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[filmscanners] RE: 8bits vs. 16bits/channel:cantheeyeseethedifference

> From: Roy Harrington
> Did you see my example showing that PS does in fact do more than just
> truncating 16 bits to 8 bits?   PS in a 16 to 8 bit mode change simulates
> intermediate values by dithering exactly like the gradient tool does.
> I don't know whether it's done by adding noise and truncating or a more
> elaborate algorithm but PS does make the average 8 bit gray value keep as
> much of the 16bit data that it can.

How did you get it to do this? Here's the experiment I performed.

1) Since PS doesn't let you create a 16bpc image from scratch or up-convert
an 8bpc image, I opened an existing 16bpc TIFF file.

2) Since PS won't let you fill a 16bpc image, I used Levels to squeeze the
entire dynamic range down to nothing, by setting the Output Levels range to
128 to 128. That gave me a uniform gray.

3) I used Levels again to stretch the dynamic range by setting the Input
Levels range to 126 to 129. This converts the value 128 into 2/3 of full
scale, or 170 on an 8-bit scale, but with some fractional bits.

4) I saved it with no compression, and opened the file in a hex editor. The
16bpc values were all 0xAAA9, proving that it did indeed have fractional

5) I converted the image to 8bpc. Moving the cursor over it still showed a
value of 170 on every pixel.

6) I resaved it, and the hex editor showed the 8bpc values as all being

If it had dithered, I would expect 2/3 of the values to be 171, or 0xAB.


Ciao,               Paul D. DeRocco
Paul                mailto:pderocco@ix.netcom.com

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