[FFmpeg-devel] [PATCH] IFF demuxer and 8SVX decoder

[Vitor Sessak]

Jai Menon wrote:
> On Thursday 27 March 2008 20:00:48 Michael Niedermayer wrote:
>> On Thu, Mar 27, 2008 at 11:48:10PM +0000, Jai Menon wrote:
>>> On Thursday 27 March 2008 15:23:54 Michael Niedermayer wrote:
>>>> On Thu, Mar 27, 2008 at 08:44:54PM +0000, Jai Menon wrote:
>>>>> On Wednesday 26 March 2008 21:12:26 Michael Niedermayer wrote:
>>>>>> uint8_t d = *buf++;
>>>>>>
>>>>>>> +        esc->fib_acc += esc->table[d & 0x0f];
>>>>>>> +        *out_data++ = esc->fib_acc << 8;
>>>>>>> +        esc->fib_acc += esc->table[d >> 4];
>>>>>>> +        *out_data++ = esc->fib_acc << 8;
>>>>>>> +    }
>>>>>> you can do this with one subtraction and 2 shifts less
>>>>> I still don't know how i can eliminate the two shifts?
>>>> change the table ...
>>> I could change it to int16_t, and remove the 2 shifts.....but then i
>>> would need to clip twice before adding the table value to the
>>> accumulator......in which case imho we should stick to 2 shifts.
>> Why would you need to clip?
>  Thats because the encoding scheme requires adding an 8 bit signed value (from 
> the table) to fib_acc. So if we change the table size to int16_t , we could 
> do away with the shifts but the value can't be directly added to fib_acc 
> without clipping. the av_clip macro iirc uses comparisons. you could actually 
> try making the change and running it on the compressed sample Dennis posted 
> earlier on the list. Instead of the sound sample, you will get a highly 
> distorted waveform which sounds nothing like the original sample.
> 
>> +    for(;buf_size>0;buf_size--) {
>> +        uint8_t d = *buf++;
>> +        esc->fib_acc += esc->table[d & 0x0f];
>> +        *out_data++ = esc->fib_acc << 8;
>> +        esc->fib_acc += esc->table[d >> 4];
>> +        *out_data++ = esc->fib_acc << 8;
>> +    }
> 
>>  this still does a unneeded subtraction besides the shifts
> where? is it buf_size-- ?
> 
> 
> other stuff should be fixed by the patch


Well, I've already asked once, but I'll ask again:

> +/**
> + * @file 8svx.c
> + * 8svx audio decoder
> + * @author Jaikrishnan Menon
> + * supports: fibonacci delta encoding
> + *         : exponential encoding
> + */
> +
> +#include <stdio.h>
> +#include <stdlib.h>

Why are the two includes needed?

-Vitor