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Rudi
Account closed
Registered: May 2010
Posts: 125 |
Fast way to rotate a char?
Im not talking about rol or ror, but swap bits so that they are rotated 90 degrees:
Example:
a char (and the bits can be random):
10110010 byte 1..
11010110 byte 2.. etc..
00111001
01010110
11011010
10110101
00110011
10110100 after "rotation" (rows and columns are swapped):
11001101
01011000
10100111
11111111
00101000
01010101
11011010
00100110 is it possible to use lookup tables for this or would that lookup table be too big?
or other lookuptable for getting and setting bits?
-Rudi |
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CreaMD
Registered: Dec 2001
Posts: 3034 |
I wonder if this would significantly speed up my proportional routine and help me "posthumously" win the internal compo I had with Wotnau ;-)
So which one is the fastest? I need to rotate full line of bitmap 320 bytes char by char, +90 degrees each |
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Dano
Registered: Jul 2004
Posts: 226 |
Haha, did my a proportial Routine for NewsPress back then the same way as CreaMD did. Switched to a standard one some time later as this was even faster. Guess i did the rotation the Oswald-way too back then. |
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Oswald
Registered: Apr 2002
Posts: 5017 |
don think you can beat a bitshifter with this c2p, couple hundred cycles even with axis method, while bitshifter worst case is about 170 (lsr a ror zp * 8 lines * 3 times) |
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Rudi
Account closed
Registered: May 2010
Posts: 125 |
Im trying to figure my head around this:
Since the rotation is just a bunch of bitswaps. For example swapping bit 0 and 7, bit 1 and 15, 2 and 23 etc.00 01 02 03 04 05 06 07
08 09 10 11 12 13 14 15
etc..
56 57 58 59 60 61 62 63. How would a swapbit-table look for this? The number of swaps are only 32. only 32 lda's and sta's? lets say ~8 cycles in total per swap: 8*32 = 256 cycles for all the swaps. but still I dont know if its possible to have a table for this..
Edit: hm, woops. i think perhaps 32 is wrong, since both locations need to be written to, so maybe two sta's instead of just one. |
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Cruzer
Registered: Dec 2001
Posts: 1048 |
Oswald: Why 3 times and not 8? |
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Oswald
Registered: Apr 2002
Posts: 5017 |
Quote: Oswald: Why 3 times and not 8?
coz you rotate in 2 buffers a,b:
a b
a b
a b
but if shifting is bigger than 3 you can just shift bits to opposite dir and then swap a,b (on screen). 3 is just a wild guess maybe 4 is the max, it was a long time ago. |
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Rudi
Account closed
Registered: May 2010
Posts: 125 |
There is some interesting cyclic dynamic going on when looking at a bit-swapping method for 8x8 bits:
these figures shows that the outer layer is rotated 8 times first, then the next layer is rotated 6 times and so forth:xxxxxxxx ........ ........ ........
x......x .xxxxxx. ........ ........
x......x .x....x. ..xxxx.. ........
x......x .x....x. ..x..x.. ...xx...
x......x .x....x. ..x..x.. ...xx...
x......x .x....x. ..xxxx.. ........
x......x .xxxxxx. ........ ........
xxxxxxxx ........ ........ ........ in this example one start with c0r0 and swap that with c7r0, then c1r0->c7r1, c2r0->c7r2 etc. so it goes around like a circle would. when that process is finished, the next inner layer does the same thing. until all four layers are done. and the result is a 90 deg rotation. its just one other way of looking at it. but still, a lookup-table for this? its difficult for me to see how. (im just giving out a few ideas for trying out other approach maybe). |
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ChristopherJam
Registered: Aug 2004
Posts: 1378 |
Problem with bitshifter is it only deals with one bit at a time.
c2p exchanges four bitpairs in only 20-30 cycles. The expensive part is moving bits to matching positions within the two bytes they're being exchanged between... |
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Digger
Registered: Mar 2005
Posts: 421 |
Nice read. But what fx do you have in mind Rudi? |
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Rudi
Account closed
Registered: May 2010
Posts: 125 |
Quote: Nice read. But what fx do you have in mind Rudi?
My initial thinking was to make a sine-scroller, where the sinus-transform is done in chunky and then transformed using c2p. Of course the c2p-routine has to be fast enough for this. It would also bring out some new ideas for other vertical effects (that are first done with horisontal tricks, and then transformed using c2p). Unfortunately there seem to be no _really_ fast c2p routine.
Im still figuring out how that 4x4, 2x2 plus 1x1 rotator works. or that is; looking at every detail of the bit swaps.
Edit: hires-sinescroller. |
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