[FFmpeg-devel] [PATCH] ALS decoder

[Michael Niedermayer]

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 


[...]
> +/** 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


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

No great genius has ever existed without some touch of madness. -- Aristotle
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