[Qgis-user] Geo Raster Background
L.W.
eaglelw at gmx.de
Fri Oct 1 14:38:39 PDT 2021
I use PDF as well, there you can not save a background-color or
something like that.
This should be better an option in QGIS to "set the colors of the black
recangles" ;-)
thanks all.
Am 01.10.2021 um 23:19 schrieb Nicolas Cadieux:
>
> 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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