Forgive me for butting in, but let me give it a try.
Given I have GeoTiffs in
- Create new folder to store output files (e.g.
- Start OSGeo4W Shell
cd "C:\GIS\Tiff_folder" to move to the input folder.
for %f in (*.tif) do gdal_translate -of GTiff -ot Byte -scale 100 500 1 255 -co BIGTIFF=IF_NEEDED "%f" C:/GIS/MyOutputs/%f
If successful, your converted grey images will appear in newly created
MyOutputs folder, with the same filename as original GeoTiffs.
[EDIT] I have just realized you have .ecw files... so it has to be like below, and you might have to adjust filenames afterwords:
for %f in (*.ecw) do gdal_translate -of GTiff -ot Byte -scale 100 500 1 255 -co BIGTIFF=IF_NEEDED "%f" C:/GIS/MyOutputs/%f.tif
-scale 100 500 1 255 according to your input file. (My example case was DEMs within 100m-500m). As commented by user30184, it is probably difficult part of this task... how best you can define the greyscale range.