Legacy ImageMagick Discussions Archive

Can I use convert to write only the upper (or lower) 8 bits of a 16-bit grayscale TIF? That is, will the following work?
I don't want any normalization to occur. When I go from 16 to 8, how do I know which 8 bits it will use?

Can I use convert to write only the upper (or lower) 8 bits of a 16-bit grayscale TIF?

Yes.

That is, will the following work?

No.

The main difficulty with your approach is that when converting 16-bit=>8-bit, the important bits are the ones at the top, not the bottom. And we can't just strip the bottom bits, because 16-bit=>8-bit(#9ABC) = #99, not #9A.

16-bit=>8-bit(#9ABC) = #99, not #9A

I give up. How do you get #99 from the top bits of #9ABC?

Is there documentation of the rules for converting from 16 to 8 bit?

Sorry, I meant to say 16-bit=>8-bit(#9A00) = #99, not #9A.

Yes, it's documented somewhere. I forget where; you'll have to look around. And it is in the source code, of course.

To get the correct spread of values, when converting from 8-bit to 16, the 8 bits must be replicated. To ensure symmetry, the conversion is the same in the other direction.

16-bit <=> 8-bit
#0000 <=> #00
#0101 <=> #01
#0202 <=> #02
#0303 <=> #03
#0404 <=> #04
:
#9898 <=> #98
#9999 <=> #99
#9A9A <=> #9A
#9B9B <=> #9B
;
#FFFF <=> #FF

You can see that #9A00 is closer to #9999 than to #9A9A, so it converts to #99.

So it looks to me that if I want to force #XXYY to convert to #XX (regardless of YY), I should first modify it to #XXXX.
Do you agree?

Thanks a ton for your assistance!

Yes, I think that's right. This seems to be a general solution (not thoroughly tested, and there may be neater/faster solutions):

I was thinking:

Have you tried it? Does it work?

I tried it. It doesn't seem to work.

I'm not an FX expert, but I think the values of "u" are floating point in the range zero to one. Bit-wise manipulations, and coping with rounding errors, is messy.

If you really must do this with imagemagick you can use this trick. It assumes that you know the image dimensions and that you are running on a platform with working pipes (that is, linux/bsd/osx/cygwin..). If you run this in windows you can create a temporary file instead. (GRAY:foo.raw instead of GRAY:-)

You must replace 2*W and H in the commands with the actual double source width and the real source height. If your source image is 320x240 you would write 640x240.

convert gray.tiff -depth 16 GRAY:- | convert -size 2*WxH -depth 8 GRAY:- -define sample:offset=25x50 -sample 50%x100%\! upper.png
convert gray.tiff -depth 16 GRAY:- | convert -size 2*WxH -depth 8 GRAY:- -define sample:offset=75x50 -sample 50%x100%\! lower.png

Upper and lower depends on the endianess of your machine.

If you're a real ImageMagick pro (Anthony), you may figure out a way to do this in one command, or at least calculate the image size in the second convert from the source image.