Re: [Lcms-user] Missing PCS black point adjustment for perceptual transforms between version-2 and version-4 profiles (and vice versa)?

[<Mar...@li...>]

Hello,

You should activate black point compensation when mixing V4 and V2  
profiles on perceptual or saturation intents.

In some cases, lcms detects the situation and performs BPC  
automatically, for example V2->V4, this is a common case
  of embedded profiles. On others scenarios, like V4->V2, you need to  
explicitly set the BPC flag. This is because some V2 printer profiles  
are broken on black point, so making the detection automatic would  
prevent any operation with those profiles.

Anyway your best option is avoid V2 entirely, those are really  
outdated (even V4 is actually outdated!) and many are buggy.

For lcms keeping some compatibility is on, but this is low in the priorities.

I tried your sample activating BPC and works as expected.
Regards
Marti.

Quoting Maurice Luttmer <mau...@gm...>:

> Hi,
>
> I've been looking through the internals of LCMS2, in particular function
> DefaultICCintents() which converts the array of profiles, intents etc. into
> a cmsPipeline.
>
> But I'm a bit puzzled, since I would expect to see a conversion being
> introduced in case of a transformation with a perceptual rendering intent
> between a version-2 profile and a version-4 profile (or vice versa) in
> order to handle the difference in PCS black points of version-2 and
> version-4 profiles.
>
> In order to verify this, I did a test with a floating point conversion of
> RGB (0, 0, 0) from the version-4 sRGB_v4_ICC_preference.icc profile to the
> version-2 sRGB2014.icc profile (and I multiplied the output values by 255).
>
> If everything is fine, I would expect the result to also have RGB = (0, 0,
> 0).
> If there is a black point mismatch, I would expect the RGB value to
> correspond to the version-4 black point (which has a lightness of about
> 3.1).
> The resulting RGB value (times 255) that I got was (11.311, 11.311,
> 11.311), which indeed corresponds to the lightness of the version-4 PCS
> black point.
>
> This suggests that adjustments between the version-2 and version-4
> perceptual PCS black points (and vice versa) are indeed missing.
> Or am I misunderstanding something?
>
> With kind regards,
> Maurice