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Re: [SG13] Split color into separate proposal

From: JF Bastien <cxx_at_[hidden]>
Date: Fri, 28 Jun 2019 08:57:33 -0700
Thanks for raising this. It's part of the feedback we offered in
http://wg21.link/p1225 and would like to see move forward.

On Fri, Jun 28, 2019 at 4:45 AM Lyberta via SG13 <sg13_at_[hidden]>

> Hi.
> I've recently wanted to work with HDR video and found that most software
> is really broken when it comes to color. Surely, I opened p0267r9 to see
> how the color is handled and was horrified. It is making the same mistake.
> It only specifies color model.
> It is fixed to "float".
> It doesn't specify color space.
> It doesn't specify transfer function.
> It doesn't support internal HDR (values outside of [0, 1] range).
> It forces integers into range [0, 255].
> This makes it pretty useless for professional work and contemporary
> consumer video.
> I think if we don't get color right (and by that I mean not make API
> flexible enough), we'll doom developers into writing their own color
> code and effectively ignoring the standard.
> Curiously enough, p0267 mentions most properties of the color yet
> forgets them when defining the "rgba_color" class.
> So, what are those?
> Color model
> Color space
> Transfer function
> Bit layout
> So, what are the common values of those?
> Color model:
> Color space:
> ITU-R Rec. 709
> ITU-R Rec. 2020
> Transfer functions:
> ITU-R Rec. 709
> sRGB
> ITU-R Rec. 2020
> ITU-R Rec. 2100 PQ (SMPTE ST 2084)
> ITU-R Rec. 2100 HLG (ARIB STD-B67)
> HDR10+ (SMPTE ST 2094-40)
> Bit layouts:
> 8 bit integer per channel (various forms)
> 10 bit integer per channel (HDR10, HDR10+, other)
> 12 bit integer per channel (Rec. 2100)
> 16 bit integer per channel (Adobe Photoshop internally)
> 16 bit float per channel (GPUs)
> 32 bit float per channel (easiest to work with on CPU)
> It's simply impossible to cram all this complexity in a simple type. We
> need a different design.
> It looks like different color models would really need different member
> functions so we can start with a different class template for every
> color model:
> template <BitLayout BL, ColorSpace CS, TransferFunction TF>
> class rgba_color;
> Now we are getting somewhere. Notice that BitLayout, ColorSpace and
> TransferFunction are concepts.
> Now imagine something like this:
> using srgb_rgba_color = rgba_color<f32f32f32f32, rec_709, srgb>;
> This seems to be the closest approximation to rgba_color from p0267.
> Now about mixing integers and floats. I've written an audio library that
> does bit depth conversion automatically. It's all about providing a
> "Sample" type which defines min, max, whether it should clamp and usual
> arithmetic operators. It act as a "smart number" that does the right
> thing. It also takes storage type and calculation type as template
> parameters. It is common to store data as integers but to convert them
> to floating point for the same of processing. Otherwise generic DSP
> template library is impossible. As I live on the bleeding edge, I've
> already conceptified my library to a degree.
> So the list would look like this:
> concept StorageType
> concept CalculationType
> concept Sample
> template std::dsp::sample (I know that std::sample already exists, hence
> bikeshed name, I share the sadness of Guy Davidson over std::vector that
> standard library reserving good named for non math stuff).
> I think we can (and probably should) base audio proposal on
> std::dsp::sample too (and then maybe expose saturation arithmetic that
> is required when working with signed integers) but that for another thread.
> Now we can do this:
> template <std::ForwardIterator I, std::dsp::Gain G>
> constexpr void Amplify(I first, I last, G gain)
> {
> for (; first != last; ++first)
> {
> *first *= gain;
> }
> }
> On integer audio this will do saturation arithmetic, same can be applied
> to ranges of colors (pixels) if they are integers. Or what if iterators
> return video frames? We can define operator*= for video frame that will
> call operator*= on pixels that will fall back onto color that will fall
> back into the sample type that Will Do The Right Thing.
> Now we can write a fade in algorithm that automatically works with audio
> AND video by just using proper types. For audio that requires iterators
> returning frames, that is why I'm very disappointed on the current
> std::audio_buffer design, but again, for another thread.
> --
> SG13 mailing list
> SG13_at_[hidden]
> http://lists.isocpp.org/mailman/listinfo.cgi/sg13

Received on 2019-06-28 10:59:38