[gradsusr] GXyat question re. colours

Jennifer Adams jma at cola.iges.org
Fri Sep 30 18:33:14 EDT 2011 

I had remembered it backwards ... it's been so long since I have  
worked on anything cairo-related... anti-aliasing must be DISABLED for  
polygon fills. These are screen shots from the X11 rendering, not  
output from gxyat.

Here is a 'gxout shade2' plot drawn with anti-aliased polygon fills:


and here is the same thing with anti-aliasing disabled for polygon  
fills:



--Jennifer


On Sep 22, 2011, at 7:37 PM, Arlindo da Silva wrote:

> On Thu, Sep 22, 2011 at 1:56 PM, Jennifer Adams <jma at cola.iges.org>  
> wrote:
> 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.
>
> I've been doing this for some time in gxyat and I never saw this  
> artifact.  Here is a typical plot with plenty of same color polygons:
>
> http://portal.nccs.nasa.gov/cgi-hs3/hs3_wx.cgi?region=hs3&dtg=2011092212&prod=1ht&model=fp&level=300&tau=000&&region_old=hs3&dtg_old=2011092212&prod_old=1ht&model_old=fp&level_old=200&tau_old=000&&loop=0
>
> Do you have an example where this problem is apparent?
>
>    Thanks,
>
>        Arlindo
>
>
>
> 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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>> gradsusr at gradsusr.org
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>>
>>
>>
>>
>> -- 
>> Arlindo da Silva
>> dasilva at alum.mit.edu
>> _______________________________________________
>> gradsusr mailing list
>> gradsusr at gradsusr.org
>> http://gradsusr.org/mailman/listinfo/gradsusr
>
> --
> Jennifer M. Adams
> IGES/COLA
> 4041 Powder Mill Road, Suite 302
> Calverton, MD 20705
> jma at cola.iges.org
>
>
>
>
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>
>
>
>
> -- 
> Arlindo da Silva
> dasilva at alum.mit.edu
> _______________________________________________
> gradsusr mailing list
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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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