Digital BW, The Print

> > Why not?  You SHOULD be seeing only the top 8 significant bits of the
> image file.  What else would it do?
>
> You are seeing 65000 levels represented as 256.

Yes, and just how do you think it does that?  It chops off the lower 8 bits
of the data values then counts the number of pixels that have each value and
displays them.  How on earth else does it do it if not that way?  Any other
way is useless, except if it does some rounding, instead of simply chopping
off the low 8 bits.

> As I said, if Photoshop
> could draw a black line in the Histogram for every level, you'd
> need a display
> that is about 30 feet long!

Only if every leaves was individually represented, and it is not.

> With the current Histogram, you are
> viewing 256
> steps (they may be black, they may be white) but one level is
> drawn in that
> small GUI. So when you work with high bit files, the Histogram
> isn't "accurate"
> because thousands of levels are being represented as one level.

So what?  The conversion from 16 bit to 8 bit should be done the EXACT same
way as it calculates the 16 bit file to display an 8 bit histogram.

> > I don't understand how that follows.  Unless either the 16 bit histogram
> > does not simply use the top 8 bits, or the conversion from 16 to 8 bits
> > somehow does some "processing", and not simply uses the top 8 bits, then
> > they should be the same.
>
> All the additional data is there, you are just not seeing it.

What additional data?  If you convert the 16 bit file to 8 bits, there is no
"additional data".  The histogram doesn't simply take 256 points from 256
values in the 16 bit file, it does something with the data so ALL the levels
are represented, though 256 levels are combined for each level displayed.

> High bit files are like a stair case.

Huh?  They are no different than 8 bit files, just more tones.

> An 8 bit file and a 16 bit
> file can be the same
> "length". But the 8 bit file has 256 steps to get from black to
> white while the 16
> bit stair case has 64500 (plus or minus).

Yes, I understand that, I design digital imaging equipment for a living and
have been doing so for 25 years.  You simply aren't understanding what it is
I'm saying.

> > Why?  Specifically.
>
> You have more steps of data so as you move the levels, curves
> whatever, and
> then you drop down to 8 bits, Photoshop uses the BEST steps to compress
> the data down to 8 bits pre color. It uses the 645000 steps to
> end up with the
> best 256.

If I understand what you are trying to say, it's completely wrong.  How 16
bits is converted to 8 bits, is one of two ways, and both are equally as
valid.  Either the lower 8 bits (or 7 bits in PS case, as it only stores
high bit data as 15 bits) are simply "dropped" for example
1011_0010_1101_0100 becomes 1011_0010.  The other way to do it is round
up/down based on the lower N bits.  If they are greater than 1/2 the cut-off
value then you merely use the upper bits, if they are lower, then you remove
the lower bits, and subtract 1 from the upper bits you have left.

In no way is there any "uses the BEST steps to compress the data down".  If
it in fact does, that's foolish and meaningless.  How does IT know what 256
values you want?

Austin