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genericguy

C++ SDL: Draw at centre of surface after rotating

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Hi, I'm in quite a predicament. I'm using a physics engine called Chipmunk (which is awesome, by the way), which has origin of objects at their centre. Unfortunately for me SDL has the origin at the left top corner. Now I rotate my SDL_Surfaces (with rotozoom), and then I'm not sure where the centre of the object is anymore (because surfaces are square)! In case you aren't following, I drew a picture: Red dot: object origin Blue rectangle: there for perspective Black Box: SDL_Surface size In those case, it seems simply drawing the surface at x-width, y-height would work, but will that work in every case? Thanks, Lucas

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Your new width and height are (if your object is in a box):

rotated_widht = cos(alpha) * original_width - sin(alpha) * original_height
rotated_height = cos(alpha) * original_height + sin(alpha) * original_width

(Take care about + and - )
Depending on where the center of the rotation is placed, you have to perform different translation. Assuming that the center of the rotation is the top left of the original object, you have to subtract

translation_x = (rotated_widht - original_width) / 2
translation_y = (rotated_height - original_height) / 2

from your original coordinates to keep the center of the object unchanged.

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