--- a/Color.st	Sat Apr 19 18:10:04 1997 +0200
+++ b/Color.st	Sun Apr 20 12:45:23 1997 +0200
@@ -308,16 +308,29 @@
      for dithering (although, with only black, white, red, green and blue,
      dithered images look very poor)."
 
-    |colors white black red green blue|
+    |colors white black red green blue clr|
 
     (aDevice notNil and:[aDevice ditherColors isNil]) ifTrue:[
         white := (self red:100 green:100 blue:100) exactOn:aDevice.
+	white colorId isNil ifTrue:[
+	    'Color [warning]: cannot allocate white color' errorPrintCR.
+	].
         black := (self red:0 green:0 blue:0) exactOn:aDevice.
+	black colorId isNil ifTrue:[
+	    'Color [warning]: cannot allocate black color' errorPrintCR.
+	].
 
         aDevice hasColors ifTrue:[
             red := (self red:100 green:0 blue:0) exactOn:aDevice.
             green := (self red:0 green:100 blue:0) exactOn:aDevice.
             blue := (self red:0 green:0 blue:100) exactOn:aDevice.
+	    (red isNil or:[red colorId isNil
+	    or:[green isNil or:[green colorId isNil
+	    or:[blue isNil or:[blue colorId isNil]]]]]) ifTrue:[
+	        'Color [warning]: cannot allocate primary color' errorPrintCR.
+		aDevice hasColors:false.
+		red := green := blue := nil.
+	    ]
         ].
 
         aDevice == Display ifTrue:[
@@ -339,15 +352,16 @@
                 "
 
                 colors := OrderedCollection new.
-                colors add:((self gray:50) exactOn:aDevice).
+                clr := (self gray:50) exactOn:aDevice.
+		(clr notNil and:[clr colorId notNil]) ifTrue:[
+		    colors add:clr
+		].
 
                 colors add:white; add:black; add:red; add:green; add:blue.
 
                 colors add:((self red:100 green:100 blue:0) exactOn:aDevice).
                 colors add:((self red:100 green:0 blue:100) exactOn:aDevice).
                 colors add:((self red:0 green:100 blue:100) exactOn:aDevice).
-
-                colors := colors select:[:clr | clr notNil].
             ].
 
             aDevice hasGreyscales ifTrue:[
@@ -361,12 +375,13 @@
                 colors add:((self gray:33) exactOn:aDevice).
                 colors add:((self gray:67) exactOn:aDevice).
                 colors add:((self gray:75) exactOn:aDevice).
-
-                colors := colors select:[:clr | clr notNil].
             ].
 
             colors notNil ifTrue:[  
-                aDevice setDitherColors:(colors asArray).
+                colors := colors select:[:clr | clr notNil and:[clr colorId notNil]].
+		colors size > 2 ifTrue:[
+                    aDevice setDitherColors:(colors asArray).
+		]
             ]
         ]
     ]
@@ -2172,6 +2187,8 @@
 
     minDelta := 9999999.
 
+    colors isNil ifTrue:[^ nil].
+
     colors do:[:aColor |
         |cr cg cb|
 
@@ -3396,9 +3413,23 @@
             "/ dither between those two colors
         ].
     ].
+
     clr1 := clr1 exactOn:aDevice.
     clr2 := clr2 exactOn:aDevice.
 
+    (clr1 isNil or:[clr2 isNil]) ifTrue:[
+	"/ fall back to b&w dither
+        aDevice blackpixel == 0 ifTrue:[
+            clr1 := Black.
+            clr2 := White.
+            newFraction := fraction.
+        ] ifFalse:[
+            clr1 := White.
+            clr2 := Black.
+            newFraction := 1 - fraction
+        ]
+    ].
+
     ^ self monoDitherFor:newFraction 
                  between:clr1 and:clr2
                       on:aDevice into:aBlock
@@ -4058,6 +4089,6 @@
 !Color class methodsFor:'documentation'!
 
 version
-    ^ '$Header: /cvs/stx/stx/libview/Color.st,v 1.94 1997-04-04 13:45:13 cg Exp $'
+    ^ '$Header: /cvs/stx/stx/libview/Color.st,v 1.95 1997-04-20 10:45:23 cg Exp $'
 ! !
 Color initialize!