[FFmpeg-devel] [PATCH] First shot at AVCHD seeking via new seeking API

[Ivan Schreter]

Hi Michael,

Michael Niedermayer wrote:
> On Sun, Aug 30, 2009 at 10:11:21AM +0200, Ivan Schreter wrote:
> [...]
>   
>> BTW, I found out that pure seeking routine time can be reduced about factor 
>> 3 for full-HD AVCHD files, when using higher initial step size (32KB) for 
>> back-off. I will investigate how this step size could be automagically 
>> determined optimally.
>>     
>
> the initial size should be at least ByteIOContext.buffer_size which is 32kb
> by default
>
>   

So I suppose something like this should be more suitable, right?

Index: libavformat/seek.c
===================================================================
--- libavformat/seek.c  (revision 19773)
+++ libavformat/seek.c  (working copy)
@@ -342,8 +342,10 @@

     // Find keyframes in all active streams with timestamp/position 
just before
     // and just after requested timestamp/position.
-    step = 1024;
-    curpos = pos;
+    step = 32768;
+    curpos = pos - step / 2;
+    if (curpos < 0)
+        curpos = 0;
     for (;;) {
         url_fseek(s->pb, curpos, SEEK_SET);
         search_hi_lo_keyframes(s,

Interestingly, for AVCHD files with high resolution, starting with yet 
higher initial step (on the order of 128-256 KB) produces even better 
performance. I suppose, this depends on key frame distance in the file 
and frame sizes. I didn't find a good metrics yet for determining the 
best backoff initial step size. But this can be addressed later as 
performance optimization.

Shall I commit the above as a preliminary optimization? Obviously, 
starting with 1024 bytes backoff is not that good currently.

Thanks & regards,

Ivan