Moving sprite according to rotation

Author

Message

x3meblue

15

Years of Service

User Offline

Joined: 11th Dec 2008

Location: Nuremberg, Germany

Posted: 17th May 2015 08:30

Link

Hello!

I make a tank game and want the tank to move according to its current rotation. I got it working with this code:

+ Code Snippet

if GetRawKeyState(KEY_S) = 1
	dec player.x, player.speed * Cos(player.rotation-90)
	dec player.y, player.speed * Sin(player.rotation-90)
elseif GetRawKeyState(KEY_W) = 1
	inc player.x, player.speed * Cos(player.rotation-90)
	inc player.y, player.speed * Sin(player.rotation-90)
endif

The question is, why i need to subtract 90 from the rotation?

Normally the formula is:
x += speed * Cos(angle)
y += speed * Sin(angle)

Is there a bug in AGKs Sin and/or Cos functions?

AGK2 Tier 1 / GameGuru / DB PRO / ProMotion 6.5

Back to top

Profile PM

baxslash

Valued Member

Bronze Codemaster

17

Years of Service

User Offline

Joined: 26th Dec 2006

Location: Duffield

Posted: 17th May 2015 11:48

Link

At a guess because of the angle of your image. Rotate the image so the tank points right maybe?

Using AppGameKit V2 Tier 1

Back to top

Profile PM Email

MikeHart

AGK Bronze Backer

21

Years of Service

User Offline

Joined: 9th Jun 2003

Location:

Posted: 17th May 2015 13:11

Link

It is no bug, its just a normal behaviour. You will find that feature in a lot of development tools.

-----------------------------------------------------------
Using AGK2 Tier 1

Back to top

Profile PM

Maindric

15

Years of Service

User Offline

Joined: 22nd Jul 2009

Location:

Posted: 17th May 2015 15:16

Link

This is because a circle starts at the far right (0 degrees or 0 * pi), and from the sound of it, your tank sprite is normally facing up (90 degrees or .5 * pi). Subtracting 90 from the existing sprite sets it to the correct 0 degrees.

http://upload.wikimedia.org/wikipedia/commons/3/3b/Circle_cos_sin.gif

This link kind of shows what I am talking about.

Back to top

Profile PM

x3meblue

15

Years of Service

User Offline

Joined: 11th Dec 2008

Location: Nuremberg, Germany

Posted: 17th May 2015 16:35

Link

Thanks for the answers. Yes, my tank is facing up and it's initial rotation is set to 0°. Ok, then i was thinking wrong, because i thought the 0 degrees are pointing up and not to the right. Thanks again.

AGK2 Tier 1 / GameGuru / DB PRO / ProMotion 6.5

Back to top

Profile PM

Coilwinder

11

Years of Service

User Offline

Joined: 28th Feb 2013

Location: Norway

Posted: 28th May 2015 02:02 Edited at: 28th May 2015 02:07

Link

If you go the other way (getting a direction from X and Y speed components for example), one exception is the function AtanFull().

Then 0 degrees is pointing up, and not to the right. The right is then 90 degrees. So if you use that function, you'd have to subtract 90 (degrees) to get it compatible with the other geometric functions (Sin, Cos etc.). (Why this is so I don't know, it's either a bug or a feature I guess

)

You could also use ATan(y/x), which would be "correct" (positive X direction / right = 0 degrees), but then you'd have to add a couple of checks; to avoid divide-by-zero and correct the direction according to the sign of the X component.

Apologies for any typos and strange grammar.

Back to top

Profile PM

CodeTrasher

12

Years of Service

User Offline

Joined: 18th Jul 2012

Location: Tampere, Finland

Posted: 28th May 2015 12:54

Link

As far as I know, the default direction in AppGameKit for 0 degree angle is {x = 0, y = -1}, which basically points up in 2D AppGameKit space (or any 2D game space, really). So, in order to get your tank sprite to face upwards in 0 degree, you just need to create/draw your sprite image to face upwards. Then you shouldn't have to subtract 90 degrees from the angle but to use your sprite angle directly.

Back to top

Profile PM Email

Sorry your browser is not supported!

AppGameKit Classic Chat / Moving sprite according to rotation