[FFmpeg-devel] [PATCH] H.264/AVCHD interlaced fixes

[Michael Niedermayer]

On Mon, Feb 16, 2009 at 10:47:00AM +0100, Ivan Schreter wrote:
> Michael Niedermayer wrote:
> > of course ANY method that scans the whole is unacceptable and i mean
> > unacceptable in the sense that no argument will help you, we will
> > not read the whole frame to extract 5 fields from the headers in the
> > first 20 bytes
> >   
> True. So I suppose something like this would suit your requirements:
> 
> +    for(;;){
> +        int src_length, dst_length, consumed, bit_length;
> +        buf = ff_find_start_code(buf, buf_end, &state);
> +        /*for(; buf + 3 < buf_end; buf++){
> +            // This should always succeed in the first iteration.
> +            if(buf[0] == 0 && buf[1] == 0 && buf[2] == 1)
> +                break;
> +        }
> +        buf += 3;*/
> +        if(buf >= buf_end)
> +            break;
> +        --buf;
> +        src_length = buf_end - buf;
> +        switch (buf[0] & 0x1f) {
> +        case NAL_SLICE:
> +        case NAL_IDR_SLICE:
> +            // Do not walk the whole buffer just to decode slice header
> +            if (src_length > 20)
> +                src_length = 20;
> +            break;
> +        }
> +        ptr= ff_h264_decode_nal(h, buf, &dst_length, &consumed, 
> src_length);
> +        if (ptr==NULL || dst_length < 0)
> +            break;
> +        while(ptr[dst_length - 1] == 0 && dst_length > 0)
> +            dst_length--;
> +        bit_length= !dst_length ? 0 : (8*dst_length - 
> ff_h264_decode_rbsp_trailing(h, ptr + dst_length - 1));
> +
> +        init_get_bits(&h->s.gb, ptr, bit_length);
> +        nal = h->nal_unit_type;
> [...]
> +        buf += consumed;
> +    }
> 
> 
> In this case, everything up to slice header must be decoded fully (no 
> way around it, since we need to get to the slice header, but it's 
> normally just peanuts),

i dont see why, ff_find_start_code() should atomatically skip over it
not really better but also not worse and it avoids the special case for
slice/idr

also the whole ff_h264_decode_rbsp_trailing seems unneeded unless i
miss something


> and then for slice itself, we will decode only 
> first 20 bytes or so. I need to check exact size needed, but 20 bytes 
> should be actually enough. The loop terminates at slice header.
> 
> Please let me know, if you are happy with it, so I can update the patch 
> appropriately.

it does look like the correct direction
and thanks for not giving up even though some of the things i might
have said where a little offensive ...

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Democracy is the form of government in which you can choose your dictator
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