The driver is probably doing some sort of color management. If you deliver 8
bit image data to the driver, then it will most likely have step sizes at some
point in a ramp that are equivalent of 7 bit data and that should be visible.
With respect to the bit depth between the app and the driver, you would
probably need to take the word of the app that it’s delivering 16 bit data. But
yeah, basically you need to print an 8 bit version and a 16 bit version and
they both should be “16 bit files” to see if you can see a difference. I
wouldn’t print an 8 bit file and a 16 bit file as those might be handled
differently.
On Oct 15, 2018, at 10:04 AM, Jason Campbell <campbell.jj76@xxxxxxxxx> wrote:
I’ve been following this chat the last few posts. I think my question out of
it all is: can one tell whether an app actually delivers 16 bit data to the
driver? I have an Epson P5000 that has a 16 bit driver. Assumedly it was in
response to the target audience (photogs) wanting to push 16 bit precision
into it (e.g. Lightroom, etc). But how does one assess whether the app
itself is printing using 16 bit data or just pushing the same old 8 bit stuff
into a 16 bit box? I assume there is no test that will pull that off short
of looking at source or getting the software author to state it?
From: "argyllcms-bounce@xxxxxxxxxxxxx
<mailto:argyllcms-bounce@xxxxxxxxxxxxx>" <argyllcms-bounce@xxxxxxxxxxxxx
<mailto:argyllcms-bounce@xxxxxxxxxxxxx>> on behalf of Ben Goren
<ben@xxxxxxxxxxxxxxxx <mailto:ben@xxxxxxxxxxxxxxxx>>
Reply-To: "argyllcms@xxxxxxxxxxxxx <mailto:argyllcms@xxxxxxxxxxxxx>"
<argyllcms@xxxxxxxxxxxxx <mailto:argyllcms@xxxxxxxxxxxxx>>
Date: Monday, October 15, 2018 at 12:07 PM
To: "argyllcms@xxxxxxxxxxxxx <mailto:argyllcms@xxxxxxxxxxxxx>"
<argyllcms@xxxxxxxxxxxxx <mailto:argyllcms@xxxxxxxxxxxxx>>
Subject: [argyllcms] Re: testing
On Oct 14, 2018, at 5:12 PM, Graeme Gill <graeme@xxxxxxxxxxxxx
<mailto:graeme@xxxxxxxxxxxxx>> wrote:
* The number of levels the screening process supports, depends
on its details. For fixed screens this can never be more than
the number of dots that make up the screen times the number of
dot sizes/densities (i.e. for a bi-tonal printing process to
support 16 bit, it needs to have 65538 dots, and you need to
average the density over all those dots).
Just to put this in perspective...you need a 256 x 256 dot array to get those
65K dots. If your printer is an ancient 300 DPI LaserWriter, you’re looking
at just barely finer than one pixel per inch resolution to get 16 bit
grayscale. That’s okay if you’re making a freeway billboard, but not for your
passport photo....
A 2400 DPI modern inkjet does rather better, of course...but you’re still not
quite at 10 PPI for the full expression of 16 bits.
So how is it that these printers do so amazingly well in real-world printing?
Black magic, especially in the halftoning / screening algorithms...made
possible in no small part by the multiple ink channels. You might only get 16
bits at 10 PPI with black ink only...but throw in three other neutral inks of
different densities and you can get a lot more. Add in some judicious use of
the other inks, and your fudging is plenty more than “good enough.”
But if you wanted “true” 16-bit printing at 300 PPI with a 3-color ink set,
you’d need a printer that has something on the order of 70,000 DPI — not
gonna happen any time soon, if ever. Fortunately, there’s no need for it — in
no small part because human visual acuity ain’t that great, either.
Think back to that 300 DPI LaserWriter example. Imagine a letter-sized piece
of paper, half of which has one dot per 256x256 pixel, the other half two
dots. Assume a modern-style stochastic halftoning screen so the dots aren’t
laid out in an obvious grid pattern. What’re your chances of being able to
tell which half of the page is lighter, which is darker? And that’s in an
example where the dots are naked-eye visible.
(Again, not to play down the significance of 16-bit color — just to put it in
perspective and to note that it’s not a magic cure-all. It can matter very
much, just not necessarily in all the situations where folk wisdom might
claim it does.)
Also a last bit of perspective: whereas 16 bits needs 65K dots, 8 bits only
needs 256 dots and can be done in a 16-dot grid. Your 300 DPI LaserWriter can
do almost 20 PPI at 8 bits, and your 2400 DPI modern inkjet can do 150 PPI.
And if you’ve got a dozen ink channels to throw at 150 PPI, you’re not all
that far off from a smartphone’s “retina” display — _plus_ you can lay the
dots down at 2400 DPI precision. Put the two together, and....
Cheers,
b&
