[ncl-talk] Small issue plotting highways

[Dave Allured - NOAA Affiliate]

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> wrote:

> Thanks, I'll check it out.
>
> On Tue, Jun 22, 2021 at 11:25 AM Rick Brownrigg <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> 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.govhttp://www.weather.gov/rah
>>>
>>>
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