Hi,
It's quite simple, you just need to set the nodata value (what you call
the background-color) in the original file and then save it. The
complication is that this requires that the datatype for the file
accepts the no data value. This part is harder to figure out for your
not used to dealing with datatypes. Perhaps, what should be asked for
is a check to see if the nodata in the translate tool check in
compatibility with the datatype format. Another possibility is to allow
for setting the nodata value (other than zero) in the Georeferencer tool
(for both the input file and the output file).
Nicolas
On 2021-10-01 4:04 p.m., L.W. wrote:
sounds like rocketscience ... for a simple task to set the
background-color ...
may be I am searching the web where I have to place my feature request
for this ...
Am 01.10.2021 um 21:39 schrieb Nicolas Cadieux:
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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Nicolas Cadieux
https://gitlab.com/njacadieux
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