[Ffmpeg-devel] TIFF LZW decoder (was: [PATCH] change gif demuxer to gif decoder)

Michael Niedermayer michaelni
Wed Oct 25 13:11:15 CEST 2006


On Wed, Oct 25, 2006 at 07:34:18AM +0300, Kostya wrote:
> On Tue, Oct 24, 2006 at 12:51:02PM +0200, Michael Niedermayer wrote:
> > Hi
> > 
> > On Tue, Oct 24, 2006 at 12:24:15PM +0300, Kostya wrote:
> > > On Tue, Oct 24, 2006 at 07:35:01AM +0300, Kostya wrote:
> > > > 
> > > > As I understand, it may not need the wrapper, just two additional fields in
> > > > structure: GetCode() pointer and dict_limit (when dictionary size reaches it,
> > > > it should be expanded). I also tend to pass buffer and size to the structure
> > > > instead of using bytestream.
> > > > 
> > > > I intend to present this scheme to the end of week.
> > >  
> > > Here it is. 
> > 
> > i think that the LZWState struct should not be in the "public"
> > header for lzw, but rather in either lzw.c (or lzw_internal.h)
> > a "typedef void LZWState;" in lzw.h should be enough ...
> I think that would cause unneeded complexity (decoders should know
> the size of LZWState anyway).

the problem with knowing the size is that code which uses lzw becomes 
dependant on the specific size, for libavcodec it doesnt matter much, just
2-3 files more which need to be compiled if the size changes but if lzw would
be used from outside libavcodec (or libavutil if we move it there) then
we couldnt change LZWState in any way without breaking binary compatibility

iam not sure is there some interrest in having lzw in libavutil and useable
from outside? if no then id accept the LZWState in lzw.h even though i dont
think its good design, its not like 1 pointer dereference per compressed
block would have a meassureable impact on speed or that 1 av_malloc in
ff_lzw_decode_init() and a av_free() call at the end would be alot of
complexity (all IMHO)

and about complexity ....
id suggest to replace the get_code function pointer by a simple

not only is it simpler but it also should be faster as it can be
inlined while a function pointed to by a function pointer can not (easily)
which means copy arguments to the stack, read function pointer, call it,
save clobbered registers, read arguments from the stack, ..., restore
clobbered registers, return

Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

In the past you could go to a library and read, borrow or copy any book
Today you'd get arrested for mere telling someone where the library is

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