[XviD-devel] XVID dithering in YV12-to-RGB555

Radek Czyz radoslaw at syskin.cjb.net
Wed Jun 30 13:56:43 CEST 2010


Hi,

 > Is there a specification which requires specific dithering patterns 
when decoding?

No, mpeg specification only says how to decode to YV12 colourspace (and 
even that specification is not pixel-perfect). All xvid's colourspace 
conversions are not really part of decoder, they're convenience 
functions for library users. I don't know if there are ARM-based devices 
that need RGB555, but if there are, it's completely up to you what kind 
(if any) dithering they might want.

Regards,
Radek

On 30/06/2010 8:08 PM, Frederik Lotter wrote:
> Hi,
>
> I am optimizing the colorspace conversion using the ARM NEON SIMD instruction set.
>
> The current dithering approach does not scale well using SIMD. The current approach adds the least 3 significant bits of a r,g,b pixel value to the next pixel in the row below, and then add this pixel's least significant bits to the next pixel in the row above. Etc....
>
> |/|/|/|/|/| ....
>
> However the sequential inheritance manner of this makes it inefficient for SIMD.
>
> Is there a specification which requires specific dithering patterns when decoding?
>
> Is there any patterns used in other architectures SIMD architectures what you can suggest? Currently my SIMD Colorspace conversion process 32 pixels in a single iteration over two lines:
>
> [xxxxxxxxxxxxxxxx] 16 pixels
> [xxxxxxxxxxxxxxxx] 16 pixels
>
> Dithering ideally should be contained to the current processed pixels otherwise large amounts if pixel data must be temporarily saved on the stack - which I want to avoid at all costs.
>
> The sequential manner of the zigzag pattern above cannot be done quickly on SIMD registers containing the r,g,b values.
>
> -- -- --
> /  /  /
> -- -- --
>
> I was wondering if I could simplify the dithering to groups of 4 pixels.
>
> Regards,
> Fred
>
>
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