secure fax (Re: email Crypto- third party)

[Pete Chown]

Tom.A.Parker@icl.com wrote:

> However you can't do a straight overlay because it
> doesn't give an XOR.
> 
> Black + Black = Black
> Black + White = Black as well.

If you don't mind halving the resolution, you could use two dots on the
fax for each dot in the real image.  Then you get the XOR like this:

0 xor 0 = 0      [*-] on transparency [*-] on fax gives [*-]: grey
1 xor 0 = 1      [-*] on transparency [*-] on fax gives [**]: black
0 xor 1 = 1      [*-] on transparency [-*] on fax gives [**]: black
1 xor 1 = 0      [-*] on transparency [-*] on fax gives [-*]: grey

In this table, a * represents a black pixel while a - represents a white
pixel.  So for example in the top line a black pixel followed by a white
pixel is overlayed with an identical pair of pixels.  Looking down
through the transparency you therefore see a black, white pair.  This
will show up as a mid tone grey.  On the second line the black pixels
are in different places so you see two black pixels when looking down.

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