Re: ISO 21496‐1:20XX Gain Maps

Thanks for putting that discussion on the public record, Rich.

On 2023-11-15 13:24, Richard Geldreich wrote:
> (I sent this earlier to Chris, without CC'ing the PNG Working Group.)
>
> >Gain Maps are an (allegedly) efficient way to provide an SDR baseline
> image plus an HDR alternate image. The claim of storage efficiency has
> not been backed up by any measurements that I have seen.
>
> FWIW: From a glTF, real time rendering, video game, and browser 
> texturing perspective, unless Gain Maps /are 100% lossless/ with 
> respect to the interchange of IEEE 754-2008 half float pixels  (FP16), 
> this isn't a compelling solution for our use cases. We need no loss 
> whatsoever for denormals, all valid normals, signed values, all valid 
> 5-bit exponents, NaN's, and Inf's.
>
> PNG is a lossless format, and we need the ability to losslessly 
> interchange half float source textures and images while preserving 
> compatibility with existing 16-bit PNG software. We also need the 
> ability to store half float data in any colorspace, or even in no 
> colorspace: such as normal maps, which the use of SDR PNG's is 
> pervasive, or height maps, etc. (Both normal and height maps are 
> commonly viewable SDR PNG content right now.)
>
> I searched this doc for "half" and found nothing. The only mention of 
> quality was with respect to the SDR image. Gain Maps also look more 
> complex vs. just storing remapped half values in the PNG 
> file, involving a resample stage for example. The source data for this 
> approach (the "Image File" in this diagram from the doc below) would 
> be an HDR image, /even a "haLf" float PNG image/.
> image.png

-- 
Chris Lilley
@svgeesus
Technical Director @ W3C
W3C Strategy Team, Core Web Design
W3C Architecture & Technology Team, Core Web & Media