multiple rasters && mapview

[Jonathan Coles]

Am Freitag, den 14.01.2005, 22:13 +0100 schrieb Bernhard Herzog:
> Jonathan Coles <jonathan at jpcoles.com> writes:
> 
> > when i first implemented the raster code i discovered that the bounding
> > box of the projected image is different (usually larger) than the
> > bounding box returned from layer.LatLongBoundingBox(). this is because
> > the image may curve outward between the two extreme x points, but the
> > extreme of the outward curve is not recognized as the extend of the
> > bounding box.
> [...]
> > if you can figure out how to correctly determine the projected bounding
> > box size, then i think you will be able to a) decrease the rendering
> > time since gdal isn't making an image for the whole window and b) easily
> > allow for multiple raster layers without using masks.
> 
> It can't be done properly without masks because as you note the boundary
> of the projected image is not rectangular.  It usually has curved edges
> and even if it were rectangular, it's may not be axis aligned.  The only
> way to draw that properly while only drawing the part of the window
> actually covered by the projected image is to return an image with an
> appropriate alpha channel or a mask bitmap in addition the actual image.

i think what i had in mind about not needing a mask is different than
what you are thinking. what i meant to say was, if you are simplying
trying not to have gdal generate a large image that covers the screen,
and are content to have a smaller image that may contain patches of "no
data" pixels, then it isn't necessary to use a mask, just a better
bounding box. 

i agree with you, however, that a mask (or alpha channel) is necessary
to get a "tight" image where there are no "no data" pixels.

> The strategy I used in Skencil 0.6 is to have one raster image, let's
> call it C for "canvas") the size of the window.  When a raster image is
> to be rendered, it is rendered into C while also determining which
> pixels are actually covered by the rendered image.  

i'm sending out a email in a few minutes that discusses a solution. gdal
warp actually supports "no data" and i can tell DrawBitmap what color to
consider the mask color. apply the patches and run the
iceland_sample_raster to see what i mean.

--jonathan

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 Jonathan Coles                               http://www.jpcoles.com
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