Re: MMX speedup for Floyd Steinberg error diffusion

On 7 May, 18:56, "Maarten Kronenburg"  <spamt...@[EMAIL PROTECTED]
> wrote:
> "rep_movsd"  wrote in message
> > Hi
>
> > I recently implemented an animated GIF encoder to convert any video
> > into a GIF ( I know it doesnt seem really useful, but thats what the
> > client wanted ).
>
> > I implemented floyd steinberg error diffusion to improve quality of
> > the quantized output.
>
> > The code is in Visual C++ and I use 32 bpp pixels for ease in
> > processing.
>
> > The diffusion process takes up about 1/2 the time required to encode a
> > frame and it seems to be the right place to optimize.
>
> > Heres how the algorithm works - pseudocode
>
> > for each pixel in the image
> > {
> >      palColor = Get color in palette thats nearest to current pixel
> >      get signed difference between actual color and palette color for
> > R G and B components
> >      add difference (with unsigned saturation) to surrounding pixels
> > with a weightage :
> >           EAST -> 7/16
> >           SOUTHWEST -> 5/16
> >           SOUTH -> 3/16
> >           SOUTHEAST -> 1/16
> > }
>
> > Now I thought I may be able to do this fast with MMX , subtracting the
> > 3 r, g, b bytes at once ( I use 32 bpp pixels, but alpha channel is
> > ignored )
>
> > I can do a packed subtract to get the differences, but thereafter the
> > multiplication and ****fting(for division by 16) seem to be impossible.
>
> > Moreover, how can one do a unsigned saturated addition of signed
> > difference values to the unsigned pixel values?
>
> > Any ideas anyone?
>
> The 4 32-bit words can be stored in a single 128-bit xmm register.

The OP asked for MMX which uses 64-bit regs. Are you sure a post-MMX
solution will work for him?

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