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 13:36:40 +1100
Martin,
I'm not sure what you mean by "trivial". I guess what I am saying is that you could, in theory, create a profile with 400% TAC and still be within gamut for a specific printing condition however, protocol prevents me from saying how it would turn out on any conventional printing process. I would like to emphasise that TAC should be specified during profile creation. By simply changing it in the image with the ISO Coated v2 profile I would say that you are, in effect, editing the profile which will no longer be a valid representation of the printing condition. This is only going to produce Lab values which are 'extrapolated' from the profile. It doesn't tell you anything about the reality of final printed product and its appearance.
As for gamut view, I would like to qualify what I have said already. If you have a profile assigned that is a valid representation of the printing condition you are targeting, whatever the TAC may be, gamut view should give you a fairly accurate preview of the end result ( even though there are still problems with the rendering of shadows on screen as you have already noted in another discussion). You still have to remember that TAC is all about shadows where the maximum values for each channel is present. In other words, 4-colour blacks. Having Black Point Compensation applied is probably more important for rendering shadows correctly ( depending on the Rendering Intent). TAC doesn't tell you much about clean saturated colours and everything else in between which is where differences in gamut are most apparent.
Mark
> On 12 Dec 2014, at 11:40 am, Martin Orpen <email@hidden> wrote:
>
>> On 12 Dec 2014, at 00:20, Mark Stegman <email@hidden> wrote:
>>
>> 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.
>
>
> But this would be trivial for Adobe…
>
> Almost as trivial as them making the Desaturate command (shift-command-U) work in CMYK — so I won’t hold my breath :-)
>
> If I feed CMYK values into ISO Coated v2 (300) I can get Lab values back.
>
> At 300% TAC the L values are around 9.8.
>
> At 400% TAC you get L values of 8.7 — which aren’t achievable, but they are still returned by the profile.
>
> Why can’t anything lower than the L value at the TAC limit be given a gamut warning?
>
> --
> Martin Orpen
> Idea Digital Imaging Ltd
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