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jamiefuller
Account closed
Registered: Mar 2018
Posts: 25 |
Simple "look at" function, avoiding atan2
Hi All,
I am writing a game that requires the player character to aim his gun at a movable point around the screen, I would usually calculate the angle using atan2 but don't want the overhead of a fully blown atan2 routine ( http://codebase64.org/doku.php?id=base:8bit_atan2_8-bit_angle )
The player can be any random x,y and the crosshaor can be any other random x,y
The gun sprite only needs to point in one of 8 directions so I can't help think that it must be easier to calculate/table. but I'm drawing a major blank :(
Any pointers or suggestions would be much appreciated.
Thanks |
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jamiefuller
Account closed
Registered: Mar 2018
Posts: 25 |
thanks so much JackAsser, That gives me some info to work through.
The only issue I can think of is that the octants are slightly offset to that,
see this image, excuse my terrible scribble :)
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ChristopherJam
Registered: Aug 2004
Posts: 1409 |
Just set
x'=x+y/2
y'=y-x/2
before using Jackie's method. Your thresholds will be 4° off, but I don't think that'll be noticeable.
(or, (x*2.5+y, y*2.5-x) if you want to just be 0.7° out) |
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JackAsser
Registered: Jun 2002
Posts: 2014 |
CJam applies a simple rotation matrix. Ideally you want to mul by sin(0.5) but sin(0.5) ~= 0.5 |
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jamiefuller
Account closed
Registered: Mar 2018
Posts: 25 |
Damn you guys are good!, I'm still struggling to get my head around the first reply :)
Thanks to you both. |
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Martin Piper
Registered: Nov 2007
Posts: 722 |
Calculate dx and dy. Shift the values, or lookup, so they're both fitting in 4 bits. You can vary the shift to maintain nearby accuracy or fit far away tests as long as you shift the same for both values.
Combine them into a 8 bit value.
Use that to index into a direction table.
The final lookup gives you the ability to finely tweak the direction sprite for whatever skewed quadrants you want. |
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jamiefuller
Account closed
Registered: Mar 2018
Posts: 25 |
Thanks everyone for your ideas, honestly some of them were well over my head but certainly put me on the right path.
This is the code I ended up with, which works well for my case needs. it doesn't compensate for my "offset" yet but it still looks pretty good and I may just skip that bit.
Huge thanks again everyone!
pointgun
lda x1
sec
sbc x2
sta dx
bcs @at10
eor #$ff
@at10
sta absdx
rol octant
lda y1
sec ; this bit just compensates for different starting co-ordinates
sbc #$0c ; this bit just compensates for different starting co-ordinates
lsr ; this bit just compensates for different starting co-ordinates
sec
sbc y2
sta dy
bcs @at20
eor #$ff
@at20
sta absdy
rol octant
sec
sbc absdx
rol octant
lda octant
and #$07
tax
lda guntab,x
sta 2046 ;store anim frame in sprite
rts
guntab BYTE $8E,$8F,$8D,$8C,$91,$90,$92,$93
x1,y1,x2,y2,dx,dy,absdx and absdy and octant are all just zeropage variables. |
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ChristopherJam
Registered: Aug 2004
Posts: 1409 |
Nice.
I was thinking an alternate way to solve the offsets problem is to just have your artwork pointing the gun NNE, ENE, ESE, SSE etc :)
(oh, and warning - your "this bit" section is subtracting y2 from half of y1 at the moment; you'll probably want to make a halved version of y2 to subtract instead if you keep that) |
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jamiefuller
Account closed
Registered: Mar 2018
Posts: 25 |
Quote: Nice.
I was thinking an alternate way to solve the offsets problem is to just have your artwork pointing the gun NNE, ENE, ESE, SSE etc :)
(oh, and warning - your "this bit" section is subtracting y2 from half of y1 at the moment; you'll probably want to make a halved version of y2 to subtract instead if you keep that)
Yeah the "this bit" section adjusts for the fact that y1 and y2 have very differing co-ordinate systems in my game, y2 only has 2 pixel resolution so to speak, so is multiplied by 2 when drawing the sprite later on :)
I also just noticed that the code above can be optimised quite a bit by removing the "sta dx", "sta dy" and the "sta absdy" as none of them are used for anything else later on.
:) |
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TWW
Registered: Jul 2009
Posts: 545 |
In which case you only need to LDA X1 CMP X2 |
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jamiefuller
Account closed
Registered: Mar 2018
Posts: 25 |
Quote: In which case you only need to LDA X1 CMP X2
Good point, and I had to double check my code to make sure :)
but in this case while I am not using dx later, I am using dx to calculate absdx, so I still need to do the subtraction to calculate dx, just not store it anywhere, only store absdx.
But I think I could probably do that for the last subtraction
sec
sbc absdx
which calculates absdy-absdx and only uses the sign of the output. I'll give it a try :) |
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