[gradsusr] GXyat question re. colours

[Arlindo da Silva]

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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>>
>>
>
>
> --
> 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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>


-- 
Arlindo da Silva
dasilva at alum.mit.edu
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