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<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'>Ok, here is how a GDAL caller can detect that a file is signed
8-bit:<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'>>>> band.GetMetadata('IMAGE_STRUCTURE')<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'>{'PIXELTYPE': 'SIGNEDBYTE'}<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'>Thanks to Frank W for giving me a clue about this a week ago
that I promptly forgot about. Sorry about that.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'>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.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'>Thanks,<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'>Jason<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'><o:p> </o:p></span></p>
<div>
<div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'>
<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span
style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> Jason Roberts
[mailto:jason.roberts@duke.edu] <br>
<b>Sent:</b> Wednesday, September 23, 2009 4:18 PM<br>
<b>To:</b> 'gdal-dev'<br>
<b>Subject:</b> RE: [gdal-dev] GetStatistics and signed 8-bit IMGs and GeoTIFFs<o:p></o:p></span></p>
</div>
</div>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'>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).<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'>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? <o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'>Thanks again,<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'>Jason<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'><o:p> </o:p></span></p>
<div>
<div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'>
<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span
style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>
gdal-dev-bounces@lists.osgeo.org [mailto:gdal-dev-bounces@lists.osgeo.org] <b>On
Behalf Of </b>Jason Roberts<br>
<b>Sent:</b> Wednesday, September 23, 2009 3:11 PM<br>
<b>To:</b> 'gdal-dev'<br>
<b>Subject:</b> [gdal-dev] GetStatistics and signed 8-bit IMGs and GeoTIFFs<o:p></o:p></span></p>
</div>
</div>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'>Is band.GetStatistics() supposed to work with signed 8-bit
integer rasters with the HFA or GTiff driver?<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'>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:<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'>>>> a = numpy.array([[-128, -1, 0, 1, 127]],
dtype='int8')<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'>>>> a<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'>array([[-128, -1,
0, 1, 127]], dtype=int8)<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'>>>> b = numpy.cast['uint8'](a)<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'>>>> print b<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'>[[128 255 0 1 127]]<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'>>>><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'>>>> gdal.UseExceptions()<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'>>>> driver = gdal.GetDriverByName('HFA')<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'>>>> dataset = driver.Create(r'C:\Temp\test_int8.img',
5, 1, 1, gdal.GDT_Byte, ['PIXELTYPE=SIGNEDBYTE'])<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'>>>> band = dataset.GetRasterBand(1)<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'>>>> band.WriteArray(b)<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'>0<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'>>>> del band, dataset, driver<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'>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:<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'>>>> dataset = gdal.Open(r'C:\Temp\test_int8.img',
gdalconst.GA_ReadOnly)<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'>>>> band = dataset.GetRasterBand(1)<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'>>>> band.GetStatistics(False, True)<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:#003399'>[0.0, 255.0, 102.2, 95.199579831005551]<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'>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?<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'>Thanks very much for your help,<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'>Jason<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana","sans-serif";
color:#003399'><o:p> </o:p></span></p>
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