--- a/PNGReader.st	Fri Feb 24 02:05:07 2017 +0100
+++ b/PNGReader.st	Sat Feb 25 19:31:46 2017 +0100
@@ -1546,7 +1546,7 @@
 
 writeImageDataChunk
     |compressedByteStream compressedBytes zlibWriter idx 
-     row32|
+     row32 bytesPerPixel|
 
     bytesPerScanline := self bytesPerRow.
     
@@ -1555,30 +1555,38 @@
     zlibWriter binary.
 
     (mask notNil and:[photometric ~~ #palette]) ifTrue:[
-        "/ on-the-fly expand mask into the alpha channel.
+        (depth == 32) ifTrue:[
+            "/ on-the-fly put mask into the alpha channel.
+            
+            bytesPerPixel := 4
+        ] ifFalse:[
+            "/ for now - only support depth24 + mask
+            self assert:(depth == 24).
+            bytesPerPixel := 3.
+        ].
         
-        "/ for now - only support depth24 + mask
-        self assert:(depth == 24).
+        "/ on-the-fly place mask into the alpha channel.
+
         row32 := ByteArray new:(4 * width).
-        self assert:(bytesPerScanline == (3 * width)).
-        
+        self assert:(bytesPerScanline == (bytesPerPixel * width)).
+
         idx := 1.
         0 to:height-1 do:[:y |
             |dstIdx maskRow|
 
             "/ expand rgb to rgba
             maskRow := mask rowAt:y.
-            
+
             dstIdx := 1.
             1 to:width do:[:x |
                 row32 at:dstIdx put:(data at:idx).
                 row32 at:dstIdx+1 put:(data at:idx+1).
                 row32 at:dstIdx+2 put:(data at:idx+2).
                 row32 at:dstIdx+3 put:((maskRow at:x) == 0 ifTrue:[0] ifFalse:[16rFF]).
-                idx := idx + 3.
+                idx := idx + bytesPerPixel.
                 dstIdx := dstIdx + 4.
             ].
-            
+
             zlibWriter nextPutAll:#[0].       "/ no filter
             zlibWriter nextPutAll:row32.
         ].
@@ -1604,7 +1612,7 @@
             outStream nextPutAll:compressedBytes
         ]
 
-    "Modified: / 17-02-2017 / 17:02:16 / cg"
+    "Modified: / 25-02-2017 / 14:27:34 / cg"
 !
 
 writePaletteChunk