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<div class="moz-cite-prefix">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?<br>
Just guessing here, obviously.<br>
<br>
<br>
On 10/1/2021 2:36 AM, L.W. wrote:<br>
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<blockquote type="cite"
cite="mid:45a3df7d-3a06-f7ee-3af5-14d05a135a60@gmx.de">
<p>Thanks for answering, but I think this is not the way I want to
go ...</p>
<p>Or, I do not have written detailed.<br>
</p>
<p>I have a raster image e.g. a white rectangle, after setting the
points, QGIS rotats this image, let's say, by 45 degrees.</p>
<p>The corners of the imported image are now black triangles.</p>
<p>I do want to have e.g. red triangles ...</p>
<p>Regards<br>
</p>
<p><br>
</p>
<div class="moz-cite-prefix">Am 01.10.2021 um 09:50 schrieb
Charles Dixon-Paver:<br>
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cite="mid:CAK2KqidN6RU469NhC2Rgw=DhiNZbdEe9J24J-VUXN_X4022w6A@mail.gmail.com">
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<div dir="ltr">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.<br>
<br>
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.<br>
<br>
The simplest solution I'm aware of is probably to follow this
workflow in QGIS:<br>
<ul>
<li>Start a new QGIS project</li>
<li>Set all black pixels to transparent in the transparency
tab of the raster layer properties</li>
<li>Digitize a "bounding area" - you can do this with a
memory layer</li>
<li>Move your vector layer to render underneath the raster</li>
<li>Give the vector a simple black symbology</li>
<li>Use the "Convert map to raster" tool from the processing
toolbox</li>
</ul>
<div>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.</div>
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