Re: prevent OCR

* Anders Thulin <[EMAIL PROTECTED]>:
> 
> Bob wrote:
> 
>> I was thinking of typing the text in a graphics editor and saving it
>> as GIF format, but how could I modify the final image so it makes OCR
>> impossible (or at least hard).

Impossible. As long as a human reader is able to comfortably read it,
any state-of-the-art OCR software should be able to handle it.

>    Some ideas:
> 
>    Reduce resolution.  OCR needs something on the order of 200 dpi,
> but it's possible for a person to read text with much lower resolution,
> though with increasing diffculty. If you can ensure that each
> instance of the same glyph becomes different each time, it will
> be a bit tricky to retrieve the text, I think.

I think it's quite possible to do OCR on a lot less dpi than 200.
 
>    Write the text in colour A, on background in colour B, and select A
> and B so that they translate to the same greyscale.  (This works
> very well in other contexts, too: I used to have a pure grey-scale monitor,
> on which very saturated colours tended to convert to black. So
> web sites using black text on fully saturated yellow background, tended to
> be fully black. Unless the OCR program has some good colour-to-grey
> conversion, it will get confused. Are they that good? Haven't tested ...)

Just load the picture into any graphics program and juggle the palette
for maximum contrast.
 
>    But, if there's the slightest difference in grey scale, the text *can* be
> retrieved, using simple histogram-based techniques.

You don't even have to resort to this, see above.
 
>> Not that I am against OCR, it is very useful, but it is to protect
>> some important document I want to make available to some people.
>> It's ok if it is difficult to convert to text, but I don't want to
>> send some plain text that anybody could cut/paste anywhere.
> 
>    It's just a question of who the adversary is: a general user, someone
> who is prepared to go to a little or to a lot of trouble to get at the
> text, or perhaps even a fully funded government organization? You
> probably can't protect yourself against the latter ...

The best one might be able to come up with is a java applet which displays
the graphic and can't be readily saved (sometimes used to "protect" images
like on http://www.johnnycash.com/photos/gallery_intro.htm, although this
one here is unprotected: http://www.johnnycash.com/billboard.htm).

Still, it is possible to capture the image. And, while fully funded,
I don't have the resources of a government organization. ;)