Re: We cannot resize pgm file.

On Fri, Mar 18, 2011 at 05:34:13PM +0900, ?$BNS ?$BCR;R wrote:
> Dir Sir,
> 
> We are using ImageMagick-6.5.9-10.
> When we convert with -resize option, we find the following problems:
> 
> 1) % convert -resize 48x14 f1.pgm f2.pgm
>    % display f2.pgm
> 
>    When display command displays the converted image, f2.pgm, the
> image cannot be displayed well.
> 
> 2)  % convert -resize 48x14 -compress none f1.pgm feep3.pgm
>     % display f3.pgm
> 
>    When display command displays the converted image, f3.pgm, 
>    the image is blurred.
> 
> 
> f1.pgm is as follows:
> 
> % cat f1.pgm 
> P2
> 24 7
> 63
> 0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
> 0 12  0  0  0  0  0 29 29 29 29  0  0 46 46 46 46  0  0 63  0  0  0  0
> 0 12  0  0  0  0  0 29  0  0  0  0  0 46  0  0  0  0  0 63  0  0  0  0
> 0 12 12 12  0  0  0 29 29 29  0  0  0 46 46 46  0  0  0 63 63 63 63  0
> 0 12  0  0  0  0  0 29  0  0  0  0  0 46  0  0  0  0  0 63  0  0 63  0
> 0 12 12 12 12  0  0 29 29 29 29  0  0 46 46 46 46  0  0 63 63 63 63  0
> 0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
> %
> 
> We want to know how to fix these problems.
> 
> Best Regards,
> ----
> Tomoko Hayashi 
> E-Mail: [email protected]

Interesting..

I use here ImageMagick-6.6.7.10 on FreeBSD ia64.

Following from the man page
http://netpbm.sourceforge.net/doc/pgm.html

I created this "plain" PGM:

TZAV> cat feep.pgm 
P2
# feep.pgm
24 7
15
0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
0  3  3  3  3  0  0  7  7  7  7  0  0 11 11 11 11  0  0 15 15 15 15  0
0  3  0  0  0  0  0  7  0  0  0  0  0 11  0  0  0  0  0 15  0  0 15  0
0  3  3  3  0  0  0  7  7  7  0  0  0 11 11 11  0  0  0 15 15 15 15  0
0  3  0  0  0  0  0  7  0  0  0  0  0 11  0  0  0  0  0 15  0  0  0  0
0  3  0  0  0  0  0  7  7  7  7  0  0 11 11 11 11  0  0 15  0  0  0  0
0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
TZAV> 

Note the magic number "P2", indicating "plain" PGM
as opposed to "raw". Refer to the above man page
for more info.

I get identical results with either

TZAV> display -resize 2400x700 feep-p5.pgm

or

TZAV> convert -resize 2400x700 -compress none feep.pgm feep-large.pgm

I wouldn't call this blurring, but interpolation,
just as expected.

However, using

TZAV> convert -resize 2400x700 feep.pgm feep-large-compress.pgm

produces "raw" PGM, with magic number "P5".

Displaying this file gives much coarser interpolation,
because only 4 bit representation is used:

TZAV> identify feep*
feep-large-compress.pgm PGM 2400x700 2400x700+0+0 4-bit Grayscale DirectClass 1.68MB 0.070u 0:00.070
feep-large.pgm[1] PGM 2400x700 2400x700+0+0 8-bit Grayscale DirectClass 4.661MB 0.359u 0:00.359
feep.pgm[2] PGM 24x7 24x7+0+0 4-bit Grayscale DirectClass 519B 0.000u 0:00.000
TZAV> 

My files:

http://seis.bris.ac.uk/~mexas/feep.pgm
http://seis.bris.ac.uk/~mexas/feep-large.pgm
http://seis.bris.ac.uk/~mexas/feep-large-compress.pgm

-- 
Anton Shterenlikht
Room 2.6, Queen's Building
Mech Eng Dept
Bristol University
University Walk, Bristol BS8 1TR, UK
Tel: +44 (0)117 331 5944
Fax: +44 (0)117 929 4423