[Qgis-user] Geo Raster Background

Fri Oct 1 14:19:22 PDT 2021

Hi,

It's quite simple, you just need to set the nodata value (what you call 
the background-color) in the original file and then save it.  The 
complication is that this requires that the datatype for the file 
accepts the no data value. This part is harder to figure out for your 
not used to dealing with datatypes.   Perhaps, what should be asked for 
is a check to see if the nodata in the translate tool check in  
compatibility with the datatype format. Another possibility is to allow 
for setting the nodata value (other than zero) in the Georeferencer tool 
(for both the input file and the output file).

Nicolas

On 2021-10-01 4:04 p.m., L.W. wrote:
>
> 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

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

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