[gdal-dev] Question on scaling between tif and vrt

[Stephen Woodbridge]

Even,

Thanks!

I didn't set his up, I just inherited the maintenance of it and I'm just 
trying to understand how it works and eventually make changes to it and 
fix some of the processing that does not seem to be grounded in a good 
process, which is looking like more and more of the code. :(

I get the point about the color table values need to be values from 0-n 
and you need 0-n entries in the color table.  Or you need to use LUT 
like you explained before.

What I was trying to understand is more along the lines of

input tif is Int16
vrt is declared as Byte

How does GDAL deal with this? There are no errors or warnings. gdalinfo 
is reporting a Byte range of values for the vrt. I'm wondering how it is 
interpreting an arbitrary Int16 value into a Byte?

So some of the obvious guesses are:

1. it is scaling them (you say no on this)
2. it is reading the Int16 and only using only the low or high order 
byte as the value?
3. something else?

Thank you for your patience with all my questions.

-Steve

On 5/19/2014 5:48 PM, Even Rouault wrote:
> Le lundi 19 mai 2014 23:42:06, Stephen Woodbridge a écrit :
>> Hi all,
>>
>> I have a .tif file with -ot Int16 and an associated .vrt file that is
>> applying a color palette and is type Byte.
>>
>> How does if decide on how to scale the pixel values?
>>
>> Does it compute the range and then just scale and offset that values to
>> the new range?
>
> Not completely sure to understand what you really mean, but I'd say no scaling
> nor offseting.
> Color table is plain and stupid. You need to provide as many entries in the
> color tables as values in the input file, and the index of the color table must
> match the value of the pixel in the source band. And this is not possible in
> your case since the minimum value is negative (as said in previous related
> discussion), and the color table entries are indexed starting at 0.
>
> --> use LUT
>
>>
>> Below are the gdalinfo for each.
>>
>> Thanks,
>>     -Steve
>>
>> gdalinfo -stats MODIS_SST_EAST-201405190900.tif
>> Driver: GTiff/GeoTIFF
>> Files: MODIS_SST_EAST-201405190900.tif
>>          MODIS_SST_EAST-201405190900.tif.aux.xml
>> Size is 8800, 6600
>> Coordinate System is:
>> GEOGCS["WGS 84",
>>       DATUM["WGS_1984",
>>           SPHEROID["WGS 84",6378137,298.257223563,
>>               AUTHORITY["EPSG","7030"]],
>>           AUTHORITY["EPSG","6326"]],
>>       PRIMEM["Greenwich",0],
>>       UNIT["degree",0.0174532925199433],
>>       AUTHORITY["EPSG","4326"]]
>> Origin = (-125.000000000000000,54.931818190930159)
>> Pixel Size = (0.009090548548341,-0.009058565004476)
>> Metadata:
>>     AREA_OR_POINT=Area
>> Image Structure Metadata:
>>     INTERLEAVE=BAND
>> Corner Coordinates:
>> Upper Left  (-125.0000000,  54.9318182) (125d 0' 0.00"W, 54d55'54.55"N)
>> Lower Left  (-125.0000000,  -4.8547108) (125d 0' 0.00"W,  4d51'16.96"S)
>> Upper Right ( -45.0031728,  54.9318182) ( 45d 0'11.42"W, 54d55'54.55"N)
>> Lower Right ( -45.0031728,  -4.8547108) ( 45d 0'11.42"W,  4d51'16.96"S)
>> Center      ( -85.0015864,  25.0385537) ( 85d 0' 5.71"W, 25d 2'18.79"N)
>> Band 1 Block=8800x1 Type=Int16, ColorInterp=Gray
>>     Min=-10004.000 Max=6035.000
>>     Minimum=-10004.000, Maximum=6035.000, Mean=-673.234, StdDev=268.660
>>     Overviews: 4400x3300, 2200x1650, 1100x825, 550x413, 275x207
>>     Metadata:
>>       STATISTICS_MAXIMUM=6035
>>       STATISTICS_MEAN=-673.23378700069
>>       STATISTICS_MINIMUM=-10004
>>       STATISTICS_STDDEV=268.66013989253
>>
>> gdalinfo -stats -noct MODIS_SST_EAST-201405190900.vrt
>> Driver: VRT/Virtual Raster
>> Files: /maps/wms/data/MODIS_SST_EAST/MODIS_SST_EAST-201405190900.vrt
>>          /maps/wms/data/MODIS_SST_EAST/MODIS_SST_EAST-201405190900.tif
>> Size is 8800, 6600
>> Coordinate System is `'
>> Origin = (-125.000000000000000,54.936579299999998)
>> Pixel Size = (0.009090548056818,-0.009058564515152)
>> Corner Coordinates:
>> Upper Left  (-125.0000000,  54.9365793)
>> Lower Left  (-125.0000000,  -4.8499465)
>> Upper Right ( -45.0031771,  54.9365793)
>> Lower Right ( -45.0031771,  -4.8499465)
>> Center      ( -85.0015886,  25.0433164)
>> Band 1 Block=128x128 Type=Byte, ColorInterp=Palette
>>     Minimum=0.000, Maximum=255.000, Mean=1.885, StdDev=21.797
>>     Metadata:
>>       STATISTICS_MAXIMUM=255
>>       STATISTICS_MEAN=1.8849891356749
>>       STATISTICS_MINIMUM=0
>>       STATISTICS_STDDEV=21.796961177215
>>     Color Table (RGB with 256 entries)
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>