gdal_calc : fails with value 0?

Question

I working to invert values of a one-band tif files (download sample).

If i use 254 it works:

$ gdal_calc.py -A ./shadedrelief.tmp.tif  --outfile=whited.tmp.tif  --calc="254*(A>180)" 
0 .. 10 .. 20 .. 30 .. 40 .. 50 .. 60 .. 70 .. 80 .. 90 .. 100 - Done
$ gdallocationinfo -valonly ./whited.tmp.tif 1 1   # we expect 254
254
$ gdal_calc.py -A whited.tmp.tif --outfile=inverted.tif --calc="255-A"
0 .. 10 .. 20 .. 30 .. 40 .. 50 .. 60 .. 70 .. 80 .. 90 .. 100 - Done
$ gdallocationinfo -valonly ./inverted.tif 1 1   # we expect 1 (because 255-A = 255-254)
1 

But the value 255 is troublesome:

$ gdal_calc.py -A ./shadedrelief.tmp.tif  --outfile=whited.tmp.tif  --calc="255*(A>180)" 
0 .. 10 .. 20 .. 30 .. 40 .. 50 .. 60 .. 70 .. 80 .. 90 .. 100 - Done
$ gdallocationinfo -valonly ./whited.tmp.tif 1 1   # we expect 255 
255
$ gdal_calc.py -A whited.tmp.tif --outfile=inverted.tif --calc="255-A"
0 .. 10 .. 20 .. 30 .. 40 .. 50 .. 60 .. 70 .. 80 .. 90 .. 100 - Done
$ gdallocationinfo -valonly ./inverted.tif 1 1   # we expect 0 (because 255-A = 255-255)
255    # <============================ THIS OUTPUT IS NOT EXPECTED.

As far as I know, RGBA range is [0-255]. So...

Why does gdal_calc fails with value 0?

---

Edit: and for some reason, the 254-based whited.tmp.tif only has 2 type of pixels, so end final.tif also only has transparent [255,0] or black [0,255]. I expected a continuous gradation between the both.

Answer 1

try --NodataValue=None, and if it doesn't work, you can change the pixel depth to --type='Int16'

Answer 2

gdalinfo -mm helps to understand, see the 2 last lines:

$ gdalinfo -mm ./shadedrelief.tmp.tif 
Driver: GTiff/GeoTIFF
Files: ./shadedrelief.tmp.tif
Size is 1980, 2011
Coordinate System is `'
Origin = (67.000000000000000,37.500000000000000)
Pixel Size = (0.016161616161616,-0.016161616161616)
Image Structure Metadata:
  INTERLEAVE=BAND
Corner Coordinates:
Upper Left  (  67.0000000,  37.5000000) 
Lower Left  (  67.0000000,   4.9989899) 
Upper Right (  99.0000000,  37.5000000) 
Lower Right (  99.0000000,   4.9989899) 
Center      (  83.0000000,  21.2494949) 
Band 1 Block=1980x4 Type=Byte, ColorInterp=Gray
    Computed Min/Max=1.000,255.000
  NoData Value=0

the value 0 is reserved for NoData pixels. So the range is reduced to [1-255].