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dcouzin
Participating Frequently
August 3, 2009
Answered

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

  • August 3, 2009
  • 3 replies
  • 118528 views

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

. 

    Correct answer Gyno-jiz

    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?


    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

    3 replies

    Chris Cox
    Chris Cox
    Legend
    August 4, 2009

    Just turn off dithering in color settings.

    D
    dcouzinAuthor
    Participating Frequently
    August 4, 2009

    Right, this was the solution from XXX mentioned above.

    D
    dcouzinAuthor
    Participating Frequently
    August 3, 2009

    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

    D
    dcouzinAuthor
    Participating Frequently
    August 3, 2009

    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.

    G
    Gyno-jiz
    Inspiring
    August 3, 2009

    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.

    D
    dcouzinAuthor
    Participating Frequently
    August 3, 2009

    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?

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