[FFmpeg-devel] [PATCH] lavc/hevc Parse SEI_TYPE_MASTERING_DISPLAY_INFO and propagate contents into the AVMasteringDisplayMetadata side data.

[Neil Birkbeck]

I guess png is another example; 100000 is the denominator for the 32-bit
integer:
https://github.com/FFmpeg/FFmpeg/blob/b58cfa616c169c90166938608e7135cdab5820e0/libavcodec/pngenc.c#L290

In the standards, the primaries and white point coords are usually only
referenced in 2-4 significant figures (ISO/IEC 23001-8:2013(E) has them all
in one doc). The display primaries can be different from these, but I don't
think you'd see them measured with any more precision (more precision is
unlikely to matter for this metadata anyway).

It seems that most are in favor of AVRational, so I'll change the avutil
struct float internal fields to rational.





On Mon, Jan 25, 2016 at 1:43 PM, Hendrik Leppkes <h.leppkes at gmail.com>
wrote:

> On Mon, Jan 25, 2016 at 10:37 PM, Michael Niedermayer
> <michael at niedermayer.cc> wrote:
> > On Fri, Jan 22, 2016 at 02:54:21PM -0800, Neil Birkbeck wrote:
> >> Hmm. I don't have a good idea of how likely it is for this conversion to
> >> float (by dividing a constant) to not be bit-exact on different
> >> architectures (compilers?) when there should not be any other math
> >> transforming the metadata (other than the conversion back to the integer
> >> coding for cases like hevc, which for a given architecture is possible
> >> without loss). The fact that this could happen at all does make it
> annoying
> >> in terms of bit-exact test expectations across arch, and this is the
> main
> >> concern, right? (for this type of metadata, it is really a hint to
> >> TVs/algorithms, and some will ignore it altogether)
> >
> > bitexactness is one concern, also theres the issue with what is ideally
> > correct.
> > that is what are the ideal values dictated by various standards
> > that hardware (cammeras, ...) aim at ?
> > are these rational or float or what can represent them better ?
> >
>
> Both HEVC and the HDMI Info Frames use fixed-point integers (the same
> scales too, apparently), I do not know of the formats anything else
> uses.
> Maybe we should be using AVRationals?
>
> I would argue that MKV storing floats is a terrible idea, and someone
> should bonk them over the head and store fixed point as well.
>
> - Hendrik
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