comparison of Cairo-generated pngs to pngs created using ImageMagick's "convert"

Hi Rick, Joe, and all,

   I'm also looking into using the new png driver for my production
system (TC model guidance plots). I haven't completed the testing for
the guidance plot style yet, but I've just conducted a test for another
type of plot that has a lot of detail (small fonts, lots of primitives,
etc.), so I thought I'd report my results and add a few more questions.

Here's what I did to make the Cairo png files:
     wks_type = "png"
     wks_type@wkWidth = 1600 ; default is 1024 by 1024
     wks_type@wkHeight = 1600

     wks = gsn_open_wks(wks_type, foodir, fooname) ; Open a
workstation.

The resulting filenames are prefixed with "cairo" and a number
indicating the wkWidth and wkHeight. I've made the files available for
viewing at:
ftp://ftp.ucar.edu/pub/mmm/jvigh/cairo_test/
In that directory, you can also compare file sizes.

To compare with the result of ImageMagick's 'convert' command, I
generated a postscript file and then used the following syntax:
     convert -geometry 600x600 -density 500 -trim -rotate 270 $base".ps"
$base".png"

The resulting filenames have the prefix "convert" and two numbers
representing the geometry and density values used, respectively.

In general, I find the Cairo-generated plot files to be larger and
crisper than the "converted" plot files for similar values of geometry.
A plot generated with a wkWidth of 800 in the Cairo driver is actually
larger than a "converted" plot created with a geometry of 1024. However,
the "converted" plots seem a bit fuzzier than the Cairo plots. Also,
polylines seem to show up brighter and thicker in the "converted" plots
than in the Cairo plots. So depending on your plot style and desired
aesthetic, it seems there could be advantages to either method. The
postscript file seems to have the thicker polylines and brighter colors,
with sharper fonts. So besides the fuzziness, it seems the "converted"
plot files are more faithful to the postscript version (as viewed in
ghostview).

To see a good example, compare:
    
ftp://ftp.ucar.edu/pub/mmm/jvigh/cairo_test/convert_temporal_1600_500.png
to
    ftp://ftp.ucar.edu/pub/mmm/jvigh/cairo_test/cairo_temporal_1024.png

As far as file sizes go, it seems that for comparable size, the
Cairo-generated plot is less than half the size of the "converted" plot
file. Using higher density values in 'convert' has little affect on the
resulting file size, but does add to the processing time.

I noticed that if the Cairo-generated png is blown up to a large size,
say 1600 x 1600, the image file looks rather disturbed because the
browser displays a reduced version. The full version is actually still
clear however.

So in light of all this, I have the following questions and comments:

- The smaller png file sizes produced by the Cairo driver are certainly
nicer from a production standpoint (if you're going to be making and
storing thousands of these).
- It would be nice if the line thickness/colors of primitives didn't
"wash out" as much in the Cairo driver.
- Are these the only settings to interact with the driver for png
images, or are there other settings/resources that can be tweaked (like
antialias or density) in the driver? Perhaps internally, the width of
primitives could be boosted a bit when the Cairo driver is being used.
- Finally, is there any way to get rid of the "000001" in the png file
that is produced?

Thanks!
Jonathan

Rick Brownrigg wrote:
> Hi Joe,
>
> The PNG output is created directly with a native driver based upon
> cairographics. So yes, it sounds like it might be of value in your
> situation and save you some hassle.
>
> Rick
>
> On Jun 3, 2010, at 3:32 PM, Joe Grim wrote:
>
>
>> Hi,
>>
>> I was wondering if you could tell me how the new PNG output is created
>> in NCL 5.2.0? Is it first created in a vector graphics format (e.g.,
>> cgm) and then converted to PNG internally? Or is it created natively,
>> skipping a conversion step?
>>
>> The reason I ask this question is that I work on a project that
>> currently uses NCL and RIP to create thousands of vector graphics
>> files,
>> and then we need to convert them all using ImageMagick's "convert"
>> program. If the newest version of NCL skips the conversion step in
>> creating the PNG files, this might save us a lot of computing time.
>>
>> Thank you in advance for your response.
>>
>> Joe Grim
>> --
>> NCAR - RAL
>> FL2 Rm 2068
>> (303) 497-8397
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Received on Thu Jun 3 18:17:08 2010