Michael,
It turns out that gdal_translate was using 255.999 since the origins in
2002 :
https://github.com/OSGeo/gdal/commit/47d859efb25da249b5d62eb8619c36134ae76572#diff-d57b4553312c88a736821448adc155e6797653b0d902bd42f9310572aa71f0b0R215
I suspect the reasoning was that, if you rounded values down (floor) and
used 255 as the scaleDstMax, and you remapped [0,65535] to [0,255] for
example, then 65534. / (65535. / 255) = 254.996.. would be rounded to
254. So you would only get 255 as the output for 65535 as the input,
which seems "unfair". But GDAL actually rounds to closest, so if using
dstMax = 65535, all input values in range [65535 - 128, 65535] get
scaled to 255, which is OK to me. Of course that last "bucket" and the
first one at 0 (input values in [0,128]) are half-width of the ones in
the middle (input values in [129,129+256] get scaled to 1) but I don't
think we can do much about that (discrete values vs intervals)
One downside of using dstMax=255.999 is that if your input range is [0,
255], then -scale was not the identity (254 got transformed to 253),
which is kind of an issue! Hence I've changed dstMax to be 255 in
https://github.com/OSGeo/gdal/pull/8367
I've also noticed during testing a more subtle consistency issue when if
you did for example "gdal_translate autotest/gcore/data/byte.tif out.asc
-scale 0 1 0 1.5 -of aaigrid", you would get values outside of the [0,
255] range because the VRT machinery was ignoring the constraints of the
VRTRasterBand data type (Byte), and was only honouring the buffer data
type of the RasterIO() request from the AAIGrid driver (Int32, always
used when translating from source bands having integer data types, but
with the reasoning that if you query a Byte band then you would get
always value in [0,255] range)
Even
Le 08/09/2023 à 04:20, Michael Sumner a écrit :
Hi, I'm expecting `gdal_translate -scale` to emit values in the range
0,255 but it seems to be targeting 0,256. (All works as expected when
using explicit src_min src_max dst_min dst_max).
To see the output range on a simple example (see code at link below in
case email garbles):
from osgeo import gdal
ds = gdal.Translate("/vsimem/scl.tif",
"autotest/gcore/data/float32.tif", options = "-scale")
ds.GetRasterBand(1).ComputeRasterMinMax()
## (0.0, 255.99899291992188)
Is that expected, am I missing something? I've tried variations on the
input values and output types.
https://gist.github.com/mdsumner/ee4103d8616b9aa341e82c46b44a8c8c
Cheers, Mike
--
Michael Sumner
Research Software Engineer
Australian Antarctic Division
Hobart, Australia
e-mail: mdsum...@gmail.com
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My software is free, but my time generally not.
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