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

[Anthony Thyssen]

On Mon, 02 Feb 2009 07:16:38 -0500 (Eastern Standard Time)
Robert Funnell <robert.funnell at mcgill.ca> wrote:

| 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 left 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
| 
Note that IM will only use the most significate bits if the output image
format is only using 8 bits values.  So you will not need to right shift
them.

That is if the 12 bit value are currently converted to 16 bit values
either by -evaluate LeftShift, or by IM itself. then IM will handle
any further processing correctly.

Try  saving using      -depth 8 output.png
or using      png8:output.png    for a 8bit palette PNG image.


  Anthony Thyssen ( System Programmer )    <A.Thyssen at griffith.edu.au>
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