[Qgis-developer] geotiff export using print composer

[Marco Hugentobler]

Hi Stefan

>1) Make the mapRenderer rectangle cover the entire paper in the composer

In your case where you need only the map for the printout (no scalebar, 
legend, ...) and you use programming anyway, I think it would be better 
to do the rendering directly with QgsMapRenderer and not using the 
composer classes.

Regards,
Marco

On 21.02.2012 09:08, SBL wrote:
> Dear all
>
> For the GarminCustomMap plugin I use the print composer (through python) to
> export a (high resolution) geotiff.
>
> Because the mapRenderer does not fill the entire paper in the composer
> (although the paper size is set according to the mapRenderer size), there is
> a reasonable offset / displacement of the resulting geotiff when using the
> output of the print composer directly in combination with the coordinates of
> the mapRenderer.
>
> In a workaround I managed to improve the precision of the georeferencing by
> removing the empty cells around the mapRenderer (remove cells starting from
> bottom / right border, until the black/grey pixels of border line of the
> mapRenderer are reached). See code here:
> http://hub.qgis.org/projects/garmincustommap/repository
> http://hub.qgis.org/projects/garmincustommap/repository
>
> This approach has (mainly) three backdraws:
> 1) The border is not always rendered identically. E.g. the color (which is
> used to identify the border line) can vary depending on the color of nearby
> objects in the map (border gets grey when there are no objects close to the
> border).
> 2) Depending on the output DPI the number of pixels in the border lines
> varys, resulting in less precision in georeferencing with increased output
> DPI.
> 3) The resulting image has a black borderline which can be disturbing when
> geotifs are exported as tiles...
>
> My question is now: do you have any suggestions for tackling these issues?
> Solutions I`ld very much appreciate are (starting with the favourits):
> 1) Make the mapRenderer rectangle cover the entire paper in the composer
> (with exactly matching corners). (Any Ideas how to do that?)
> 2) Having the border of the mapRenderer rendered always in 100% black and
> with a fixed number of pixels (regardless output DPI). Are there any
> possibilities to define this in the settings for the composer/mapRenderer?
> 3) Having the border of the mapRenderer rendered always in 100% black and
> with the number of pixels in a fixed relation to the output DPI. Would it be
> possible to define that?
>
> Thanks for helping!
>
> Cheers
> Stefan
>
>
>
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-- 
Dr. Marco Hugentobler
Sourcepole -  Linux&  Open Source Solutions
Churerstrasse 22, CH-8808 Pfäffikon SZ, Switzerland
marco.hugentobler at sourcepole.ch http://www.sourcepole.ch
Technical Advisor QGIS Project Steering Committee