[FFmpeg-devel] [PATCH/RFC] Remove triplication of compute_lpc_coefs() function

[Vitor Sessak]

Michael Niedermayer wrote:
> On Thu, Aug 28, 2008 at 02:28:50PM +0200, Vitor Sessak wrote:
>> Hi all.
>>
>> I've finally found a more or less clean way of doing $subj. I also want to 
> 
> going back to this patch ...
> 
> after some tests, ive not been able to find a compute_lpc_coefs() with floats
> that results in acceptable compression
> btw, my tests gave:
> 27063400
> vs.
> 27731934
> which is a little worse than the 0.4% quoted ... (so much for the, it doesnt
> matter)
> 
> using http://www.gnu.org/fun/jokes/eternal-flame.ogg
> 
> so lets see if we can improve this patch ...
> 
> [...]
> 
>> +    if ((autoc[max_order] == 0 || autoc[0] <= 0) && fail)
>> +        return -1;
> 
> fail should be first

done (but I wonder if gcc would be so stupid not to optimize this out in 
an inline function)

> 
>> +
>> +    for(i=0; i<max_order; i++) lpc_tmp[i] = 0;
>> +    err = autoc[0];
>> +
>> +    for(i=0; i<max_order; i++) {
>> +        if (normalize) {
>> +            r = -autoc[i+1];
>> +            for(j=0; j<i; j++) {
>> +                r -= lpc_tmp[j] * autoc[i-j];
>> +            }
>> +            r /= err;
>> +        } else
>> +            r = -autoc[i];
>> +
>> +        if (ref)
>> +            ref[i] = fabs(r);
>> +
>> +        err *= 1.0 - (r * r);
>> +
>> +        i2 = (i >> 1);
>> +        lpc_tmp[i] = r;
>> +        for(j=0; j<i2; j++) {
>> +            tmp = lpc_tmp[j];
>> +            lpc_tmp[j] += r * lpc_tmp[i-1-j];
>> +            lpc_tmp[i-1-j] += r * tmp;
>> +        }
>> +        if(i & 1) {
>> +            lpc_tmp[j] += lpc_tmp[j] * r;
>> +        }
>> +
> 
>> +        if (lpc)
>> +            for(j=0; j<=i; j++) {
>> +                lpc[i][j] = -lpc_tmp[j];
>> +            }
> 
> is this really needed?
> cant a simple
> lpc+= stride;
> be used to decide if space gets reused or all intermediates get exported?

It could, but I won't make the code simpler. lpc_tmp (which is not a 
temporary var, actually) will be modified in the next loop iteration, so 
one have to do some memcpy'ing anyway. (Yes, I plan to rename lpc_tmp 
after this patch is commited).

About the minus operation, it'll require changes to both aac and ra288...

>> +
>> +        if (err < 0 && fail)
>> +            return -1;
> 
> fail should be first

done.

New patch attached. I've also diff'ed it against lpc.c so you can see 
what actually changed in the function.

-Vitor
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