[Qgis-user] Geo Raster Background

[Charles Dixon-Paver]

I just loaded an unreferenced png with black content into qgis, then set
the nodata value to 99999, then did a fake georeference with arbitrary
coordinates, and the result seemed to be rotated without any background
pixels. Using 3.20 on Windows. The geoereferencer has a checkbox for
setting nodata to 0, so uncheck that first. Otherwise, if you're still
unable to get the "triangles" to change, my previous messages should still
apply.

On Fri, 1 Oct 2021 at 18:27, L.W. <eaglelw at gmx.de> wrote:

> yes, maybe to orange, a color that is not in the original image, so I can
> do this color transparent.
>
>
> Am 01.10.2021 um 18:25 schrieb David Strip:
>
> When georeferencing, aren't the areas outside the original image, eg, the
> "triangles" resulting from rotation, treated as no data? Wouldn't this
> issue be resolved by setting the no data value to something other than
> black?
> Just guessing here, obviously.
>
>
> On 10/1/2021 2:36 AM, L.W. wrote:
>
> Thanks for answering, but I think this is not the way I want to go ...
>
> Or, I do not have written detailed.
>
> I have a raster image e.g. a white rectangle, after setting the points,
> QGIS rotats this image, let's say, by 45 degrees.
>
> The corners of the imported image are now black triangles.
>
> I do want to have e.g. red triangles ...
>
> Regards
>
>
> Am 01.10.2021 um 09:50 schrieb Charles Dixon-Paver:
>
> There's not really any way for a GIS system to automatically differentiate
> between black background pixels and black data pixels. QGIS supports users
> setting custom pixel values to render as transparent pixels, but it's left
> up to users to ensure that their data and background values are different.
>
> From my view, there are a couple of options when it comes to fixing this
> issue in your data. One is to use an image processing tool like GIMP or
> Photoshop effectively, but that requires retaining the geographic
> information and knowing how to reprocess the raster accordingly.
>
> The simplest solution I'm aware of is probably to follow this workflow in
> QGIS:
>
>    - Start a new QGIS project
>    - Set all black pixels to transparent in the transparency tab of the
>    raster layer properties
>    - Digitize a "bounding area" - you can do this with a memory layer
>    - Move your vector layer to render underneath the raster
>    - Give the vector a simple black symbology
>    - Use the "Convert map to raster" tool from the processing toolbox
>
> There are a wide number of variations on that workflow that are possible
> of course, but I think that's the easiest to get started.
>
>
>
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