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:
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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