--- a/Depth32Image.st	Mon Aug 01 23:30:02 2016 +0100
+++ b/Depth32Image.st	Fri Sep 02 17:42:18 2016 +0100
@@ -207,6 +207,8 @@
 
 pixelAtX:x y:y
     "retrieve a pixel at x/y; return a pixelValue.
+     The interpretation of the returned value depends on the photometric
+     and the colormap. See also Image>>atX:y:)
      Pixels start at x=0 , y=0 for upper left pixel, end at
      x = width-1, y=height-1 for lower right pixel.
      The pixel value contains r/g/b/a in msb order (i.e. r at high, a at low bits)"
@@ -249,6 +251,8 @@
 
 pixelAtX:x y:y put:aPixelValue
     "set the pixel at x/y to aPixelValue.
+     The interpretation of the pixelValue depends on the photometric
+     and the colormap. (see also: Image>>atX:y:put:)
      Pixels start at x=0 , y=0 for upper left pixel, end at
      x = width-1, y=height-1 for lower right pixel"
 
@@ -286,6 +290,26 @@
 
 !Depth32Image methodsFor:'converting rgb images'!
 
+computeAlphaValuesFromMask:aMaskImage
+    "convert a mask into alpha values;
+     masked pixels get an alpha value of 0,
+     unmasked of 255"
+
+    |alphaMask|
+
+    alphaMask := self alphaMaskForPixelValue.
+    self valuesFromX:0 y:0
+                 toX:width-1 y:height-1
+                  do:[:x :y :pixelValue |
+        (aMaskImage pixelAtX:x y:y) ~~ 0 ifTrue:[
+            self pixelAtX:x y:y put:(pixelValue bitOr:alphaMask).
+        ] ifFalse:[
+            self pixelAtX:x y:y put:(pixelValue bitClear:alphaMask).
+        ].
+    ].
+    photometric := #rgba
+!
+
 rgbImageAsTrueColorFormOn:aDevice
     "return a truecolor form from the rgba or argb-picture."
 
@@ -781,7 +805,7 @@
         ^ self asMonochromeFormOn:aDevice
     ].
 
-    form := Form width:width height:height depth:usedDeviceDepth onDevice:aDevice.
+    form := Form imageForm width:width height:height depth:usedDeviceDepth onDevice:aDevice.
     form isNil ifTrue:[
         'Depth32Image [warning]: display bitmap creation failed' errorPrintCR.
         ^ nil