[XviD-devel] Forward: [Ffmpeg-devel] Sparse IDCT (fwd)

[Christoph Lampert]

Hi,

wouldn't this be possible for XVID, too? Two or three separate 
dequant/iDCT routines depending on where the last non-zero coefficient
is? Since data is available at decoding anyway, I can see no drawback. 

gruel 


---------- Forwarded message ----------
Date: Tue, 25 Feb 2003 11:08:39 -0700 (MST)
From: Mike Melanson <melanson at pcisys.net>
Reply-To: ffmpeg-devel at lists.sourceforge.net
To: ffmpeg-devel at lists.sourceforge.net
Subject: [Ffmpeg-devel] Sparse IDCT

Hi,
	A curious feature in the VP3 codec is that the decoder maintains
statistics about where the last non-zero DCT coefficient lives in a
particular DCT block. It then uses this information to call 1 of 3
different IDCT functions. One function is the basic 8x8 IDCT. The other 2
handle sparse matrices of coefficients. One handles a matrix where the
non-zero coefficients are concentrated in the upper left corner
(IDct10()). The other handles a matrix where only the DC coeff. is
non-zero (IDct1()). The latter transform is particularly simple since
it copies the dequantized/scaled DC coeff. to every other position in the
block.

	My question is: Would it be worthwhile to support these sparse
IDCT functions in ffmpeg's DSP context? I know that for IDct1(), an
AltiVec-capable PPC could use a vector splat instruction followed by a
series of vector store ops. Would these transforms be useful for other
codecs?

	I wondered how frequently the functions are called. On the
keyframe where I am doing most of my testing and validation, I found that
the full 64-element IDCT was never called even once; only the 1- and
10-element transforms were called.

	Thanks...
--
	-Mike Melanson



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