how to convert 16-bit to 8-bit without dithering

[dcouzin]

I have 16-bit RGB tiffs which I must convert to 8-bit Windows bitmaps.

When I do Image > Mode > 8 Bits/Channel, Photoshop introduces dithering.  It doesn't ask.  It just does it.

Of course Photoshop dithers to try to simulate the fine 0-32768 of the 16-bit tiff with with the coarse 0-255 of the 8-bit bitmap.

But the nature of my pictures don't allow dithering.  It can't have pixels that are identical to neighbor pixels become unidentical.

So I want the simplest kind of 16-bit to 8-bit conversion -- pixel-by-pixel -- each 16-bit level getting replaced with the nearest 8-bit level.

If there's no way to turn off dithering in Image > Mode > 8 Bits/Channel is there some other route to the needed 8-bit bitmaps?

Thanks

. 

[dcouzin]

The following is nearly a workaround:

1. In Image > Image Size choose nearest neighbor interpolation and then add a zero after the width and height numbers.

2. Do Image > Mode > 8 Bits/Channel.

3. In Image > Image Size choose bilinear interpolation and then remove a zero after the width and height numbers.

There are still traces of dithering. This could be improved by multiplying the dimensions by 20 instead of by 10, but for my image size 2000x1500 I doubt my system could handle that.

Still looking for a real solution.

[Gyno-jiz]

When I copy from a 16-bit doc to an 8-bit doc, the dithering seems to disappear.

Save for web also seems to avoid dithering on conversion to 8-bit.

[dcouzin]

J Maloney, I'm sorry I don't understand your suggestions.  What is it to "copy from a 16-bit doc to an 8-bit doc"?  My Photoshop abilities are very limited.

Also, where in Photoshop CS4 v.11.0 do I find "Save for web" and what else does "Save for web" do to an image?

[Gyno-jiz · Accepted solution]

Create a new 8-bit doc and copy the file from your 16-bit original to the new bit doc using copy (or copy merged) and paste.

Save for web strips almost everything but image data from the file. Probably best to copy/paste if you need dithering for color space conversions, or do as suggested and change the color settings.

Save for web (and devices) is in the file menu.

http://help.adobe.com/en_US/Photoshop/11.0/WS9E2C7F1A-87C0-4dae-9C0C-0C2B3C566F84.html

[dcouzin]

Thanks J Maloney,

The copy-paste method nicely avoids dithering.

I'll study your second method too.

[dcouzin]

Someone answered me privately with the solution:

   To: dcouzin

   From: XXX

   Sent: Aug 3, 2009

   Subject: Private Message: No dithering wanted

   What happens if you choose in Color Settings

   No Dithering for Images (8 bit) ?

   Best regards --XXX

That's it!

Thank you XXX

[Chris Cox]

Just turn off dithering in color settings.

[dcouzin]

Right, this was the solution from XXX mentioned above.