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May 1997

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1997.05.09 15:27 "To ALL TIFF MAILING LIST MEMBERS", by Sanjay G Prabhudesai
1997.05.12 05:44 "Re: To ALL TIFF MAILING LIST MEMBERS", by Rainer Wiesenfarth

1997.05.12 05:44 "Re: To ALL TIFF MAILING LIST MEMBERS", by Rainer Wiesenfarth

Sanjay G. Prabhudesai wrote:
> Hello Everybody,
> [...]
> I have a customer who is bent on using Group 4 compression for reasons not
> known to me. 

(flame on - I hope the customer's name isn't Microsoft - flame off)

Maybe you should tell your customer that it MAKES ABSOLUTELY NO SENSE
to compress an image with anything other than the TWO COLORS black
and white with Group 4. This has - as repeatedly stated - a number
of reasons:

1. The file you generate would NO LONGER BE A TIFF FILE. You can
   name it xxx.tif, you can use the tags and so on, but you
   MUST NOT CALL IT "TIFF"!!! If you want to generate a TIFF
   file, you have to follow the standard. This results in
   either converting the image to greyscale or to switch to
   an other compression scheme.

2. No other TIFF reader would read and interpret your images
   correctly. What you intend to do is telling TIFF that your
   image is 8 times the width. As a result, each 'normal'
   TIFF reader would display it in black and white only,
   horizontally stretched by a factor of 8. You will not
   be able to recognise anything in this image.

3. As the Group 4 compression is designed primary for
   bilevel images like fax sheets, it is optimised for a
   longer sequence of bits of the same value. As you
   encode your 8-bit color values in a scanline in addition
   to the image pixels, you will get a - from Group 4's
   sight - bad mixture of 0s and 1s. The compression ratio
   would not justify the effort made for compression.
   Maybe you should create a sample of this and show
   it to your customer, along with the same image compressed
   using other methods.

> [...]
> While trying this approach using libTIFF writer I got the following error
> message:
>  "Bits per sample must be 1 for Group 3/4 encoding/decoding".

You have to set PhotometricInterpretation to MinIsWhite and
to 1.

> [...]
> Regards,
> Sanjay Prabhudesai

Greetings Rainer

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