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

[Hiro]

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.

[snibgo]

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.

[Hiro]

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!

[Hiro]

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

[snibgo]

Yes, that is correct.