Legacy ImageMagick Discussions Archive

Hi,

I'd like to convert a 16-bit PPM file to two 8-bit PPM files, one is from MSB 8bits and the other from LSB 8bits.

In the following example I use a bash function, tohex(), which convert decimal numbers into hexadecimal numbers.

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bash$ type tohex
tohex is a function
tohex () 
{ 
    perl -nae 'print join(" ", map {sprintf "%x", $_ } @F), "\n"' "$@"
}
bash$ tohex
65535 64192 65280            
ffff fac0 ff00

To get MSB 8bit, I did follows;

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bash$ echo "P3 1 1 65535 64192 64192 64192" | convert - -evaluate And 65280 -depth 16 -compress None PPM:-|tohex
0
1 1
ffff
fa00 fa00 fa00

This is what I expected.

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bash$ echo "P3 1 1 65535 64192 64192 64192" | convert - -evaluate And 65280 -depth 8 -compress None PPM:-|tohex
0
1 1
ff
f9 f9 f9

I don't understand why I get 0xf9 instead of 0xfa.

To get LSB 8bit, similary

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bash$ echo "P3 1 1 65535 64192 64192 64192" | convert - -evaluate And 255 -depth 16 -compress None PPM:-|tohex
0
1 1
ffff
c0 c0 c0
bash$ echo "P3 1 1 65535 64192 64192 64192" | convert - -evaluate And 255 -evaluate LeftShift 8 -depth 16 -compress None PPM:-|tohex
0
1 1
ffff
c000 c000 c000

This is what I expected, but...

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bash$ echo "P3 1 1 65535 64192 64192 64192" | convert - -evaluate And 255 -evaluate LeftShift 8 -depth 8 -compress None PPM:-|tohex
0
1 1
ff
bf bf bf

Why I get 0xbf instead of 0xc0?

What is wrong? Is there a good way to get 8bit PPM from 16bit PPM?

Same results on both my Linux and Cygwin environments.

On Linux

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bash$ convert -version
Version: ImageMagick 6.2.5 03/04/15 Q16 http://www.imagemagick.org
Copyright: Copyright (C) 1999-2005 ImageMagick Studio LLC

On Cygwin

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$ convert -version
Version: ImageMagick 6.9.9-11 Q16 i686 2017-09-30 http://www.imagemagick.org
Copyright: c 1999-2017 ImageMagick Studio LLC
License: http://www.imagemagick.org/script/license.php
Features: Cipher DPC OpenMP
Delegates (built-in): autotrace bzlib cairo fftw fontconfig fpx freetype gslib jbig jng jp2 jpeg lcms lzma pangocairo png ps rsvg tiff webp x xml zlib

Thanks in advance.

Conversion from 8 to 16 bits works by replicating the byte, not by appending a zero byte. So 0xff becomes 0xffff, 0xab becomes 0xabab, etc. Similarly in the opposite direction.

Arithmetically, this is done by multiplying (or dividing) by 0x0101, which is decimal 257.

Thank you for your quick reply.

Code: Select all

bash$ echo "P3 1 1 65535 64192 64192 64192" | convert - -evaluate And 255 -evaluate Multiply 257 -depth 8 -compress None PPM:-|tohex
0
1 1
ff
c0 c0 c0
bash$ echo "P3 1 1 65535 64192 64192 64192" | convert - -evaluate RightShift 8 -evaluate And 255 -evaluate Multiply 257 -depth 8 -compress None PPM:-|tohex
0
1 1
ff
fa fa fa

Works great!

Thanks a lot!

0xffff/0xff = 257
0x10000/0x100=256

Yes, that is correct.

Legacy ImageMagick Discussions Archive

converting a 16-bit PPM file to two 8-bit PPM files

converting a 16-bit PPM file to two 8-bit PPM files

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Re: converting a 16-bit PPM file to two 8-bit PPM files