[magick-users] Converting 16-bit GRAY files to 8 bits

[Robert Funnell]

On Mon, 2 Feb 2009, Anthony Thyssen wrote:

> On Sun, 01 Feb 2009 08:58:36 -0500 (Eastern Standard Time)
> magick-users-bounces at imagemagick.org wrote:
>
> | I am trying to convert 16-bit GRAY files to 8-bit JPEG or PNG format.
> | Since the original images are actually 12-bit, I assume that I need to
> | right-shift each pixel, so I'm using a command like
> |
> | convert -size 512x512+3416 -depth 16 -evaluate RightShift 4 -endian LSB gray:c_vf1169.fre c_vf1169_rs4_lsb.png
> |
> | The effect seems to be that the high-order byte is simply ignored when
> | the right-shift is done. Could someone please explain what I'm doing
> | wrong? I'm using IM 6.2.4 Q16 as distributed by Debian.
> |
> | I apologize if this is really obvious, but I haven't been able to
> | figure it out or find an answer.
> |
> perhaps the 12 bit values in 16 bit integers are already right shifted?

The 12 bits are indeed right shifted in their 16-bit representation. 
What I wanted to do was to right shift the 12 bits into an 8-bit value 
for use in an 8-bit PNG.

In looking into this further in response to your response, I've 
discovered that PNG does support 16-bit greyscale, but I don't want 
that. I've tried some additional manoeuvres but without success, and I 
haven't figured out how to tell IM whether to produce 8-bit or 16-bit 
PNG.

- Robert