[FFmpeg-devel] [RFC] Specifying KEYINT (-g) has no effect in libx264

[Baptiste Coudurier]

On 05/24/2011 07:22 AM, Etienne Buira wrote:
> On Mon, May 23, 2011 at 01:32:15AM -0700, Baptiste Coudurier wrote:
>> On 5/22/11 8:33 PM, Michael Niedermayer wrote:
>>> On Sun, May 22, 2011 at 10:40:07PM +0200, Stefano Sabatini wrote:
>>>> Hi,
>>>>
>>>> see issue #157.
>>>>
>>>> The problem was possibly introduced in:
>>>>
>>>> commit 0140d3f0921e5cbb6ea8706acb0307f7ff57a133
>>>> Author: Baptiste Coudurier <baptiste.coudurier at gmail.com>
>>>> Date:   Sat Apr 16 16:50:50 2011 -0700
>>>>
>>>>     In libx264 wrapper, add -preset and -tune options
>>>>
>>>> and depends on the fact that x264_param_default_preset() which is
>>>> called in x264_init(), calls x264_param_default() which defaults the
>>>> already set options.
>>>>
>>>> Indeed order of options matters, with the current mechanism options
>>>> are set in the relevant contexts, and globally processed in the object
>>>> initialization, so the order specified on the commandline is basically
>>>> ignored.
>>>>
>>>> In the case of libx264 we may process profile/preset *before* the
>>>> other options set in the context, so that specific settings will
>>>> override profile/preset information.
>>
>> No, order doesn't matter, as long as a preset is specified, it will
>> override other options.
>>
>>>> This has the problem that most of the times user-set options are
>>>> changed to medium profile or libx264 will complain&exit, see:
>>>>
>>>> thus making fine-tuned user options pointless (since they are changed
>>>> to medium preset and/or will be rejected by libx264).
>>
>> No, you can still specify fine-tuned options without a preset.
>>
>>> I see 3 obvious solutions
>>>
>>> A. is mplayers to just use -x264opts which ive added
>>>
>>> B. keep track of which options have been set by the user and which are
>>>    default.
>>
>> 'B' is how it should be done. It's trivial, just remap "g" in the
>> libx264.c the way weightp was mapped.
> 
> That covers the cli case, but libavcodec users might have surprises.

Well, if libavcodec users want to use preset, or per codec options, they
have to use AVOptions.

> What about:
> x264_param_default()
> apply_avcodeccontext_values(blindly)
> if (preset or tune specified) {
>     x264_param_default_preset()
>     apply_avcodeccontext_values(if_different_than_default)
> }

We want sane defaults and I don't think that will work since you cannot
distinguish between defaults and user specified the default value.

> Also, I think we can drop check_default_settings (which looks like to
> contain a "100" vs "1000" typo when checking bitrate btw).

Indeed.

-- 
Baptiste COUDURIER
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