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

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

Post by Hiro » 2018-02-28T08:20:01-07:00

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.

Code: Select all

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;

Code: Select all

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.

Code: Select all

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

Code: Select all

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...

Code: Select all

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

Code: Select all

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

Code: Select all

$ 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
Posts: 10568
Joined: 2010-01-23T23:01:33-07:00
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Location: England, UK

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

Post by snibgo » 2018-02-28T08:37:27-07:00

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.
snibgo's IM pages: im.snibgo.com

Hiro
Posts: 3
Joined: 2018-02-28T07:18:22-07:00
Authentication code: 1152

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

Post by Hiro » 2018-02-28T08:56:08-07:00

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
Posts: 3
Joined: 2018-02-28T07:18:22-07:00
Authentication code: 1152

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

Post by Hiro » 2018-03-01T08:17:46-07:00

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

snibgo
Posts: 10568
Joined: 2010-01-23T23:01:33-07:00
Authentication code: 1151
Location: England, UK

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

Post by snibgo » 2018-03-01T08:47:30-07:00

Yes, that is correct.
snibgo's IM pages: im.snibgo.com

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