On 16/10/2017 19:37, Alejandro Tejada via use-livecode wrote:
Hi Bob,

Bob Sneidar wrote:
I didn't post any code I don't think, but I will certainly
take some credit for having done so! ;-)
In fact, you are not late! :-D
But I am too late - the file is called "...Final Version ..." :-) :-)
Let's have a look anyway....
Please, take a look at the script of
Atkinson Dither 04 (Fastest Version)
and make it faster.
OK, did that :-)

I've been aware of this interesting discussion, but had no time to look at it at all (until tonight). Sorry to be late to the party.

I made a few changes, as follows:

1. (Just on principle).
I would change all these reference to "char" to "byte" when we are really talking about bytes in binary data.

Probably doesn't make any difference to performance - the engine will most likely realize that there aren't any Unicode strings involved - but it's just "right" to call them bytes, and to use the byte functions to manipulate them :-)

2. there's a function ImgToCh which extracts a single channel of data out the image data. It does this with a cute 'delete' method - but that is over-thinking the problem. You can (i.e. the engine can) access a single byte within a byte string in constant time (it's surely a single offset in the C library).

so instead of

repeat untiltempVar isempty
  putchar1oftempVar aftertResult
end repeat

we can simply do

*repeat* withi = 1tothenumberofcharsintempVar step4*
  put*bytei oftempVar aftertemp*
end* *repeat*

(I kept the initial delete to select which channel, just because it's so clever :-) - but that could have been removed and the repeat changed to
*   repeat* withi = tChannel+1tothenumberofcharsintempVar step4*

3. The result of that function is then passed to another function that converts the byte string into a sequential array of numbers (and that's all that's done with the byte string).
So those two should be combined into a single pass - to get :

Function ImgToChToArrayNum2 tImageData, tChannel
   -- extract a single channel's data, and convert to sequential array of numbers
   puttImageData intotempVar
   deletebyte1totChannel oftempVar
   repeat withi = 1tothenumberofbytesintempVar step4
      putbytetonum(bytei oftempVar) intotResult[tCounter]
   end repeat
end ImgTochToArrayNum2

This takes the time for this part of the whole process down from 250ms to about 75ms, and therefore the total process time down from around 750-800ms to 600-650ms (on my aging MBP).

Trying repeated runs, the times do seem to vary more than usual - not sure why.

I haven't tackled the second half (i.e. the actual dithering bit yet - maybe tomorrow).

-- Alex.

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