--- a/Image.st	Thu Feb 16 20:42:11 2017 +0100
+++ b/Image.st	Fri Feb 17 11:23:19 2017 +0100
@@ -10538,6 +10538,8 @@
 !
 
 createMask
+    "create a mask filled with one's (i.e. all pixels opaque)"
+    
     |maskArray bytesPerMaskRow|
 
     bytesPerMaskRow := (width+7) // 8.
@@ -10545,6 +10547,35 @@
     maskArray := ByteArray new:(bytesPerMaskRow * height) withAll:2r11111111.
     mask := ImageMask width:width height:height fromArray:maskArray.
     ^ mask
+
+    "Modified (comment): / 17-02-2017 / 07:59:07 / cg"
+!
+
+createMaskForPixelValue:pixelValue
+    "create or modify the mask to be off wherever the pixelValue appears.
+     If there is already a mask, pixels which are already masked remain so.
+     This is a helper for image readers which use a pixel index as mask,
+     instead of a separate mask plane.
+     
+     This is a slow fallback, if it turns out to be timing relevant,
+     redefine in concrete image classes (especially Depth8Image)"
+
+    |mr|
+    
+    mask isNil ifTrue:[
+        mask := self createMask.
+    ].
+    0 to:height-1 do:[:y |
+        mr := mask rowAt:y.
+        0 to:width-1 do:[:x |
+            (self pixelAtX:x y:y) == pixelValue ifTrue:[
+                mr at:x+1 put:0
+            ].
+        ].
+        mask rowAt:y putAll:mr
+    ].
+
+    "Created: / 17-02-2017 / 11:19:09 / cg"
 !
 
 darkened