I assume your code is for backward filter (from paeth to raw).
I tried with test data: y =. i. 5 10
'height width'=. $y
pixels=. ,y
but it ran into error
|index error: scanlinepair
| xformimage=.1 1&}.;.0 scanlinepair/\.array
Also the bpp meant comparison with bpp byte apart. this bpp can
be 1 to 7 depending on pixel format. so that each of the current
and above line can be reshaped or partition before processing
the 2x2 blocks.
Пн, 01 сен 2014, Henry Rich написал(а):
> Untested. Here is a framework. It's not pretty, but this
> computation is just not parallelizable.
>
> NB. convert to array, reverse, add bottom/right 0
> array =. ({.~ >:@$);.0 (height, width) $ pixels
>
> NB. y is 2x2 of (unused,above),:(left,upper-left)
> NB. Result is Paeth predictor
> paeth =. ({~ (i. <./)@:|@:(- +`-/))@:(}.@,)
> NB. y is 2x2 of (new pixel diff,above),:(left,upper-left)
> NB. Result is (new pixel total,next upper-left (same as input 'above')
> pixels2x2 =. ({. + 2 {. 256 | paeth)
> NB. y is is a pair of scanlines (current,above)
> NB. transpose, then decompress on each 2x2 producing a table of
> NB. (uncompressed value,above). Discard the above part, leaving a list
> scanlinepair =. 0 {"1 pixels2x2/\.@|:
> NB. Operate on each scanline-pair; reverse and remove added row/col
> xformimage =. 1 1&}.;.0 scanlinepair/\. array
>
>
> Henry Rich
>
>
> On 9/1/2014 1:44 AM, bill lam wrote:
> >Context for discussion, section 6.6 of this png rfc
> >
> >* http://tools.ietf.org/html/rfc2083
> >
> >The forward filter seems trivial, but the backward filter seems
> >tricky to me if not using loop. Any idea on a loop free
> >solution?
> >
> >
> >NB. Paeth Predictor
> >paeth=: 3 : 0
> >p=. +/ 1 1 _1 * y
> >y{~ (i.<./) |p-y
> >)
> >
> >forward=: 4 : 0
> >prev=. 10$ 3 5 8
> >iy=. i.10
> >pae=. 256&| iy - paeth"1 ((-x)}.(x#0),iy),.prev,.((-x)}.(x#0),prev)
> >)
> >
> > 3 forward''
> >253 252 250 3 3 3 3 3 3 3
> > 4 forward''
> >253 252 250 0 1 0 4 2 0 6
> >
> >NB. backward ??
> >NB. 4 backward 4 forward ''
> >
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regards,
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