made it work with Ruby 1.9

Ruby 1.9 has a different treatment of "character" data type.

In 1.8.7, it's an integer. 1.9.2 treats it as one-char string.
Thus, for 1.9.2 to work, it is necessary to insert the conversion
functions ord and chr at appropriate places. 1.8.7 will treat ord
on integers as no-op. chr is always invoked on integers, as is
correct. This will make a slight performance hit, as an additional
function call is performed for each pixel access.

lib/imageruby/bitmap/rbbitmap.rb

@@ -41,10 +41,10 @@ def get_pixel(x,y)
       index = (y*@width + x)
       pointindex = index*3
       Color.from_rgba(
-        @array[pointindex+2],
-        @array[pointindex+1],
-        @array[pointindex],
-        @alpha ? @alpha[index] : 255
+        @array[pointindex+2].ord,
+        @array[pointindex+1].ord,
+        @array[pointindex].ord,
+        @alpha ? @alpha[index].ord : 255
         )
 
     end
@@ -66,13 +66,13 @@ def set_pixel(x,y,color)
 
       index = (y*@width + x)
       pointindex = index*3
-      @array[pointindex+2] = color.r
-      @array[pointindex+1] = color.g
-      @array[pointindex] = color.b
+      @array[pointindex+2] = color.r.chr
+      @array[pointindex+1] = color.g.chr
+      @array[pointindex] = color.b.chr
 
       if color.a != 255 then
         @alpha = "\xff"*(@height * @width) unless @alpha
-        @alpha[index] = color.a
+        @alpha[index] = color.a.chr
       end
 
       self