[A83] Re: Angles
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[A83] Re: Angles
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Original Message
From: "Sebastiaan Roodenburg"<sebastiaan@rimsystems.nl>
Subject: [A83] Re: Angles
Date: Thu, 20 Sep 2001 15:19:42 +0200
It doesn't have to be that accurate, really. I don't wanna have to mess
with all that trig stuff =) Or lookup tables, or anything of the sort.
I'd rather stick to bit-level math.
here is the source to the angle routine in pheonix:
fire_big_bullet:
inc hl
inc hl
ld a,(player_y)
sub (hl) ;I'm guessing this is the enemy Y
ret c
srl a
srl a
ld b,a ; B = (PY - EY)
srl a
add a,b
ret z
ld b,a ; B = (3/8) * (PY - EY)
dec hl
dec hl
ld a,(player_x)
sub (hl) ; A = PX - EX
inc hl
inc hl
jr c,fire_big_left
call get_angle
ld a,c
deploy_big:
ld bc,(which_shot)
bit 7,c
jr nz,deploy_huge
add a,27
ld de,big_ebullet_data
ld (de),a
jr fire_enemy_bullet
get_angle:
ld c,0
sub b
ret c ; DX < (3/8) DY -> straight down
inc c
sub b
ret c ; DX < (3/4) DY -> slight angle
inc c
sub b
ret c ; DX < (3/2) DY -> 45 deg angle
sub b
ret c
inc c ; DX > (3/2) DY -> high angle
ret
can anyone, maybe even patrick davidson himself, explain this to me?
>
>i think the most simple way to do it would be like this:
>
> l = distense between the two points ( == sqrt( (enemy_x-player_x)^2 +
>(enemy_y-player_y)^2) ), devided by the speed ( == 3 for 3 pixels per
>frame )
> dx = (enemy_x-player_x) / l
> dy = (enemy_y-player_y) / l
>
>then when you want to let your bullet travel, let is start at (player_x,
>player_y), and add (dx,dy) to that point every frame...
>
>you could use some fixed-point values to speed up a bit... i'm not sure if
>it is posible to use a look-up-table for the SRQT thing...
>
>---
>
>an other way to do it, would be to use sin/cos/tan calculations...
>
>angle = arctan( (y2-y1)/(x2-x1) )
>dx = 3 * sin(angle)
>dy = 3 * cos(angle)
>
>the sin and cos can be calculated with a single lookuptable ( cos(a) ==
>sin(a + .5 Pi) ), using fixed-point values..
>
>> -----Oorspronkelijk bericht-----
>> Van: assembly-83-bounce@lists.ticalc.org
>> [mailto:assembly-83-bounce@lists.ticalc.org]Namens Joe Pemberton
>> Verzonden: Thursday, September 20, 2001 14:55
>> Aan: assembly-83@lists.ticalc.org
>> Onderwerp: [A83] Angles
>>
>>
>>
>> How would I determine the angle needed for an enemy bullet fired from
>> (enemy_x), (enemy_y) so it would hit the player (player_x),
>> (player_y) at a
>> constant speed of 3 pixels per frame? I saw the routine in Pheonix that
>> did this but I can't understand it. help?..
>>
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