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

Mon, 19 May 2014 17:23:15 -0700 

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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