
+if ! $ASEPRITE -b sprites/bg-index-3.aseprite -sheet "$d/sheet.aseprite" -data "$d/sheet.json" ; then
+    exit 1
+fi
+cat >$d/compare.lua <<EOF
+local original = Sprite{ fromFile='sprites/bg-index-3.aseprite' }
+assert(original.transparentColor == 3)
+local sheet = Sprite{ fromFile='$d/sheet.aseprite' }
+assert(sheet.transparentColor == 3)
+EOF
+$ASEPRITE -b -script "$d/compare.lua" || exit 1

sprites/README.md

@@ -10,3 +10,5 @@
   three layers each one (called "a", "b", "c" too). There is a
   combination of visibilities ("b/b", "c", "c/a", and "c/b" are
   hidden).
+* `bg-index-3.aseprite`: Indexed, 4x4, two layers ("fg" and "bg")
+  with a special transparent index color (= palette index 3).