[Qgis-user] Geo Raster Background

[Gabriel De Luca]

 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. (eaglelw at 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. <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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