Moving sprite according to rotation

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x3meblue

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Joined: 11th Dec 2008

Location: Nuremberg, Germany

Posted: 17th May 2015 08:30

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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

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baxslash

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Joined: 26th Dec 2006

Location: Duffield

Posted: 17th May 2015 11:48

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

Using AppGameKit V2 Tier 1

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MikeHart

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Posted: 17th May 2015 13:11

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

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Using AGK2 Tier 1

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Maindric

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Posted: 17th May 2015 15:16

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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.

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x3meblue

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Joined: 11th Dec 2008

Location: Nuremberg, Germany

Posted: 17th May 2015 16:35

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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

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Coilwinder

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Joined: 28th Feb 2013

Location: Norway

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

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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.

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CodeTrasher

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Location: Tampere, Finland

Posted: 28th May 2015 12:54

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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.

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