[Openicc] seeking for advice on rendering intent and black point compensation

[Hal V. Engel]

On Wednesday 03 October 2007 09:03:18 Alastair M. Robinson wrote:
> Hi,
snip
>
> As for the default intent for such conversions, I would prefer either
> Perceptual or Relative Colorimetric with BPC *ON* to be the default.  If
> you have an image which is in a "print" colour space, its black point
> will be much lighter than the "pure" black of an additive RGB working
> space like sRGB - so a relative colorimetric transform from a printer
> colour space to a working space will result in a washed out, faded look.
>   That's the effect Black Point Compensation is designed to minimise.
>
> It's also entirely possible that an advanced user would want to choose
> the intent on a per-image basis - one might want to treat an image from
> a scanner differently from one that's ready for printing, for instance,
> so I would be in favour of being able to choose the intent when being
> asked whether to convert.

I agree with this since I do use different intents when doing color space 
conversions depending on the circumstances.  I would feel hamstrung if I 
could not choose the conversion intent.

>
> >>> (2) The rendering intent used for the display color correction can be
> >>> configured in the Preferences dialog. Would it make sense to reuse this
> >>> setting for the conversion when loading an image?
>
> Here I disagree with the consensus so far - conversion from an embedded
> profile to the working space, and conversion from the working space to
> the display profile are two separate operations, and their settings
> shouldn't be tied too closely.
>
> Using the current display settings as a *default* for the conversion
> settings is fine - as long as it can be overridden.
>
> Having to change your *display* settings to force a different intent to
> be used when loading an image is not logical.

I agree with this as well.  There should be a separate conversion intent 
available either as a preferences setting or as part of the conversion dialog 
or both (IE. a default setting in preferences that can be overridden in the 
conversion dialog).

>
> >> Be warned about using relative colorimetric without BPC. Your users will
> >> seldom expect such results.
>
> Agreed - although in the case of different working spaces (where both
> profiles have "pure" black) - AdobeRGB -> sRGB for instance it makes no
> difference, the difference with something like USWebCoatedSWOP (or any
> printer profile for that matter) -> sRGB is very pronounced.
>
> > I increasingly think that user interfaces need to offer _five_ rendering
> > intents, with a strong emphasis placed on the first two shown, eg:
> >
> > Perceptual
> > Relative Colorimetric with BPC
> > -- Rarely Needed --
> > Relative Colorimetric without BPC
> > Absolute Colorimetric
> > Saturation
>
> Agreed, once again.  I already use five-intents rather than
> four-intents-and-a-checkbox in PhotoPrint, and may well take up your
> idea of emphasising Perceptual and RC/BPC.

I also agree since having a BPC check box implies that BPC will do something 
with intents other than relative colorimetric when this is not the case.  
Many users find this confusing.

>
> > The "Use black point compensation" option would then be removed
> > entirely. (Or are there circumstances when it does make sense for other
> > intents?)
>
> As far as I know the BPC flag is simply ignored with intents other than
> Relative Colorimetric - at least with lcms.
>
> All the best,
> --
> Alastair M. Robinson
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