[XviD-devel] lossless rescale

[Christoph Lampert]

On 2 Aug 2002, Florin Andrei wrote:

> Recently, i've seen on Freshmeat an announcement of a software that
> claims to perform "lossless JPEG rescale".

Do you have a link for that? 

> According to the author, the logic is like this: normally, an
> application like Gimp, when rescaling a JPEG image, it actually performs
> a conversion to plain bitmap, applies the rescale algo to the bitmap,
> then converts the bitmap back to JPEG. Because JPEG is a lossy format,
> the whole process brings a noticeable loss of quality.
> Now, if the program knows "everything" about JPEG, a transformation
> could be applied without going through decoding to bitmap / recoding to
> JPEG. Therefore, the loss of quality is much smaller, if any.

I have no idea how we want's to do that. JPEG is block based, everything 
is done on 8x8 block level. When he rescales, these blocks change size 
and coefficients have to be recalculated for all blocks. 
Even if this is done without converting to a bitmap, calling it "lossless
rescaling" is almost criminal. 
Lossless cropping (haha), when boundaries are multiples of width 8 would
be possible. 

> I wonder if a similar thing can be done to DivX. Is it possible, at
> least theoretically?

No. I'd say it's just as impossible as for JPEG. 

gruel

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