Simple assembler maths problem

[mcgeezer]

If you read some of my posts on here you'll know that I'm not scared to tell people how bad I am at maths, what most people find simple I find incredibly hard to work out.

Take this one for example which is pretty simple for Super Sprint.

Say I have a range 0-400 for the car throttle.

And I have a steer setting which goes from -300 to +300 (0 is when the car wheels are centred).

When the car is idle (no throttle), the steer should quickly interpolate (I think that's the right expression) to 0, when the car is going full throttle the steer should very slowly interpolate back to 0.

Been working on it a while now and I can't get it right.

Any ideas anyone?

[DMWCashy]

What do you have so far?,

If I understand you correctly, I would have a set point for the target steering position, and then move the current steering position towards the target set point by a delta amount(some fraction created by the current speed) each game frame.

Then you can just fine tune the delta equation until you get something that feels right.

[LeCaravage]

Why not set car throttle in a unsigned byte (0-255) ? And same goes for steer but signed this time.
The way I understand it, if you have enough memory I'd go for a 256 bytes (may be more if more precision is needed) table of dx, where dx is the amount of steering indexed with throttle.

[mcgeezer]

Originally Posted by DMWCashy

What do you have so far?,

If I understand you correctly, I would have a set point for the target steering position, and then move the current steering position towards the target set point by a delta amount(some fraction created by the current speed) each game frame.

Then you can just fine tune the delta equation until you get something that feels right.

What I have at the moment... (which isn't right).

.steer_to_centre:
	move.l	carCurrentThrottle(a4),d0	; 400
	lsr.l	#8,d0
	moveq	#8,d1			; 0=0, 1=1, 2=4, 3=8, 4=16
	lsl.w	d0,d1
	
	tst.w	carSteerMod(a4)
	beq	.exit
	bmi	.steer_decr_left
	bra	.steer_decr_right

; If the car is not moving then the decrease is instant
.steer_decr_left:
	add.w	d1,carSteerMod(a4)
	bra	.exit

.steer_decr_right:
	sub.w	d1,carSteerMod(a4)
	bra	.exit

[DMWCashy]

Using a divide would normally be needed to make this feel right else it would be too linear,

I think @LeCaravage is on the right track here, to save the need for expensive division, use a lookup table.

I wouldn't even think you would require 256 lookups, if max throttle is 400, I would divide it by 16 so you can shift, and that would give you 25 lookups.

Then just create the lookup table values in a spreadsheet and play around until it feels right.

[Don_Adan]

Originally Posted by mcgeezer

What I have at the moment... (which isn't right).

.steer_to_centre:
	move.l	carCurrentThrottle(a4),d0	; 400
	lsr.l	#8,d0
	moveq	#8,d1			; 0=0, 1=1, 2=4, 3=8, 4=16
	lsl.w	d0,d1
	
	tst.w	carSteerMod(a4)
	beq	.exit
	bmi	.steer_decr_left
	bra	.steer_decr_right

; If the car is not moving then the decrease is instant
.steer_decr_left:
	add.w	d1,carSteerMod(a4)
	bra	.exit

.steer_decr_right:
	sub.w	d1,carSteerMod(a4)
	bra	.exit

Perhaps you can reduce number of branchs too.

tst.w carSteerMod(a4)
beq .exit
bpl.b .steer_decr_right
neg.w d1
.steer_decr_right
add.w d1,carSteerMod(a4)
bra .exit

[a/b]

This is your problem:

	move.l	carCurrentThrottle(a4),d0	; 400
	lsr.l	#8,d0

After that d0 will be either 1 (if throttle >= 256) or 0 (otherwise). You need more levels, so either use bfffo (020+) to find the highest set bit in d0, or shift it say 4 times and then use that as index in a small logarithmic table.