<div dir="ltr">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.</div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Fri, 1 Oct 2021 at 18:27, L.W. <<a href="mailto:eaglelw@gmx.de">eaglelw@gmx.de</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div>
<p>yes, maybe to orange, a color that is not in the original image,
so I can do this color transparent.</p>
<p><br>
</p>
<div>Am 01.10.2021 um 18:25 schrieb David
Strip:<br>
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<blockquote type="cite">
<div>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">
<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>Am 01.10.2021 um 09:50 schrieb
Charles Dixon-Paver:<br>
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<blockquote type="cite">
<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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