Re: Photoshop's handling of 8- and 16-bit images
Re: Photoshop's handling of 8- and 16-bit images
- Subject: Re: Photoshop's handling of 8- and 16-bit images
- From: email@hidden
- Date: Thu, 23 Aug 2001 17:36:23 EDT
In a message dated 8/23/01 3:58:53 PM, email@hidden writes:
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1) Open 8-bit, Save as 8-bit -- All original values remain intact.
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2) Open 16-bit, Save as 16-bit -- Half the tones are lost.
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All input values are subjected to the following two operations:
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a) scale by factor 32768/65535 and round to nearest integer.
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b) scale above result by 65535/32768 and round to nearest integer.
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The net result of this is that half of the original tone values are lost,
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specifically from 65536 possible tones in the original down to 32769
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possible tones in the output. The output has no odd pixel values for tones
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less than or equal to 32768, and has no even pixels for tones greater than
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32768. You cannot discover this within Photoshop because Photoshop never
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displays the underlying 16-bit pixel values. The output covers the full
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16-bit range, but the histograms are "combed" (i.e. every other value is
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missing). So just opening and saving a 16-bit image in Photoshop
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essentially discards one bit of data.
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3) Open 16-bit, Save as 8-bit -- This operation is performed with a
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noise-driven dither during conversion down to 8-bits. This dithering
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technique helps "hide" banding or posterization effects that may result
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if
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the low 8-bits were simply discarded.
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4) Open 8-bit, Save as 16-bit -- each 8-bit value is copied into both halves
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of the 16-bit value. This 16-bit value is then subjected to the two steps
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described above (2a and 2b). So the low 8-bits of the output are not empty
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as you might think.
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All of the above fits with my previous understanding of Photoshop 8 and 16
bit processing... and all of it seems to be the lesser of evils compared to
the alternatives.
C. David Tobie
Design Cooperative
email@hidden