[FFmpeg-devel] [PATCH] ALS decoder

[Thilo Borgmann]

Michael Niedermayer schrieb:
> On Thu, Sep 24, 2009 at 05:03:14PM +0200, Thilo Borgmann wrote:
>> Revision 23 attached.
> 
> [...]
>> +/** Reformat block sizes from log2 format to direct form. Also assure that the
>> + *  block sizes of the last frame correspond to the actual number of samples.
>> + */
>> +static void reconstruct_block_sizes(ALSDecContext *ctx, uint32_t *div_blocks)
>> +{
>> +    unsigned int b;
>> +
>> +    // The last frame may have an overdetermined block structure given in
>> +    // the bitstream. In that case the defined block structure would need
>> +    // more samples than available to be consistent.
>> +    // The block structure is actually used but the block sizes are adapted
>> +    // to fit the actual number of available samples.
>> +    // Example: 5 samples, 2nd level block sizes: 2 2 2 2.
>> +    // This results in the actual block sizes:    2 2 1 0.
>> +    // This is not specified in 14496-3 but actually done by the reference
>> +    // codec RM22 revision 2.
>> +    // This appears to happen in case of an odd number of samples in the last
>> +    // frame which is actually not allowed by the block length switching part
>> +    // of 14496-3.
>> +    // The ALS conformance files feature an odd number of samples in the last
>> +    // frame.
>> +
>> +    for (b = 0; b < ctx->num_blocks; b++)
>> +        div_blocks[b] = ctx->sconf.frame_length >> div_blocks[b];
>> +
>> +    if (ctx->cur_frame_length == ctx->last_frame_length) {
> 
> if(ctx->cur_frame_length != ctx->sconf.frame_length) {
> and last_frame_length becomes removeable 
> 

Yes.

> 
> [...]
>> +/** Decodes an ALS frame.
>> + */
>> +static int decode_frame(AVCodecContext *avctx,
>> +                        void *data, int *data_size,
>> +                        AVPacket *avpkt)
>> +{
>> +    ALSDecContext *ctx       = avctx->priv_data;
>> +    ALSSpecificConfig *sconf = &ctx->sconf;
>> +    const uint8_t *buffer    = avpkt->data;
>> +    int buffer_size          = avpkt->size;
>> +    int invalid_frame, size;
>> +    unsigned int c, sample, ra_frame, bytes_read, shift;
>> +
>> +    init_get_bits(&ctx->gb, buffer, buffer_size * 8);
>> +
>> +    // In the case that the distance between random access frames is set to zero
>> +    // (sconf->ra_distance == 0) no frame is treated as a random access frame.
>> +    // For the first frame, if prediction is used, all samples used from the
>> +    // previous frame are assumed to be zero.
>> +    ra_frame = sconf->ra_distance && !(ctx->frame_id % sconf->ra_distance);
>> +
> 
> 
>> +    // the last frame to decode might have a different length
>> +    if (ctx->num_frames && ctx->num_frames - 1 == ctx->frame_id)
>> +        ctx->cur_frame_length = ctx->last_frame_length;
> 
> ctx->cur_frame_length= FFMIN(samples - ctx->frame_id*(uint64_t)sconf->frame_length, sconf->frame_length);
> and ctx->num_frames becomes unneeded and can be removed

Isn't this way slower than before?
Basically yes, I would suggest:

if (...)
    ctx->cur_frame_length = samples -
ctx->frame_id*(uint64_t)sconf->frame_length;

which gets rid of ctx->last_frame_length but should still be faster for
all but the last frame?

Thanks!

-Thilo