Re: rgbimage plots with linked axes

[Dima K. <gn...@di...>]

sfeam <sf...@us...> writes:

> On Thursday, 31 August 2017 22:44:01 Dima Kogan wrote:
>> Hi. I just stumbled on another bug with rgbimages. If somebody knows
>> what's causing it, that'd be appreciated.
>
> This is a very complicated one.
> I'll look at it in more detail over the weekend.
> For now I'll just note:
>
> - The script basically does not work with 5.2 or current cvs, so I assume
> you are using 5.0?

I was using "gnuplot 5.1 patchlevel 0", but it wasn't a release. It was
built from the latest sources as of 2016/11/18. I just tried on the
latest sources, and the problem is mostly the same.


> - To make the script work with 5.2 I can move the range outside the plot command:
>
>     set xrange [-120:-80]
>     set yrange [20:60]
>     plot "montage_40_-99_1300miles_4.png" binary filetype=png flipy ...

Yep. I had to do this to make it work with the latest sources. Does that
make sense? This smells like a bug too.


> - I'm not sure whether or not I see the same thing as you on zooming.
>   At first glance it looks to me that the zoomed-in placement is correct
>   (point is on top of LA) in the zoom view even though it was not in the
>   original full view.  The closer in I zoom the closer to perfect the superposition
>   becomes.   Is that what you see or do you see something else?

That's what I see too: I zoom on an area that does NOT have the circle
in it (but that SHOULD have it), and I see the zoomed circle post-zoom.


> - So I guess my question is, would it be a correct description to say that
>   the plot is correct but only after a zoom operation?  If so, that sounds
>   easier to track down than  "it's always wrong".

I can't tell. There could be a systematic error, but this is a low-res
map and doesn't have enough detail.


> - One possible problem occurs to me.  The coordinates mapped to an
>   image pixel describe the center of the pixel, not a corner.
>   You may have constructed your mapping with that in mind, or maybe not.
>   If not then perhaps adjusting the mapping function will fix everything.

It's true that I wasn't super careful about this distinction, but I
don't see 0.5 pixels of error, I see 35 pixels of error.

I was hoping that this one wouldn't be tough to track down since one
would expect the rgbimage to be plotted against y2 normally, and against
y2 via a mapping. But here the NORMAL y2 coordinates are becoming
broken. But admittedly I haven't spent much time in looking at the
sources on this one.