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<p>Hi,<br>
</p>
<p>Start by setting the nodata value in the input raster.
(raster/conversion/translate/Assign a specific nodata value to
output bands). Make sure you select a data type that allows your
nodata value. For example, if you select <span style="color:
rgb(0, 0, 0); font-family: Lato, 'Open Sans', 'Lucida Grande',
'Segoe UI', Arial, sans-serif; font-size: 14.399999618530273px;
font-style: normal; font-variant-caps: normal; font-weight:
normal; letter-spacing: normal; orphans: auto; text-align:
start; text-indent: 0px; text-transform: none; white-space:
normal; widows: auto; word-spacing: 0px;
-webkit-tap-highlight-color: rgba(0, 0, 0, 0.4);
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); display: inline !important; float: none;">Byte - Eight bit
unsigned integer, and a no data value of <b>-999</b>, -999 does
not exist in that data type so the nodata values will be 0
therefore affecting all black pixels in the image. Select
something like int16 of float32... and a no data value of -999
for the test. Load the new raster and in the layer properties,
make sure you have been successful changing the nodata values.<br>
</span></p>
<p><span style="color: rgb(0, 0, 0); font-family: Lato, 'Open Sans',
'Lucida Grande', 'Segoe UI', Arial, sans-serif; font-size:
14.399999618530273px; font-style: normal; font-variant-caps:
normal; font-weight: normal; letter-spacing: normal; orphans:
auto; text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: auto; word-spacing: 0px;
-webkit-tap-highlight-color: rgba(0, 0, 0, 0.4);
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); display: inline !important; float: none;">Then, when
georeferencing, make sure you <b>DO NOT select "Use 0 for
transparency when needed".</b> The nodata from the
georefrenced file will be taken from the original file (-999).
Therefore, pixels with the value 0 in the image will not be
affected.</span></p>
<p><span style="color: rgb(0, 0, 0); font-family: Lato, 'Open Sans',
'Lucida Grande', 'Segoe UI', Arial, sans-serif; font-size:
14.399999618530273px; font-style: normal; font-variant-caps:
normal; font-weight: normal; letter-spacing: normal; orphans:
auto; text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: auto; word-spacing: 0px;
-webkit-tap-highlight-color: rgba(0, 0, 0, 0.4);
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); display: inline !important; float: none;">Nicolas<br>
</span></p>
<p><span style="color: rgb(0, 0, 0); font-family: Lato, 'Open Sans',
'Lucida Grande', 'Segoe UI', Arial, sans-serif; font-size:
14.399999618530273px; font-style: normal; font-variant-caps:
normal; font-weight: normal; letter-spacing: normal; orphans:
auto; text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: auto; word-spacing: 0px;
-webkit-tap-highlight-color: rgba(0, 0, 0, 0.4);
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); display: inline !important; float: none;"><br>
</span></p>
<div class="moz-cite-prefix">On 2021-10-01 2:33 p.m., L.W. wrote:<br>
</div>
<blockquote type="cite"
cite="mid:eb05aed0-f992-6244-eda9-3346417c8424@gmx.de">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<p>Hi,</p>
<p>nope this isn't what I want ... it is the same as setting
transparent color manually.</p>
<p>I am using PDF and TIF for georeferencing.</p>
<p>Regards</p>
<p>Without no-data-0 or transparent-color:<br>
</p>
<p><img src="cid:part1.AD6213E5.2C5E9644@gmail.com" alt=""
class=""></p>
<p>Set transparent-color to 0,0,0 or use no-data-0:<br>
</p>
<p><img src="cid:part2.9BF694E6.21B2D177@gmail.com" alt=""
class=""></p>
<p><br>
</p>
<div class="moz-cite-prefix">Am 01.10.2021 um 19:26 schrieb
Charles Dixon-Paver:<br>
</div>
<blockquote type="cite"
cite="mid:CAK2KqicgxUWu=Vv04hmZA3JhEJWbfyyEg=8b_BoVcJt+CeA7EA@mail.gmail.com">
<meta http-equiv="content-type" content="text/html;
charset=UTF-8">
<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"
moz-do-not-send="true">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>
</div>
<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>
</div>
<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>
</div>
<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>
</div>
</blockquote>
</blockquote>
<br>
</blockquote>
</div>
</blockquote>
</div>
</blockquote>
<br>
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<pre class="moz-signature" cols="72">--
Nicolas Cadieux
<a class="moz-txt-link-freetext" href="https://gitlab.com/njacadieux">https://gitlab.com/njacadieux</a></pre>
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