Re: [Gnuplot-bugs] rgbimage shift-rotates the image along the x-axis

[fredfriend <van...@il...>]

I'm sorry to learn that I've lied to gnuplot! This was never my intention.

Cutting off the header works indeed. 

Part of the problem, to my opinion, is that I (any probably more users)
don't know the differences between all these bitmap/pixmap types and the
gnuplot manual is not very descriptive about how to tailor a given
bitmap/pixmap so it gets accepted by gnuplot. It's probably not the role of
gnuplot to describe file formats, but if it doesn't know what to do with the
headers, where other programs do, then my conclusion (as a non-expert) is
that gnuplot doesn't read (properly) PNM, PPM or whatever format.

So if gnuplot only accept images without the first (4) header lines, then I
think it's a good thing to say/hint that in the manual.

Gnuplot is a very versatile and useful plotting program and I appreciate all
the work done by all the authors!

and thank you for your fast response!



Hans-Bernhard Bröker-2 wrote:
> 
> fredfriend wrote:
> 
>> I'd like to overplot data with gnuplot via "rgbimage". The image that is
>> inserted is however always shift-rotated.
> 
> No, it's just shifted, because you make incorrect assumption about the 
> contents of a PNM image file.
> 
>> The cropped image is then saved as PNM image 
> 
> Strictly speaking, there is no such thing as saving an image as a PNM 
> file.  PNM is a family of three formats.  You save as either PBM, PGM or 
> PPM.
> 
>> I looked at the imported image (the PNM) with different programs and
>> there's
>> no shift apparent in the image itself, so I think it's a gnuplot thing.
> 
> You used all kinds of program that know about the PNM family of image 
> file formats.  But you told gnuplot that the file was a raw binary image 
> without _any_ format.  It's not, so you lied to gnuplot.  The behaviour 
> you see is the direct result of that lie.
> 
> The shift you see is the PNM file format header.  The way you told 
> gnuplot to read that file, it has to assume this header was part of the 
> image data.  It's not, and so all other file positions are off by its
> size.
> 
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