[FFmpeg-devel] [RFC] libavfilter audio - af_resample channel conversion

[Michael Niedermayer]

On Wed, Jul 07, 2010 at 04:31:04AM -0700, S.N. Hemanth Meenakshisundaram wrote:
> On 07/04/2010 07:57 AM, Stefano Sabatini wrote:
>> On date Thursday 2010-07-01 01:42:51 -0700, S.N. Hemanth Meenakshisundaram 
>> encoded:
>>    
>>> On 06/25/2010 05:10 PM, Stefano Sabatini wrote:
>>>      
>>>> On date Friday 2010-06-25 03:52:45 -0700, S.N. Hemanth Meenakshisundaram 
>>>> encoded:
>>>>        
>>>>> [...]
>>>>>          
>>>>        
>>> Hi All,
>>>
>>> Here is the working af_resample.c and associated Makefile and
>>> allfilters.c changes.
>>>      
>> [...]
>> All these are duplicated of existing code. I'm not against this if
>> this will get us with a working/testable audio filtering framework,
>> but put at least a FIXME somewhere.
>>
>> Also if someone can suggest a better solution to share this code it
>> would be even better.
>>
>>    
>
>
> Hi All,
>
> Here is an idea for a generalized channel conversion function for 
> af_resample:
>
> We could have a table of gain tables corresponding to various channel 
> conversions as below:
>
> +static const struct {
> +    int64_t in_channel_layout;
> +    int64_t out_channel_layout;
> +    const float gain[8][8];
> +} channel_mixing_table[] = {
> +    {
> +        .in_channel_layout = CH_LAYOUT_5POINT0,
> +        .out_channel_layout = CH_LAYOUT_STEREO,
> +        .gain = {
> +            {1, 0, 0.5, 0.7, 0},
> +            {0, 1, 0.5, 0, 0.7},
> +        },
> +    }
> +};
>
>
>
> The table above currently has a single gain table for 5.0 to stereo 
> downmixing.
>
> We would need to look up an appropriate gain table only when input or 
> output channel layout changes. When these changes occur, we can also fix 
> the conversion function to be called to be one of ten function as below 
> (based on planar/non-planar and sample format).
>
> +/* Generic channel conversion functions in case of short sample format */
> +static void generic_planar_convert_short(short **outputs, short **inputs, 
> float **gains, short in_channels, short out_channels, int samples_nb)
> +{
> +    int i, j, k;
> +
> +    for (j = 0; j < out_channels; j++)
> +        for (k = 0; k < in_channels; k++)
> +            for (i = 0; i < samples_nb; i++)
> +                outputs[j][i] += gains[j][k] * inputs[k][i];
> +}
> +
> +static void generic_packed_convert_short(short **outputs, short **inputs, 
> float **gains, short in_channels, short out_channels, int samples_nb)
> +{
> +    int i, j, k;
> +
> +    for (i = 0; i < samples_nb; i++)
> +        for (j = 0; j < out_channels; j++)
> +            for (k = 0; k < in_channels; k++)
> +                outputs[i][j] += gains[j][k] * inputs[i][k];
> +}
> +
>
> And 4*2 other functions for the other 4 sample formats (planar and 
> non-planar versions for each)
>
> The for loops and multiply-add statements could be factorized as a macro if 
> necessary for concise representation across the 10 functions. Only the 
> order of the loops and the pointer types of data would change between the 
> functions and we would cover all sample formats and any channel conversion 
> for which a gain table of conversions exists.
>
> I am testing this out for a couple of conversions now.
>
> One potential big drawback of this is the large size of the table of gain 
> tables. However, if we add these gain tables as and when necessary, perhaps 
> it is acceptable?

this would need ~272n^2/2 bytes
this seems a bit big to me
also multiplying by 0,1,0.5 is not efficient, but without seeing the whole
table its hard to say which way to store it would be best

also you might want to look at liba52/downmix.c but beware its GPL so you
cant just copy code from there, just design ideas

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

The bravest are surely those who have the clearest vision
of what is before them, glory and danger alike, and yet
notwithstanding go out to meet it. -- Thucydides
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