[Qgis-user] Geo Raster Background

[Charles Dixon-Paver]

You can translate pixel values with the raster calculator (available under
the raster menu). Black pixels will have a value of 0,0,0 on RGB bands and
red pixels would have a value of 255,0,0 so it's just a matter of changing
the values accordingly on those bands.

The raster calculator will contain an if expression in the next QGIS
release, but if you're using an existing release you will have to use a bit
of a quirky syntax to achieve that.

So if you select the red band and use an expression similar to ( "Image at 1"
=  0 )  *  255 it should work. If you need assistance on raster calculator
syntax gis stack overflow has a large number of Q&A on the topic.

If you need info on a particular pixel value at each band, just use the
identify features tool (it works on rasters too).

Note that the previous statement about the GIS identifying valid data
values from background pixel values still applies to the calculator.


On Fri, 1 Oct 2021 at 10:36, L.W. <eaglelw at gmx.de> 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.
>
> Regards
>
>
>
> On Fri, 1 Oct 2021 at 09:28, L.W. <eaglelw at gmx.de> wrote:
>
>> Hi,
>>
>> how to change the backgroundcolor of a georeferenzed raster image?
>>
>> When a raster image is rotated by qgis after setting all points the
>> backgroundcolor is black RGB(0,0,0).
>>
>> Set black to transparent is difficult when there is black text.
>>
>> regards
>>
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