[gdal-dev] GetStatistics and signed 8-bit IMGs and GeoTIFFs

[Jason Roberts]

Ok, here is how a GDAL caller can detect that a file is signed 8-bit:

 

>>> band.GetMetadata('IMAGE_STRUCTURE')

{'PIXELTYPE': 'SIGNEDBYTE'}

 

Thanks to Frank W for giving me a clue about this a week ago that I promptly
forgot about. Sorry about that.

 

So. does anyone know whether band.GetStatistics is supposed to work for
these signed 8-bit files? If not, I guess I will write a workaround so my
app will calculate statistics itself for signed 8-bit files.

 

Thanks,

Jason

 

From: Jason Roberts [mailto:jason.roberts at duke.edu] 
Sent: Wednesday, September 23, 2009 4:18 PM
To: 'gdal-dev'
Subject: RE: [gdal-dev] GetStatistics and signed 8-bit IMGs and GeoTIFFs

 

Someone suggested that I try this with 1.6.2 without saying whether it might
be a bug or by design. The behavior is the same with 1.6.2, using binaries
produced by Tamas Szekeres (thanks Tamas, for providing those on your
website).

 

Back to the original question: is it a bug or by design? How does the GDAL
caller determine through GDAL that a given file is signed 8-bit or unsigned
8-bit? 

 

Thanks again,

Jason

 

From: gdal-dev-bounces at lists.osgeo.org
[mailto:gdal-dev-bounces at lists.osgeo.org] On Behalf Of Jason Roberts
Sent: Wednesday, September 23, 2009 3:11 PM
To: 'gdal-dev'
Subject: [gdal-dev] GetStatistics and signed 8-bit IMGs and GeoTIFFs

 

Is band.GetStatistics() supposed to work with signed 8-bit integer rasters
with the HFA or GTiff driver?

 

With GDAL 1.6.0, I can successfully create a signed 8-bit raster with the
HFA or GTiff driver using the PIXELTYPE=SIGNEDBYTE option:

 

>>> a = numpy.array([[-128, -1, 0, 1, 127]], dtype='int8')

>>> a

array([[-128,   -1,    0,    1,  127]], dtype=int8)

>>> b = numpy.cast['uint8'](a)

>>> print b

[[128 255   0   1 127]]

>>> 

>>> gdal.UseExceptions()

>>> driver = gdal.GetDriverByName('HFA')

>>> dataset = driver.Create(r'C:\Temp\test_int8.img', 5, 1, 1,
gdal.GDT_Byte, ['PIXELTYPE=SIGNEDBYTE'])

>>> band = dataset.GetRasterBand(1)

>>> band.WriteArray(b)

0

>>> del band, dataset, driver

 

When I look at that raster with ArcGIS, it shows it is signed 8-bit integer
and the five values show up properly. But when I open it with GDAL and call
band.GetStatistics(), it looks like the calculation is performed using
unsigned 8-bit integers:

 

>>> dataset = gdal.Open(r'C:\Temp\test_int8.img', gdalconst.GA_ReadOnly)

>>> band = dataset.GetRasterBand(1)

>>> band.GetStatistics(False, True)

[0.0, 255.0, 102.2, 95.199579831005551]

 

I am wondering if this is a bug or by design. If by design, is there any way
my application can tell through GDAL that it is a signed 8-bit file rather
than an unsigned 8-bit file?

 

Thanks very much for your help,

 

Jason

 

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