Re: [Lcms-user] Question about sRGB and rendering intent

[Aaron B. <bo...@gm...>]

Hi Marti,
Thank you, I will continue to ignore gAMMA and cHRM. I think I can get away
without an ICC profile, as I already indicate sRGB
color space in the image header. My guess is that perceptual was simply
chosen in the original file because
it equals zero, which seems to be a sane default value, even if it isn't :)
Best,
Aaron


On Tue, Sep 14, 2021 at 3:50 PM Marti Maria <mar...@li...>
wrote:

>
> Hi,
>
> My advice: just ignore gAMMA and cHRM chunks, they are just an
> approximation to sRGB space for non color-savvy readers. Embed the standard
> sRGB profile with relative colorimetric intent and this is colorimetrically
> correct and going to work in most situations.
>
> In the embedded profile you may use perceptual or relative colorimetric,
> but this indeed is an advanced topic. Please note that the image is already
> “in-gamut” so the meaning of the intent here is something completely
> different.
>
> if you use “relative colorimetric”: that means all RGB in the image have a
> direct correspondence with Lab. There is no color shifts or movements. The
> profile exactly identifies the Lab of each RGB triplet in a colorimetric
> way.
> if you use “perceptual”: That means the RGB has been tweaked to “look
> good” under whatever device the embedded profile represents. You can use
> the perceptual table A2B0 of the embedded profile to “undo” this preference
> mapping and obtain the original Lab values. Doing so may result in
> different appearance, but maybe close to the original image, whatever
> “original image” may mean.
>
> So, be careful with perceptual, as it can destroy images if you don’t know
> the gory details under the hood.
>
> Best regards
> Marti Maria.
>
>
> > On 14 Sep 2021, at 16:21, Aaron Boxer <bo...@gm...> wrote:
> >
> > Hello!
> > This question is not LCMS specific, but I figured this is a good place
> > to ask a general CMS question:
> >
> > I have a PNG image in sRGB space with sRGB chunk (perceptual rendering
> intent) and also gAMMA and cHRM chunks for gamma and chromaticity. I am
> converting this image to JPEG 2000
> > format, which does not store rendering intent information. When I
> convert, I ignore the gamma and chroma info.
> >
> > Does it make sense to convert perceptual rendering intent into an ICC
> profile for display ? JPEG 2000 is able to store ICC profiles.
> >
> > Any insight here would be greatly appreciated.
> > Thanks,
> > Aaron
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