Re: Images and TAC values
Re: Images and TAC values
- Subject: Re: Images and TAC values
- From: Mark Stegman <email@hidden>
- Date: Fri, 12 Dec 2014 11:20:54 +1100
Martin/Roger/Mike,
I believe Roger is correct but he needed to elaborate. I stick my neck out
and try.
The Total Area Coverage (used to be Total Ink Weight) is the sum of the
highest dot percentages in each channel of a CMYK file. These are
necessarily customised for a particular printing condition for a host of
reasons related to the specific transfer characteristics that result from
the combination of ink, substrate, press and printing process. For example,
newsprint will support a much lower TAC than sheet-fed coated as it is more
absorbent and uses inks with lower tack and viscosity to cope with the run
speeds. Laying down 'runnier' inks on 'blotting paper' generates far
greater dot gain, or TVI, and is therefore harder to control in the shadows
which soon 'plug up' if an inappropriate coated sheet-fed profile was used.
Just to complicate the dynamic it is possible to generate and use multiple
profiles, with the same TAC, for the same printing condition to produce the
same colour space, or gamut. This is one reason why standards-based
profiles are continually 'refined' - to achieve a more optimum operation
for a given printing condition and its colour space. Using the wrong
profile, with a higher TAC, would result in no greater gamut but it would
certainly result in more poorly rendered detail, especially in the midtone
to shadow region.
This means that the Gamut View in Photoshop, which must be comparing Lab
coordinates as it describes appearance, is telling you that the colours are
out of gamut or beyond the limits of the colour space for a specified
printing condition and profile. It does not tell you if that profile is
appropriate for the specified printing condition and will not care what the
TAC is. It is only concerned with the appearance of the colour.
Mark
On Fri, Dec 12, 2014 at 11:04 AM, Martin Orpen <email@hidden>
wrote:
> On 11 Dec 2014, at 23:36, Roger Breton <email@hidden> wrote:
> >
> > Mind you, one can never go over the TIL built inside a CMYK output
> profile for as long as the image is in RGB space, only at the time of
> conversion does the R=G=B=0 gets mapped to whatever 300% or 320% or
> whatever TIL embodied in an output profile.
>
>
> Which is why it seemed clear that the OP was talking about editing CMYK
> images…
>
>
> > This has nothing to do with gamut warning, Martin. In my humble
> knowledge.
>
> Really?
>
> If you edit a CMYK image which has a TAC of 300% and make it 400%
> Photoshop’s Gamut Warning should show this shouldn’t it?
>
> If you set the soft proof profile to something similar — for example the
> BasICColor version of ISO Coated v2 (300) while your image is in the usual
> ICC version it will “kind of” do it.
>
> 310% TAC is outside of the limits of the profile. Why doesn’t Photoshop’s
> gamut warning show this?
>
> --
> Martin Orpen
> Idea Digital Imaging Ltd
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