[ncl-talk] Small issue plotting highways

Wed Jun 23 15:20:54 MDT 2021

Hi Andrew, all,
   Sort of a shot in the dark here, but coincidentally, I also regularly make radar plots and overlay highways (also in gray), along with a whole bunch of other info and I have not noticed this with my png files. I also use a shapefile for the highways and more or less use your technique I think. I'm attaching two examples - one from an active day, and one from today just to demonstrate that I always see the same road behavior. I see your line thickness is very small - perhaps increasing the thickness would help a bit?

If you'd like I'd be happy to share this script, though fair warning it has a ton of stuff in so may be a bit hard to parse.

Nick Bassill, PhD

Director of Research & Development, Center Of Excellence

Affiliated with NYS Mesonet

LC SB-28, 1400 Washington Ave.,

SUNY, University at Albany, NY 12222

Contact: (518) 442-6375 | @NickPBassill | http://operations.nysmesonet.org/~nbassill/
________________________________
From: ncl-talk <ncl-talk-bounces at mailman.ucar.edu> on behalf of Dave Allured - NOAA Affiliate via ncl-talk <ncl-talk at mailman.ucar.edu>
Sent: Wednesday, June 23, 2021 4:52 PM
To: Andrew Kren - NOAA Federal <andrew.kren at noaa.gov>
Cc: ncl-talk <ncl-talk at ucar.edu>
Subject: Re: [ncl-talk] Small issue plotting highways

Andrew, the difference between your plots with and without the contour fill layer is surprising.  I can only speculate that there might be a branch somewhere inside NCL, that handles antialiasing differently, depending on which graphics layers are active.  Notice that the resolution of the lettering is also affected slightly.

I still think your best bets are to try the resource suggested by Rick, or to avoid the issue by using some form of vector graphics for output.  Thanks for showing that contrasting plot, and good luck.

On Wed, Jun 23, 2021 at 8:08 AM Andrew Kren - NOAA Federal <andrew.kren at noaa.gov<mailto:andrew.kren at noaa.gov>> wrote:
Thanks Dave and Rick. I will try that. Here is what I get on a day when there is nothing on the radar to plot. The highways show up just fine with no changes to the code. I'm wondering if it has something to do with the contours overlaid or something similar? I will also try a PDF or PS file.

On Tue, Jun 22, 2021 at 6:31 PM Dave Allured - NOAA Affiliate <dave.allured at noaa.gov<mailto:dave.allured at noaa.gov>> wrote:
Andrew, PNG is an image format that naturally consists of a grid of pixels.  The only pixellation that I see in your plot is the natural pixels in the PNG format.  Your highways are mostly very fine diagonal lines.  PNG decomposes a diagonal line into a group of small gray and black squares (pixels) on a regularly spaced pixel grid.  When the intended line is thin enough, it is difficult to get a clear representation on the pixel grid.  One of the possible results is apparently dashed lines like those I am seeing.

Try making the same plot using vector graphics such as PDF, postscript (PS), or SVG, as Rick suggested.  These vector formats preserve a true mathematical representation of line data.  The result is clean edges and fine lines that will not pixellate when you zoom in, when using proper viewing software that supports true vector mode display.

On Tue, Jun 22, 2021 at 12:15 PM Andrew Kren - NOAA Federal via ncl-talk <ncl-talk at mailman.ucar.edu<mailto:ncl-talk at mailman.ucar.edu>> wrote:
Thanks, I'll check it out.

On Tue, Jun 22, 2021 at 11:25 AM Rick Brownrigg <brownrig at ucar.edu<mailto:brownrig at ucar.edu>> wrote:
Hi Andrew,

It's odd that you are getting inconsistent results. Do you see the same thing if you write to a PS or PDF file? I don't know what's going on, but it looks like a  bit of overzealous anti-aliasing. There's a resource to control whether antialiasing is applied or not:

https://www.ncl.ucar.edu/Document/Graphics/Resources/wk.shtml

It's on by default and generally yields better results, but you might experiment with it.

Wish I had a better answer.
Rick

On Tue, Jun 22, 2021 at 8:45 AM Andrew Kren - NOAA Federal via ncl-talk <ncl-talk at mailman.ucar.edu<mailto:ncl-talk at mailman.ucar.edu>> wrote:
Hi there,

I am plotting radar data and overlaying highways onto the map. I first plot the radar analysis, then overlay the highway information by reading/plotting the shapefile.

At times, when I view the PNGs, the highways show up as blotched out (lack of detail and pixelated; see attached image). At other times, the highways show up very clear and darker as expected. Is there a reason for this? Below shows a snippet of my code that produces the radar plot and the highway overlay.

 plot = gsn_csm_contour_map(wks,radar,res)

 ; add highway polylines to the map from shapefile
 plres = True
 plres at gsLineColor = "grey48"
 plres at gsLineThicknessF = 0.2
 id = gsn_add_shapefile_polylines(wks,plot,SHAPE_DIR+"roadtrl020.shp",plres)

Thanks,

--

Andrew Kren
Meteorologist
NOAA's National Weather Service
Raleigh Forecast Office
1005 Capability Drive, Suite 300
Raleigh, North Carolina 27606
voice: 919-326-1035
mailto: andrew.kren at noaa.gov<mailto:andrew.kren at noaa.gov>
http://www.weather.gov/rah
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