[gdal-dev] Question on building multi band composite and going back to RGB GeoTiff

[Andrew C Aitchison]

On Mon, 22 Apr 2024, Raley, Nathan via gdal-dev wrote:

> Hmm, good catch.  Looking at the stats for the red band:
>
> Band 1 Block=128x128 Type=UInt16, ColorInterp=Gray
>  Min=130.000 Max=36265.000
>  Minimum=130.000, Maximum=36265.000, Mean=10415.962, StdDev=3502.933
>  NoData Value=0
>  Metadata:
>    STATISTICS_MAXIMUM=36265
>    STATISTICS_MEAN=10415.961859513
>    STATISTICS_MINIMUM=130
>    STATISTICS_STDDEV=3502.9325925887
>    STATISTICS_VALID_PERCENT=49
>
> So, is there no way to actually do this via command line, or am I going to have to write a python lib to actually read through these and determine what they should be scaled to?

There is gdal_transform -scale (and -scale_2 etc.)

What are you going to do with the resulting file ?
QGIS allows you to make these sorts of adjustments at view-time.

Remember that the max and min values present in an image may not represent
the full range and the next-door image may have a larger or smaller range
(or just a shifted range). Imagine a row of three tiles; the middle on on 
the edge of a glacier, the first entirely on the glacier and the last 
entirely off the glacier. You may wish to apply the same scaling to all 
three.

> From: Daniel Evans <daniel.fred.evans at gmail.com>
> Sent: Monday, April 22, 2024 9:29 AM
> To: Raley, Nathan <nraley at gaig.com>
> Cc: 'gdal-dev at lists.osgeo.org' (gdal-dev at lists.osgeo.org) <gdal-dev at lists.osgeo.org>
> Subject: [External] Re: [gdal-dev] Question on building multi band composite and going back to RGB GeoTiff
>
> Hi Nathan, My initial suspicion might just be that the scaling the data provider did to go from the raw data to a human-eye-friendly RGB composite isn't the conversion you're assuming. I know that with the data I regularly work with,
>
> Hi Nathan,
>
> My initial suspicion might just be that the scaling the data provider did to go from the raw data to a human-eye-friendly RGB composite isn't the conversion you're assuming.
>
> I know that with the data I regularly work with, it may be provided as Uint16, but the data range doesn't extend all the way to 65535.
>
> If you compare the values in the separate R and G images to the RGB composite, do they appear to match the conversion you're assuming, or is there a different scaling (and possibly offset)?
>
> Cheers,
> Daniel
>
> On Mon, 22 Apr 2024, 15:20 Raley, Nathan via gdal-dev, <gdal-dev at lists.osgeo.org<mailto:gdal-dev at lists.osgeo.org>> wrote:
> I currently have a RGB geotiff composite image that has a Byte datatype.  I also have individual band images for R, G, Redge, and NIR that are UInt16 datatypes.  Since I’m missing the Blue band from the individual bands, I was attempting to extract the blue band from the RGB composite image, scale it up to the UInt16 datatype, and build a composite VRT with R, G, B, NIR, RE bands included in it.  I was then attempting to extract the RGB bands from the multispec VRT in order to see if the process was working as intended, but I’m getting an extremely blue image.
>
> Can anyone shed some light as to what I may be doing wrong here?
>
> I started by building a VRT for each band:
> gdal_translate source_RGB.tif b.vrt -ot UInt16 -of VRT -b 3 -scale 0 255 0 65535
> gdalbuildvrt -b 1 r.vrt source_R.tif
> gdalbuildvrt -b 1 g.vrt source_G.tif
> gdalbuildvrt -b 1 nir.vrt source_NIR.tif
> gdalbuildvrt -b 1 re.vrt source_RE.tif
>
> I then merged the VRTs:
> gdalbuildvrt -separate multispec.vrt r.vrt g.vrt b.vrt nir.vrt re.vrt
>
> I now have a multispec.vrt with the R, G, B, NIR, and RE bands, all with a UInt16 datatype.
>
> Now, I attempted to rebuild a RGB GeoTiff from the composite VRT with something like:
> gdal_translate -ot Byte -of GTiff -b 1 -b 2 -b 3 -scale 0 65535 0 255 -co PHOTOMETRIC=RGB multispec.vrt multispec.tif
>
> Viewing the result in QGIS appears overly blue.  What am I doing wrong here?
>
> Thanks,
> Nathan
>
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-- 
Andrew C. Aitchison                      Kendal, UK
                    andrew at aitchison.me.uk