[FFmpeg-devel] Optimised math functions in FFMPEG?

[Paul Buxton]

So construct filtergraph with

   1. zscale with a linear transfer function
   2. color correction filter (I am calling it underwater correction, but
   nothing it does is actually specific to underwater images, it is simply a
   grey world based algorithm for whitebalance).
   3. zscale with desired output transfer function (probably BT709).

Is it normal that someone needs to know the characteristics of the filter
to such a degree that they know to apply the pre/post
linearisation/de-linearisation to use it?

Agree filter cannot recover red where none is present, but at recreational
dive depths there is still some red present, and this does get the image
closer to what you 'see' as a diver. I think the bigger problem with this
approach is that for scenes where there is not much other color, it can
wash out the color that is there.
Compared to other algorithms, such as histogram equalisation it seems to
perform pretty well and  without temporal instability which requires
smoothing to correct.

I can throw some comparison videos up on youtube if anyone is
interested (when it finally finishes converting).

I need to look into how the FATE testing stuff works and put together a
test, and make sure everything else fits with coding guidelines before
submittinga patch. But I think I would prefer to send the working floating
point version before working on a performance optimised int varient, unless
that would be against contribution guidelines..

Paul


On Sun, Aug 15, 2021 at 4:09 PM Paul B Mahol <onemda at gmail.com> wrote:

> On Sun, Aug 15, 2021 at 3:43 PM Paul Buxton <
> paulbuxton.mail at googlemail.com>
> wrote:
>
> > Thanks both.
> >
> > I have implemented the algorithm in this paper
> >
> >
> https://www.researchgate.net/publication/275213614_A_New_Color_Correction_Method_for_Underwater_Imaging
> >
> > Which basically converts to an L A B (not CIELAB) colorspace, subtracts
> the
> > average value of the frame from each pixel and converts back.
> >
> > Yes I did put the linearisation /de-linearisation into the filter. Not
> > applying it produces noticeably incorrect results. Is there instead a way
> > to tell the filtergraph that the filter expects and outputs linear data?
> >
>
> There is no automated thing yet, but for converting to/from linear use
> zscale filter.
>
> Also if input is 8bit depth I doubt there can be recovered much if there is
> no red component in video.
> If input is in float that is completely different story....
>
>
> >
> > I can investigate an integer version.
> >
> >
> > On Sun, Aug 15, 2021 at 2:31 PM Paul B Mahol <onemda at gmail.com> wrote:
> >
> > > On Sun, Aug 15, 2021 at 3:27 PM Nicolas George <george at nsup.org>
> wrote:
> > >
> > > > Paul Buxton (12021-08-15):
> > > > > I have my filter working nicely but it is a little slow. I have
> > > > > parallelised it where possible, but it is still a bit heavy. I am
> > > > wondering
> > > > > if there are any optimised implementations/approximations of some
> > math
> > > > > functions built in that I should use instead of the standard
> library
> > > > > implementations.
> > > > > Specifically any of
> > > > > pow()
> > > > > exp()
> > > > > log()
> > > > >
> > > > > I have done a quick grep of the code but can't see any likely
> > > contenders.
> > > >
> > > > I am not aware we have those. The kind of optimization you can do
> > > > depends a lot on your use case. Please give a little details.
> > > >
> > > > If you want to speed your filter up, the best would be to make it
> work
> > > > with integers. It would also allow to make reliable unit tests.
> > > >
> > >
> > >
> > > If you convert to/from linear light inside filter, that is wrong thing
> to
> > > do.
> > >
> > >
> > > >
> > > > Regards,
> > > >
> > > > --
> > > >   Nicolas George
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