Help applying friction

[Galaga_Freak]

Been messing around with 16 bit math and sub-pixel sprite movements and I ran into a problem when trying to apply friction to a given sprites speed.

 

I've attached the entire code below but the relevant section is this:

;-------------------------------
; Friction (broken )
;-------------------------------



ApplyFriction



.xfriction  lda xspeed+1

 bmi .doNegFrict

 lda xspeed

 bmi .doNegFrict

 

.checkPosFrict  lda xspeed+1 ;check if 0

 beq .checkLow

 jmp .doPosFrict

.checkLow	lda xspeed

 beq .yfriction

.doPosFrict;positive speed, subtract friction from speed

 sec

 lda xspeed

 sbc #FrictionLow

 sta xspeed

 lda xspeed+1

 sbc #0

 sta xspeed+1

 

 jmp .yfriction

 

;negative speed, add friction to speed

.doNegFrict	clc

 lda xspeed

 adc #FrictionLow

 sta xspeed

 lda xspeed+1

 adc #0

 sta xspeed+1

 

.yfriction
;Not done yet

 

It would seem it works as I expect when the xspeed is negative (sprite moving left) but when positive xspeed+1 > 0 friction no longer applies.

 

2 thoughts

 

1) this is probably a simple bug, but I cant seem to see it.

2) there is probably a much easier way of applying friction to a given speed than the method I am using above

 

Any suggestions?

friction.zip

[Robert M]

xfriction    lda xspeed+1 

     bmi .doNegFrict 

     lda xspeed 

     bmi .doNegFrict 

 

if xspeed and xspeed+1 form a 16-bit integer, then your should only test the high byte for the sign of the 16-bit integer not both bytes.

 

Cheers!

[Galaga_Freak]

Cool, I knew it would be something simple

 

Thanks a bunch!

[Thomas Jentzsch]

BTW: Instead of two separate routines for adding and subtracting the friction values, you can use a more general routine and add negative values. E.g.

 lda #FrictionLow; #-FrictionLow

 ldy #0          ; #-1

 clc

 adc xspeed 

 sta xspeed 

 tya

 adc xspeed+1 

 sta xspeed+1

And since you probably need more than one 16-bit addition in your code, you can define a subroutine:

 lda #FrictionLow; #-FrictionLow

 ldy #0          ; #-1

 ldx #<xspeed    ; or e.g. <xpos, <yspeed ...

 jsr Add16

 ...

Add16 SUBROUTINE

 clc

 adc $00,x

 sta $00,x

 tya

 adc $01,x

 sta $01,x

 rts

[Galaga_Freak]

BTW: Instead of two separate routines for adding and subtracting the friction values, you can use a more general routine and add negative values. E.g.

 lda #FrictionLow; #-FrictionLow

 ldy #0          ; #-1

 clc

 adc xspeed 

 sta xspeed 

 tya

 adc xspeed+1 

 sta xspeed+1

And since you probably need more than one 16-bit addition in your code, you can define a subroutine:

 lda #FrictionLow; #-FrictionLow

 ldy #0          ; #-1

 ldx #<xspeed    ; or e.g. <xpos, <yspeed ...

 jsr Add16

 ...

Add16 SUBROUTINE

 clc

 adc $00,x

 sta $00,x

 tya

 adc $01,x

 sta $01,x

 rts

 

Thanks thomas! I saw code similar to this in your cave 1k source, but didn't fully understand it at the time, I think because you were applying friction based on relative speed of the object(?). This makes much more sense now

[Thomas Jentzsch]

I think because you were applying friction based on relative speed of the object(?)...

Yup, the faster the vertical speed, the more opposite friction (1/8th of the current yspeed). This makes flying the helicopter much easier.

 

The physics are still not very exact, since air-friction isn't linear but quadratic to speed. But it works quite well for the game.