Digital BW, The Print

Thanks for the warning Dan!

I was sceptical, but I did a quick test with a greyscale gradient -- and there
was the gap, just as you said.

Can anyone who understands the innards of Photoshop suggest why it is so?

Peter Marquis-Kyle

----- Original Message -----

From: "Dan Culbertson" <danculb@...>


> Not sure why this happens but I have noticed that some grayscale images that
> I convert to RGB end up with gaps in the shadow end of the luminosity
> histogram immediately after conversion.  Gaps that were not there before
> conversion.   My solution is to convert from grayscale to multichannel,
> create two copies of the original grayscale channel, then convert from
> multichannel to RGB.  If you are using any process that requires you to
> convert your grayscale file to RGB and apply curves, you may want to try
> this additional conversion step - especially if you have noticed any
> posterization happening in the shadows.  Definitely shows up in a print of a
> grayscale gradient converted to RGB with any significant curve manipulation.

on 8/12/01 8:16 AM, Peter Marquis-Kyle at petermk@... wrote:

> Thanks for the warning Dan!
> 
> I was sceptical, but I did a quick test with a greyscale gradient -- and there
> was the gap, just as you said.
> 
> Can anyone who understands the innards of Photoshop suggest why it is so?

you might expect this kind of thing to happen if the manipulation was being
done in the wrong gamma.
-- 
John Brownlow

http://www.pinkheadedbug.com

ICQ: 109343205

Johnny Deadman wrote

> on 8/12/01 8:16 AM, Peter Marquis-Kyle at petermk@... wrote:
>
> > Thanks for the warning Dan!
> >
> > I was sceptical, but I did a quick test with a greyscale gradient -- and
there
> > was the gap, just as you said.
> >
> > Can anyone who understands the innards of Photoshop suggest why it is so?
>
> you might expect this kind of thing to happen if the manipulation was being
> done in the wrong gamma.

Johnny:

This is the quick test I did; why don't you try too (I would like to know what
you see):

1    open a new file (mode: greyscale; contents: white; size: 2000 px square)

2    fill the file with a gradient from black to white

3    look at the histogram (nice and smooth)

4    change the mode to RGB

5    look at the luminosity histogram (I see a spike and a gap near the black
end)

Cheers

Peter Marquis-Kyle

> This is the quick test I did; why don't you try too (I would like to know
> what you see):

I'm not Johnny, but this is what I see (soon to be in the images 
section of the list):
http://www.borderless-photos.com/temp/histos.gif

Two surprises here, first that the grayscale histogram was not 
linear, and second, the expected surprise after conversion.

I've got Pshop set to Europe Prepress Defaults

Tim
--
Tim Spragens
http://www.borderless-photos.com

Tim,

Your Histogram images are now in teh "File" section of the group 
homepage in the folder "Message Related Files" and the file 
description contains the # of your original post.

Very interesting. Looks like the grayscale to RGB conversion in 
Photoshop is not so great! Did you try it in 16-bit?

Martin


--- In DigitalBlackandWhiteThePrint@y..., "Tim Spragens" 
<t.spragens@c...> wrote:
> > This is the quick test I did; why don't you try too (I would like 
to know

> > what you see):
> 
> I'm not Johnny, but this is what I see (soon to be in the images 
> section of the list):
> http://www.borderless-photos.com/temp/histos.gif
> 
> Two surprises here, first that the grayscale histogram was not 
> linear, and second, the expected surprise after conversion.
> 
> I've got Pshop set to Europe Prepress Defaults
> 
> Tim
> --
> Tim Spragens
> http://www.borderless-photos.com

> Tim,
> 
> Your Histogram images are now in teh "File" section of the group
> homepage in the folder "Message Related Files" and the file
> description contains the # of your original post.
> 
> Very interesting. Looks like the grayscale to RGB conversion in
> Photoshop is not so great! Did you try it in 16-bit?
> 
> Martin

While we've got Tim working..... Have you tried a Grayscale>Lab>RGB
conversion, and Dan's multi-channel approach? ;-)

Todd

Tim (and Martin, Peter, Todd),

Dan gave us a pretty good answer in message 170. If you discuss this further, 
based on the - very useful - illustration of the histograms we now have, please 
be very spcific about the relative gammas of the:

-grayscale file you test with
-the gray space you have set up in PS 6 (or the gray settings in ps 5)
-the RGB space you are going to.

Antonis

--- In DigitalBlackandWhiteThePrint@y..., "Tim Spragens" <t.spragens@c...> 
wrote:

> I've got Pshop set to Europe Prepress Defaults
> 
> Tim

Todd,

I did the same thing as Tim, created a new Photoshop grayscale file 
(Gray Gamma 2.2) 2000 x 2000 pixels and filled it edge-to-edge with a 
gradient. I then converted it to LAB mode then to RGB mode (sRGB 
IEC61966-2.1 with a gamma of 2.2).

I repeated converting 8-bit to 16-bit to RGB mode path.

I repeated this using the Multichannel workflow Dan Culbertson 
suggested in Message #621. Grayscale convert to Multichannel mode, 
add 2 duplicate channels of the grayscale convert to RGB mode.

I have posted the histograms for all three workflows in the Files 
sections of the group homepage along with Tim's Grayscale to RGB 
conversion histograms.

It appears that the Multichannel workflow is the only one of the 
three that preserves the histogram intact.

Martin



 --- In DigitalBlackandWhiteThePrint@y..., Todd Flashner 
<tflash@e...> wrote:

> > Tim,
> > 
> > Your Histogram images are now in teh "File" section of the group
> > homepage in the folder "Message Related Files" and the file
> > description contains the # of your original post.
> > 
> > Very interesting. Looks like the grayscale to RGB conversion in
> > Photoshop is not so great! Did you try it in 16-bit?
> > 
> > Martin
> 
> While we've got Tim working..... Have you tried a Grayscale>Lab>RGB
> conversion, and Dan's multi-channel approach? ;-)
> 
> Todd

> It appears that the Multichannel workflow is the only one of the
> three that preserves the histogram intact.
> 
> Martin


Dan knows his bits.

Thanks Martin, Tim and Dan.

Todd

Sorry for the shorthand. Europe Prepress Defaults are:
RGB: Adobe RGB (1998)
CMYK: Euroscale Coated v2
Gray: Dot Gain 15%
Spot: Dot Gain 15%

> Dan gave us a pretty good answer in message 170. If you discuss this
> further, 
> based on the - very useful - illustration of the histograms we now have,
> please 
> be very spcific about the relative gammas of the:
> 
> -grayscale file you test with
> -the gray space you have set up in PS 6 (or the gray settings in ps 5)
> -the RGB space you are going to.
> 
> Antonis
> 


--
Tim Spragens
http://www.borderless-photos.com

> While we've got Tim working..... Have you tried a Grayscale>Lab>RGB
> conversion, and Dan's multi-channel approach? ;-)

I haven't. I went out shooting this afternoon and am now developing 
film. As mentioned later in the thread, Dan's approach would work. 
I am curious what happens going to LAB, though. I'll try a bit later.

Tim

--
Tim Spragens
http://www.borderless-photos.com