[gradsusr] GXyat question re. colours

[Jennifer Adams]

The anti-aliasing in the Cairo library does make the colors seem a  
little duller than when drawing without anti-aliasing. There is no  
real cure for this, although if you increase the thickness of the anti- 
aliased lines, they do brighten. For lines that we know will be  
exactly horizontal or vertical, we plan to 'snap' them to the pixel  
grid and then they won't be drawn with the anti-alias blending on the  
edges. This will make the plot frame and other rectangles look sharp  
and bright. Turning off anti-aliasing for polygon fills results in  
very faint gaps that are visible between adjacent polygons of the same  
color (e.g. in a shaded plot), so I don't recommend that. A possible  
solution is to draw the perimeter of each polygon to cover the gaps,  
but this is a performance hit. We are still thinking about the user  
interface to control all these factors. For using gxyat right now, I  
would try tweaking the line thickness and the size of your output  
image, those things may improve the appearance/dimness of the anti- 
aliased lines.
--Jennifer



On Sep 22, 2011, at 11:51 AM, Arlindo da Silva wrote:

> On Thu, Sep 22, 2011 at 4:25 AM, Becker, Bernd <bernd.becker at metoffice.gov.uk 
> > wrote:
> Arlindo,
>
> you are right, this is an artifact of antialiasing
> (see attached plot: the yellows are the same in contour and in  
> legend box)
>
> So we found the cause.
>
> But the plot looks no where near as impressive as the previous  
> output from gxyat.
>
> What can we try to do about that?
>
> Anti-aliasing is a perceptual approximation that achieves "smooth  
> lines" by blending pixels with the "environment"; it is all handled  
> by the cairo library.  I am not entirely sure why the legend gives  
> you a different color... but wait a minute!  One trick that gxyat  
> uses to avoid those funny lines in filled  contours is to turn anti- 
> alising off every time a (filled) polygon is drawn. How are you  
> creating your legend? Try using the more basic line to, move to  
> function instead of a polygon function; this way you will have the  
> same anti-aliasing used by the contour lines.
>
>    Arlindo
>
>
>
> Cheers,
> Bernd.
>
>
>
> --
> Bernd Becker   Climate Impacts Product Development
> Met Office Hadley Centre    FitzRoy Road  Exeter   Devon EX1 3PB   
> United Kingdom
> Tel.: +44 (0) 1392 884511 Fax: +44 (0)870 900 5050
> E-mail:bernd.becker at metoffice.gov.uk - http://www.metoffice.gov.uk/research/our-scientists/seasonal-to-decadal/bernd-becker
>
> http://www.metoffice.gov.uk/news/releases/archive/2011/weather-game
>
>
>
> From: gradsusr-bounces at gradsusr.org [mailto:gradsusr-bounces at gradsusr.org 
> ] On Behalf Of Arlindo da Silva
> Sent: 22 September 2011 03:10
> To: GrADS Users Forum
> Subject: Re: [gradsusr] GXyat question re. colours
>
> On Wed, Sep 21, 2011 at 7:02 AM, Becker, Bernd <bernd.becker at metoffice.gov.uk 
> > wrote:
>
> Hello,
>
> How can I force gxyat to save the colours to a gif file as displayed  
> on
> screen ?
>
> Please find attached 2 images,
> one created with printim (authentic colours) and one
> created with gxyat and observe, that the yellow in the contour plot
> differs from the
> yellow in the legend.
>
> What could be the cause of that?
>
> What can I do about it?
>
>
> This is strange. Possibly this is an artifact of anti-aliasing.  
> Could you try turning anti-aliasing off?
>
> ga-> gxyat +a file.png
>
>     Arlindo
>
>
> -- 
> Arlindo da Silva
> dasilva at alum.mit.edu
>
> _______________________________________________
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>
>
>
>
> -- 
> Arlindo da Silva
> dasilva at alum.mit.edu
> _______________________________________________
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--
Jennifer M. Adams
IGES/COLA
4041 Powder Mill Road, Suite 302
Calverton, MD 20705
jma at cola.iges.org



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