+1  A: 

With an indexed image there are only a certain number of colours (or in this case shades of grey) available - usually 256. It's probable that there aren't enough to get the full range of shades in the original image.

As having the exact shades is important I'd shift to a 24bpp image.

ChrisF  2009-07-02 10:55:52
+1: I agree, it's bound to be a scaling issue
Jon Cage  2009-07-02 11:03:11
The original image was saved in 8bpp format. We are using greyscale video cameras set to 8bpp to keep within the bandwidth of a 1394a bus at high framerate, so the original image is guarenteed to be in this format.
badbod99  2009-07-02 12:43:03
+3  A: 

The value in the 8bpp indexed image isn't the color itself (or gray value) but the index. Try to look up the color value in the palette.

Robert Paulson  2009-07-02 10:58:51
Could be this too.
ChrisF  2009-07-02 11:09:11
Took a look at the colour pallet, and the index matched the correct colour. I just assumed the pallet would be an even distribution of greyscale covering 256 colours, but actually there was a range of 64 black colours from index 192 :-)
badbod99  2009-07-02 11:24:18

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