[Qgis-user] Geo Raster Background

[L.W.]

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. <eaglelw at gmx.de
> <mailto:eaglelw at 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.
>>
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