[magick-users] RE: Averaging 8x8 blocks (Wolfgang Hugemann)

[Wolfgang Hugemann]

I've done some testing in the meanwhile, trying to perform this 
operation by the use of 16-bit images. After downloading and installing 
the current Windows 16-bit executable (6.3.5-10 Q16), I stripped the 
operation down to:

convert -depth 16 -size 64x64 xc:grey50 grey.tif
identify -verbose grey.tif >log.txt
convert grey.tif -fx "1/256" x.tif
identify -verbose x.tif >>log.txt
convert x.tif x.tif -compose multiply -composite x2.tif
identify -verbose x2.tif >>log.txt

and got strange results (see below).

Anyone who can throw some light onto this?

Wolfgang Hugemann

========
Logfile (abbreviated):

Image: grey.tif
   Format: TIFF (Tagged Image File Format)
   Class: DirectClass
   Geometry: 64x64+0+0
   Type: Grayscale
   Endianess: MSB
   Colorspace: Gray
   Depth: 9-bit
   Channel depth:
     Gray: 9-bit
   Channel statistics:
     Gray:
       Min: 256 (0.500008)
       Max: 256 (0.500008)
       Mean: 256 (0.500008)
       Standard deviation: 0 (0)
   Histogram:
       4096: (32768,32768,32768) #800080008000 grey50
...
   Tiff:document: grey.tif
   Tiff:rows-per-strip: 64
   Tiff:software: ImageMagick 6.3.5 09/18/07 Q16
...

Image: x.tif
   Format: TIFF (Tagged Image File Format)
   Class: DirectClass
   Geometry: 64x64+0+0
   Type: Grayscale
   Endianess: MSB
   Colorspace: Gray
   Depth: 9-bit
   Channel depth:
     Gray: 9-bit
   Channel statistics:
     Gray:
       Min: 2 (0.00390631)
       Max: 2 (0.00390631)
       Mean: 2 (0.00390631)
       Standard deviation: 0 (0)
   Histogram:
       4096: (  256,  256,  256) #010001000100 
rgb(0.390631%,0.390631%,0.390631%)
...

Image: x2.tif
   Format: TIFF (Tagged Image File Format)
   Class: DirectClass
   Geometry: 64x64+0+0
   Type: Grayscale
   Endianess: MSB
   Colorspace: Gray
   Depth: 16-bit
   Channel depth:
     Gray: 16-bit
   Channel statistics:
     Gray:
       Min: 1 (1.5259e-005)
       Max: 1 (1.5259e-005)
       Mean: 1 (1.5259e-005)
       Standard deviation: 0 (0)
   Histogram:
       4096: (    1,    1,    1) #000100010001 
rgb(0.0015259%,0.0015259%,0.0015259%)
  ...