[FFmpeg-devel] [PATCH] Rl2 Video decoder

[Michael Niedermayer]

On Sun, Mar 16, 2008 at 05:44:21PM +0100, Sascha Sommer wrote:
[...]

> +/**
> + * Run Length Decode a single 320x200 frame
> + * @param s rl2 context
> + * @param buf input buffer
> + * @param size input buffer size
> + * @param out ouput buffer
> + * @param stride stride of the output buffer
> + */
> +static void rl2_rle_decode(Rl2Context *s,const unsigned char* buf,int size,
> +                               unsigned char* out,int stride){
> +     int row_inc = stride - s->avctx->width;
> +     /* calculate additional video_base offset due to stride != width */
> +     int offset = (s->video_base / s->avctx->width) * row_inc;
> +     int frame_size = s->avctx->width * s->avctx->height - offset;
> +     int pos_in = 0;
> +     int pos_out = s->video_base;
> +     unsigned char* back_frame = s->back_frame - s->video_base;
> +     out += offset;
> +
> +     while(pos_in < size){
> +         unsigned char tag = buf[pos_in++];
> +         int len = 1;
> +         unsigned char val = 0;
> +         int i;
> +         if(tag >= 0x80){
> +             if(pos_in >= size)
> +                 break;
> +             len = buf[pos_in++];
> +             if(!len)
> +                 break;
> +         }
> +         if(s->back_frame)
> +             val = tag | 0x80;
> +         else
> +             val = tag & ~0x80;
> +
> +         for(i=0;i<len && pos_out < frame_size; i++){
> +             if(s->back_frame && (!tag || tag == 0x80))
> +                 val = back_frame[pos_out];
> +             out[pos_out] = val;
> +             ++pos_out;
> +             if(!(pos_out % s->avctx->width))
> +                  out += row_inc;
> +         }

modulo is as division a very slow operation so it should be avoided
and yes the 0x80 was what i meant
But there are still a few simplifcations possible
Theres are several  redundant variables, there are some checks which are
redundant as well
And i would check len against the remaining size instead of checking
against both len and the size in every iteration.
Then i also would use pointers like in / in_end instead of buf/pos_in/size
it avoids the buf+pos_in addition.

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

Republics decline into democracies and democracies degenerate into
despotisms. -- Aristotle
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