Hi, I usually do a brief adjustment on pixel values prior to georeferencing, to make sure that no pixel has the value I need to assign to nodata.
Depending on the source dataset, I usually try to replace the zeros with ones, and reserve the zero for nodata. El vie, 1 de oct. de 2021 a la(s) 15:34, L.W. (eagl...@gmx.de) escribió: > 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. <eagl...@gmx.de> 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. >> >> >> _______________________________________________ > Qgis-user mailing list > Qgis-user@lists.osgeo.org > List info: https://lists.osgeo.org/mailman/listinfo/qgis-user > Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user >
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