Does gdal_calc.py not calculate arithmetic with in the same file?

Question

I have one GeoTIFF file with 2 bands. The data type is Byte. I want to calculate (Band2-Band1)/(Band2+Band1).

But the following command provided a dark image:

gdal_calc.py -A infile --A_band 1 -B infile, --B_band 0 --outfile outfile --calc "(A-B)/(A+B)" 

Does gdal_calc.py not calculate arithmetic with in the same file?

EDIT:

I run the following code using python:

import subprocess
gcalc = 'C:\\Users\\cleo\\Documents\\gdalpys\\gdal_calc.py'

a = 'D:\\a.tif'
b = 'D:\\b.tif'
outfile = 'D:\\result.tif'
expr = '(A-B)/(A+B)'
subprocess.call([sys.executable,gcalc,'--type','Float32','-A',a,'--A_band','1','-B',b,'--B_band','0','--outfile',outfile,\
    '--calc',expr],shell=True)  
a,b,outfile = None, None,None

gdalinfo -stats is as follows:

Driver: GTiff/GeoTIFF

Files: D:\tt.tif

Size is 36001, 36001

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 = (-0.000138888888898,0.000138888888884)

Pixel Size = (0.000277777777778,-0.000277777777778)

Metadata:

  AREA_OR_POINT=Area

Image Structure Metadata:

  INTERLEAVE=BAND

Corner Coordinates:

Upper Left  (  -0.0001389,   0.0001389) (  0d 0' 0.50"W,  0d 0' 0.50"N)

Lower Left  (  -0.0001389, -10.0001389) (  0d 0' 0.50"W, 10d 0' 0.50"S)

Upper Right (  10.0001389,   0.0001389) ( 10d 0' 0.50"E,  0d 0' 0.50"N)

Lower Right (  10.0001389, -10.0001389) ( 10d 0' 0.50"E, 10d 0' 0.50"S)

Center      (   5.0000000,  -5.0000000) (  5d 0' 0.00"E,  5d 0' 0.00"S)

Band 1 Block=36001x1 Type=Float32, ColorInterp=Gray

  NoData Value=1.175494351e-038

Answer 1

This is an old question, but I've just run into what I think is a similar problem dividing two integer grids to get a percentage. The key for me was to convert them to float grids in the calculation. Just setting the output type didn't help. I am running GDAL 1.11.2. Example code is:

gdal_calc.py \
-A "_t2.tiff" \
-B "_t3.tiff" \
--calc="(A.astype(float)/B.astype(float))*100.0" \
--overwrite \
--type Float32 \
--outfile="_t4.tiff" 

A.astype(float) is numpy syntax to cast the integer grids to floats.

Answer 2

The output data data type defaults to the input data type which is Byte in your case. Your output needs to be floating point as all the values will be between -1 and 1. When the values get written to your output byte raster, they get truncated.

Use the --type option to set the output datatype.

gdal_calc.py --type=Float32 -A infile --A_band=1 -B infile, --B_band=0 --outfile=outfile --calc="(A-B)/(A+B)" 

The statistics of the output file should look something like this:

gdalinfo -stats outfile.tif
Driver: GTiff/GeoTIFF
Files: outfile.tif
       outfile.tif.aux.xml
<snip> coordinate system stuff </snip>
Band 1 Block=36001x1 Type=Float32, ColorInterp=Gray
  Min=-1.000 Max=1.000 
  Minimum=-1.000, Maximum=1.000, Mean=0.000, StdDev=0.480
  NoData Value=255
  Metadata:
    STATISTICS_MAXIMUM=1
    STATISTICS_MEAN=0.00044260699339504
    STATISTICS_MINIMUM=-1
    STATISTICS_STDDEV=0.47959178176737