[Qgis-user] Geo Raster Background

[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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