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

[Daniel Evans]

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> 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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