Hi,
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 Byte - Eight bit unsigned integer,
and a no data value of *-999*, -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.
Then, when georeferencing, make sure you *DO NOT select "Use 0 for
transparency when needed".* 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.
Nicolas
On 2021-10-01 2:33 p.m., L.W. wrote:
Hi,
nope this isn't what I want ... it is the same as setting transparent
color manually.
I am using PDF and TIF for georeferencing.
Regards
Without no-data-0 or transparent-color:
Set transparent-color to 0,0,0 or use no-data-0:
Am 01.10.2021 um 19:26 schrieb 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. <[email protected]
<mailto:[email protected]>> 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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Nicolas Cadieux
https://gitlab.com/njacadieux
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