[Qgis-user] Geo Raster Background

Fri Oct 1 13:04:55 PDT 2021

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. <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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> --
> Nicolas Cadieux
> https://gitlab.com/njacadieux
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