[Qgis-user] Geo Raster Background

[L.W.]

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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