[FFmpeg-devel] [PATCH] swscale: Only copy dstW/chrDstW of the last line in the plane in planar copy

Martin Storsjö martin
Fri Jul 16 19:33:01 CEST 2010

On Fri, 16 Jul 2010, Michael Niedermayer wrote:

> On Fri, Jul 16, 2010 at 10:25:12AM +0300, Martin Storsj? wrote:
> > On Fri, 16 Jul 2010, Martin Storsj? wrote:
> > 
> > > The attached patch makes planarCopyWrapper all lines but the last in one 
> > > block, then copies only dstW/chrDstW of the last line.
> > > 
> > > If the destination planes are offset within their buffer, copying the 
> > > whole plane in one memcpy may write outside of the destination buffer.
> > > 
> > > A concrete example: The source data is 144x144, stored in the upper left 
> > > corner of a 176x144 buffer. If this is "scaled" into 144x144 into another 
> > > 176x144 buffer, but centered into this buffer, the approach wich memcpying 
> > > the whole plane writes outside of the destination buffer.
> > 
> > Updated patch - uses the already existing length variable instead of 
> > recalculating it.
> the caller could even expect that the non last lines width..stride is
> preserved as well (example picture in picture filters.

Yes - my code that uses swscale initially assumed that too, but I changed 
it to handle being overwritten.

> what speed effect does it have to do this with all lines?
> if its slower we could pass a flag to swscale.

Haven't benchmarked anything, but I guess it depends on the dimensions. 
More memcpys (height vs 1) but fewer bytes copied in total.

> that said your patch could be applied, it fixes a bug but i dont think
> this will be the final solution

Ok with the one updated with Loren's suggestion, too, doing it all in one 

// Martin

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