Sorry for responding so slowly. I have been out of office for some days.
1. To get the 8x8 downsized block average of the pixel values in an
image do the following:
convert image.jpg -filter box -resize 128x96 average.jpg
This works fine and on JPEGs with arbitrary size when using 12.5% as
Anthony proposed.
2 To get the variance you can use the formula:
x=image pixel value
var = ave(x^2) - (ave(x))^2
As you have indicated, this approach does not work, due to clipping.
This may get solved if I could somehow tell IM to use 16bit color depth
when performing this operation.
I presume that MIFF is capable of storing 16bit color depth, as well as
TIFF. So it is probably recommendable to store the intermediate results
either as MIFF or TIFF.
But I couldn't convince IM to use 16 bit color depth. I tried to apply
the option "-storage-type long" but it wasn't recognized.
Any ideas on the solution? BTW: How do I find out about the maximum
color depth that can be handelt by my IM version?
Greetings from Germany
Wolfgang Hugemann
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