[x264-devel] Low resolution reference pictures

[saritha reddy]

Thank you for your reply, Jason.

On Tue, Mar 27, 2012 at 6:34 PM, Jason Garrett-Glaser <jason at x264.com>wrote:

> On Tue, Mar 27, 2012 at 6:09 PM, saritha reddy <saritha84 at gmail.com>
> wrote:
> > Hi,
> >
> > I've just started tracing the x264 source code. I'm still trying to
> > understand the complete code flow. I see that a reference picture for a
> > given current picture is divided into four low resolution luma pictures
> > using an interpolation technique. How is it fetched or handled in inter
> > prediction? Is it not easier to perform block matching search method on a
> > full resolution picture? What's the benefit of having them as four
> separate
> > low resolution pictures?
>
> They're not low resolution, they're full resolution.  Note that there
> *are* lowres planes, but those are for the lookahead (which I don't
> think is what you're talking about) and not in the same place.
>
> The planes are F, H, V, and C.  F is the full-pixel plane, that is,
> the actual image.  H, V, and C are the pre-calculated hpel planes.
> qpel is done by linear interpolation between pairs of planes.  hpel is
> precalculated because the hpel pixels get referenced many times over
> during subpixel motion search and the 6-tap hpel filter is slow,
> making caching it a good memory/speed tradeoff.
>
> #x264dev on Freenode IRC is open if you want more real-time answers to
> questions about the source; we pretty much always have time to help
> anyone learn if they want to.  I also recommend looking at
> http://akuvian.org/src/x264/overview_x264_v8_5.pdf , which albeit
> quite outdated, is still a good rough guide to the structure of x264.
>
> Jason
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