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Forums > C64 Coding > Fast way to rotate a char?
2017-01-04 08:32
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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2017-01-04 15:03
Rudi
Account closed

Registered: May 2010
Posts: 125
Some notes to self. altho, i dont know if it will do any miracles. maybe one can reduce it to Axiss solution, but I have no idea:

Basically what happens from a layman perspective (like me), bits are copied like this:
where each number are the bit-indices. consider the second number (after the arrow) to be the destination (of zero memory), and source (infront of arrow):

copy from (bit-index) -> to:
0->7
1->15
2->23
3->31
4->39
.
.(and so on..)
.
63->56
looking at those destination values in binary produce a pattern like this:
07: 00000|111|
15: 00001|111|
23: 00010|111|
31: 00011|111|
39: 00100|111|
47: 00101|111|
55: 00110|111|
63: 00111|111|
and
06: 00000|110|
14: 00001|110|
22: 00010|110|
30: 00011|110|
38: 00100|110|
46: 00101|110|
54: 00110|110|
62: 00111|110|
and so on...

notice last column (trits) are same, but decrease, and msbs are increasing. wether or not one can derive something out of this i dont know. of course these are just index-values and dont have any sensible meaning for copying from and to memory. cycles would be massive using this technique, but my thought was to get something sensible out of this (which i did not yet)...
2017-01-04 15:40
Slammer

Registered: Feb 2004
Posts: 416
I think its a shame that your cyclecounts for this problem is so low allready. I would like to see the spritecollision solution. Something like storing the 8 bytes on row 7 of succeding chars. Stretch the line and use the LFT trick to calculate the solution. But that would take 8 lines.
2017-01-04 15:58
tlr

Registered: Sep 2003
Posts: 1701
Quote: I think its a shame that your cyclecounts for this problem is so low allready. I would like to see the spritecollision solution. Something like storing the 8 bytes on row 7 of succeding chars. Stretch the line and use the LFT trick to calculate the solution. But that would take 8 lines.

Interesting idea! It would not necessarily take 8 lines. IIRC collisions can be retriggered during the line. You will be limited by the amount of pixels a sprite covers though.
2017-01-04 16:02
Rudi
Account closed

Registered: May 2010
Posts: 125
I dont see how sprites will solve the C2P-problem (with as few cycles as possible)? but then again, Im not too experienced with sprites on the C64. A goal would be to calculate as many chars (per frame) as possible for C2P.
2017-01-04 16:04
chatGPZ

Registered: Dec 2001
Posts: 11088
tlr: wasnt there some VIC that only clears (or retriggers?) the collisions once?
2017-01-04 19:07
tlr

Registered: Sep 2003
Posts: 1701
Quote: tlr: wasnt there some VIC that only clears (or retriggers?) the collisions once?

That was lightpen triggering. It's working very differently on 6569R1.
2017-01-04 19:22
Skate

Registered: Jul 2003
Posts: 490
I just made a rough calculation. Using 16k of precalc data (which is already terrible), each char takes 500-600 cycles to rotate. So, c2p routine looks like the best solution in every aspect. I'm still thinking an alternative approach though.
2017-01-04 19:54
Flavioweb

Registered: Nov 2011
Posts: 442
Quoting Skate
I just made a rough calculation. Using 16k of precalc data (which is already terrible), each char takes 500-600 cycles to rotate.

Just to say (...maybe i'm off-topic...) this code took 544 cicles rotate a char counterclockwise.
    *=$C000
    .FOR S=0, S<8, S=S+1
    LDA SOURCE+S
    .FOR D=0, D<8, D=D+1
    ASL
    ROL DEST+D
    .NEXT
    .NEXT
    RTS
;-------------------
SOURCE
    .BYTE %11111111
    .BYTE %10000000
    .BYTE %01000000
    .BYTE %00100000
    .BYTE %00010000
    .BYTE %00001000
    .BYTE %00000100
    .BYTE %11111111
DEST
    .BYTE %00000000
    .BYTE %00000000
    .BYTE %00000000
    .BYTE %00000000
    .BYTE %00000000
    .BYTE %00000000
    .BYTE %00000000
    .BYTE %00000000
;---------------------------------------
2017-01-04 19:57
Rudi
Account closed

Registered: May 2010
Posts: 125
This takes ~472 cycles, and is basically similar to Oswalds approach:
	;counter-clockwise rotation by 90 degrees.
	
	;column 1
	ror $74
	rol
	ror $75
	rol
	ror $76
	rol
	ror $77
	rol
	ror $78
	rol
	ror $79
	rol
	ror $7a
	rol
	ror $7b
	rol
	sta $7c
	;tot = 59 cycles

	;column 2
	ror $74
	rol
	ror $75
etc..
	ror $7b
	rol
	sta $83
;tot = 59*8 = 472 cycles
did a test in zp. so rotated (destination) bytes are in $7c to $83. this req eight bytes to be copied to zeropage ($74 to $7b) (as an example).
2017-01-04 20:35
Oswald

Registered: Apr 2002
Posts: 5007
axis' one is close to max possible I guess, go for it if understandable version is not enough :)
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