Re: rgbimage plots with linked axes

[sfeam <sf...@us...>]

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?

- 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 ... 

- At that point I see (I think) the same thing you do in that the single explicit
  point lies North of rather than exactly on top of the Los Angeles marker.

- 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?

- 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".

- 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.

	Ethan

> I'm plotting an image showing a map composed of stitched openstreetmap
> tiles (attached). The mapping between pixels in this image and lat/lon
> is linear in longitude and nonlinear in latitude. I'm plotting the image
> pixels onto x2y2, and using linked axes to get the latitude on y and
> longitude on x. The script is this:
> 
>   set x2tics auto
>   set y2tics auto
> 
>   lon0       = -123.5614
>   lon_offset = 180.0 + lon0
> 
>   zoom = 4.
>   px_to_lon(x) = x/(256. * 2.**4)*360. - 180. + lon_offset
>   lon_to_px(x) = (x-lon_offset+180.)/360. * 2.**zoom * 256.
> 
>   lat_offset_px = 1215.969
> 
>   s1_cos(x)     = (sin(x)+1)/cos(x)
>   inv_s1_cos(x) = asin( (x**2 - 1)/(x**2 + 1) )
> 
>   px_to_lat(x) = inv_s1_cos(exp((1 - (x + lat_offset_px)/(2.**zoom * 256) *2)*pi))*180./pi
>   lat_to_px(x) = (1 - log( (sin(x*pi/180) + 1.)/cos(x*pi/180) )/pi)/2. * 2.**zoom * 256 - lat_offset_px
> 
> 
>   set link x2 via lon_to_px(x) inverse px_to_lon(x)
>   set link y2 via lat_to_px(y) inverse px_to_lat(y)
> 
>   plot [x=-120:-80][y=20:60] "montage_40_-99_1300miles_4.png" binary filetype=png flipy with rgbimage notitle axis x2y2, '-' with points axis x2y2
>   60 419
>   e
> 
> 
> Note that the circle representing Los Angeles lies at pixel coordinates
> 60,419 and I'm plotting a point there. The corresponding latlon is
> roughly -118,34. The pixels are on x2y2 and the point is on x2y2, so
> this point should be rendered in that circle. I'm not seeing that at
> all, however. The point is at y2=419 as it should be, but the Los
> Angeles circle is roughly at y2=448. The x coordinates match properly.
> 
> Furthermore, zooming in makes it clear that gnuplot is confused:
> interactively zooming on the plotted point shows me the zoomed LA
> circle, even though visually the LA circle was elsewhere.
> 
> Any pointers?