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2013-05-22 12:12

Oswald

Registered: Apr 2002
Posts: 5094

accurate 3d rotation

some silly questions & a lot of text here, its been ages I have done this routine and I am wondering where could I find more accuracy.

I am using the additive method to get the rotation matrix, working with 16 bit sines.

first, I wonder why have I choosen the sinetable max value to be +/- $1fff, there's no consecutive 4 adc's to have overflow over +/- $8000, still an adc / sbc pair may turn into adc/adc because of the case when substracting a negative number? :/ not sure if sine lookups may align this way. also should I round the 16 bit table or those bits will go bad anyway? (dont remember if I have rounded it, boy its been precalculated in basic in the stone ages:)

secondly, I keep adding #$40 to rotation angles in many places before looking them up in sincos tables. I suspect I am doing something wrong here, trying to turn the lookup value into cosine, while still having a label for the cosine offset. sin+$40= cosine, cosine+(angle+#$40)= ??? why ? :D or does this thing come from the sin*cos to addition theoremes?

third: if I recall right each adc/sbc introduces 1 bit error, is this true? so after 2 adc lowmost 2 bits in the 16 bit result is unusable?

fourth: flipping sign of a 2 complement 16 bit number is this correct?:

	lda yxl    ;*-1
	eor #$ff
	clc
	adc #$01
	sta yxl

	lda yxh
	eor #$ff
	adc #$00
	sta yxh

finally, getting a cube coordinate out of the rotation matrix, only using the HI bytes:

	lda xzh    ;mpp
	clc
	adc xyh
	sec
	sbc xxh
	sta vertexes+0

should I rewrite this to gain accuracy including the low 8 bits?:

        lda xzl
        clc
        adc xyl
        sta templ
        
        lda xzh
        adc xyh
        sta temph
        ...
        etc

finally, rounding the final8 bits by looking at the low8 is a good idea? ie, if low8 (signless) msb is high, inc high8 bits?

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2013-05-23 08:34

Oswald

Registered: Apr 2002
Posts: 5094

Quote: That's the square tables for babylonian multiplication.

should made it clear in the article :)

2013-05-23 08:43

Bitbreaker

Registered: Oct 2002
Posts: 508

Quote: should made it clear in the article :)

Well, the articles base on each other, as the multiplication is used also during rotation and such. But it sure wouldn't hurt to add some comments to the code, yes :-)

2013-11-28 09:47

theWizard
Account closed

Registered: Jul 2007
Posts: 109

3d on a 64 , driller was wrong , 3d on a c64 should be illegal mate , it just looks wrong allways has allways will.

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