[Libav-user] difference between various h264 codec...

[wm4]

On Sat, 3 May 2014 23:25:31 +1000
Jean-Yves Avenard <jyavenard at gmail.com> wrote:

> Hi..
> 
> Currently updating our vdpau code that was written many moons ago, to
> use the newish hwaccel API instead...
> 
> Previously, we would specifically use the vdpau equivalent codec.
> 
> So for example, say were going to open a h264 video, we would instead
> use and open the "h264_vdpau" codec.
> 
> Since using the hwaccel_context / hwaccel in AVCodecContext, that
> method of searching explicitly the vdpau equivalent codec only works
> with mpeg1/2, h264 and vc1 fail... If I explicitly use the h264_vdpau
> codec, I get lots of errors.

There is no h264_vdpau codec anymore (except maybe the old legacy
implementation that was removed from Libav, but not FFmpeg yet).
Instead, you're supposed to use the hwaccel feature.

This is the defacto-documentation how to use it:

https://git.videolan.org/?p=ffmpeg.git;a=blob;f=ffmpeg_vdpau.c

The trick is that you open the normal h264 codec, but select a vdpau
pixelformat when the get_format callback is used.

> If I simply open the h264 codec as-is; it works, and I can see I have
> a very low CPU usage (eg 5-6% vs 90%). That tells me it all works.
> 
> But in the logs, all entries are with [h264] rather than [h264_vdpau],
> making it hard to know if I'm using vdpau or not, or if it's not
> optimal.
> 
> playing the file with mplayer (updated as-of today), the log do
> contain [h264_vdpau], so obviously they do something rather
> differently.

I don't know what they're doing currently, but they still could be
using the "old" API, or emulating it.

> The main reason I'm asking, is that I'm not 100% sure vdpau is properly used.
> 
> With high-complexity codec like h264 or vc1 it's easy to see if it's
> using vdpau: just look at the CPU usage, the difference being
> significant enough.
> 
> But with things like old wmp3 or mpeg, you can't really tell the
> difference on a modern CPU.

Well, there's nothing magic about the "new" API, it's just the old one
slightly changed. I still have no idea why it was changed at all.

> As I'm trying to prevent any regressions, I wanted to be sure.
> 
> Is it okay to simply open the codec found by default in my stream
> as-is, and FFmpeg will see automatically that hwaccel is set and do
> what needs to be done?
> 
> thanks.
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