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

[Marti M. <mar...@li...>]

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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