Digital BW, The Print

Hi Martin,

> As an example lets say the scan originally came from a 12-bit scanner and
> the 12-bit data was mapped accurately into 16-bit space (each
> 12-bit value X
> 16) so that in 16-bit space there are pixels at intervals of 16 levels or
> tones.

Well, actually that gets me to another point, thanks, but I'll answer this
one first.

What you say, isn't what happens, at least with my experience.  For raw
data, the 12 bit values simply occupy a "portion" of the 16 bit "space".
The other part is, your scan isn't going to occupy the entire 12 bit space
anyway, only a portion of that...

Why I say this, is when I look at my HDR files from my Leaf using the
histogram in PS, they do exactly as I say.  What I then have to do, is set
the setpoints FIRST, which expands the tonal range (disperses it) over the
entire 16 bit range.  Then I apply the tonal curves.

It's the setpoints that are critical....and only after that is done, is the
data expanded over the entire 16 bit range...but remember one little thing,
16 bits in PS, is really only 15 bits...

So, now that that's out of the way...my "other" point that you queued me on,
was what is the disposition of the 16 bit file that the original post was
talking about...was it a file that was "setpointed", because if it wasn't,
it has to be before really doing anything else...

> If you did a strong levels or other adjustment on this 16-bit file
> wouldn't the spacing change so that in some portions of the 16-bit range
> data points are brought closer together

Correct.

> ...even overlapping

That would have to be a VERY drastic tonal move to do that in 16 bit
space...

> giving the
> "spikes" seen in histograms

Yes, that sounds right.

> ...and in others portions of the 16-bit
> range they
> are spread farther apart than intervals of 16. To see a gap in the 16-bit
> file, 65,535 levels, with an 8-bit histogram tool it would have to be 256
> 16-bit levels wide, which sounds like a lot, but I guess you
> could push the
> data that far.

Yes, you could...but that would be one HELL of a move...

> Assuming you had a "perfect", full range, 12-bit scan there would be 4096
> tones in the original 16-bit scan but after manipulation it might
> be reduced
> to a lower number. It is still going to be much more than 256, so when you
> do a mode change to 8-bit wouldn't be likely that in mapping and throwing
> out tones that the gaps get totally or partially filled in so that the
> histogram of the 8-bit version of the file looks smoother?

No.  Mapping to 8 bits should only be a matter of lopping off lower bits.
What PS MAY do is round up or down...sigh.  That would cause the histogram
to change.


> Of course there is the question of whether the Histogram function in
> PhotoShop reads 16-bit and 8-bit files the same way.

Of course.

> What I would love to see on the Histogram display, which does not
> seem like
> it would be difficult, is first the bit depth of the file being
> analyzed and
> a display of the number of discrete tones or levels in the file. This last
> would be a great help in determining the amount of data loss caused by any
> adjustment or action.

Agreed.

I'm not sure we answered any questions, but all I can think of, is the 16 to
8 conversion is not a simple lopping off of bits...

Regards,

Austin