[Flac-dev] FLAC__stream_decoder_skip_single_frame() // how is it used?

[Brian Willoughby]

Hello all,

I would like to copy a FLAC file without decoding and then being  
forced to re-encode.  All I really need to do is run the bare minimum  
of the decoder to find FLAC frames and then I should be able to write  
the frames unchanged to the destination file (*).  libFLAC offers the  
FLAC__stream_decoder_skip_single_frame() function to provide half of  
this, but I cannot figure out how to (officially) obtain the encoded  
FLAC frame.  When performing a full decode, the write callback makes  
each frame visible to the application, but that's only after it has  
been decoded, which takes extra CPU time. What I think I need is a  
'skip' callback that just gives the frame header and pointers to the  
encoded data.

Does anyone have any hints?  I'm still looking at the flac command- 
line sources, and also digging into the libFLAC sources to see how  
things work, but I haven't found an obvious solution.  The  
documentation seems to suggest that there are valid reasons to scan  
without decoding, but the only thing that seems possible is to check  
for errors and compute the MD5 sum.  I'd actually like to go further  
than those examples and access the encoded data.

I have a feeling that my question has been asked before, or at least  
a similar topic, but searching my archives back to 2006 doesn't  
reveal anything.

* One potential issue is that the FLAC frame may not be byte-aligned,  
such that perhaps the encoded data can't simply be copied to a new,  
shorter file without shifting bits around.  Then again, perhaps the  
FLAC header is byte-aligned at the start, and only the encoded  
contents are bit-aligned.  I'm sure this particular question is  
answered in the documentation of the file format, but I wanted to  
mention it in case it affects my situation.  I'm certainly planning  
to read the format and API documentation thoroughly.

Brian Willoughby
Sound Consulting

P.S.  My desired application is to take so-called 'hidden' tracks  
from a CD and split them into two files by dropping the silence in  
between.  It's fairly common to have an 'extra' track on a CD that is  
only slightly hidden, where several minutes of silence are inserted  
after the last listed track, followed by the 'hidden' track.  I just  
purchased several lossless music titles online, and a few have these  
hidden tracks.  Of course, I'll preserve the original files in my  
backup, but it would be handy for listening if I could just dump the  
silent sections and keep two shorter tracks - this sure would save  
dead air on my music player.  My idea was that since FLAC already  
encodes silence in a special way (independent, constant.value = 0),  
then it would be far faster and simpler to detect the middle stretch  
of silence by dealing directly with the FLAC-encoded files.  I'm not  
worried about keeping 1 or 2 seconds of silence at either end, as  
would happen when copying whole FLAC frames, but really just want to  
discard most of the long silent sections so I don't have to listen to  
them.

I have code running which can detect the silent sections reliably,  
but only when fully decoding.