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Change rotation origin

C_Worm
#22343
Hey!

I made a function that rotates my 4 vertices in a rectangle.

However the origin seemes to be (0, 0), which is the top left corner.

How could i change the origin to be in the middle of the rectangle?


rotate(): 

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void rotate(float radian)
{
	objPosData[0]  	= ((x * cos(radian)) 	  - 	(y * sin(radian)));
	objPosData[4]  	= (x * cos(radian)) 	  - 	((y + h) * sin(radian));
	objPosData[8]  	= ((x + w) * cos(radian)) - 	((y + h) * sin(radian));
	objPosData[12] 	= ((x + w) * cos(radian)) - 	(y * sin(radian));
	
	objPosData[1]  	= (x * sin(radian)) 	  + (y * cos(radian));
	objPosData[5]  	= (x * sin(radian)) 	  + ((y + h) * cos(radian));
	objPosData[9]  	= ((x + w) * sin(radian)) + ((y + h) * cos(radian));
	objPosData[13] 	= ((x + w) * sin(radian)) + (y * cos(radian));
}
Edited by C_Worm on Reason: Initial post
ratchetfreak
#22344
first translate the object such that the rotation point is at the (0, 0)

then do the rotation

then do the reverse translate of step 1

C_Worm
#22346
Hmmm now the rectangle rotates around its origin, however the rectangle is shrinking and expanding in size aswell..... i can't find out why.

Why is this? 

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	void rotate(float radian)
	{
		float baseX = 0 - w/2;
		float baseY = 0 - h/2;
		float baseW = baseX + w;
		float baseH = baseY + h;

		float n0X  = (baseX * cos(radian)) - (baseY * sin(radian));    
		float n4X  = (baseX * cos(radian)) - (baseH * sin(radian));
		float n8X  = (baseW * cos(radian)) - (baseH * sin(radian));
		float n12X = (baseW * cos(radian)) - (baseY * sin(radian));     

		float n1Y  = (baseX * sin(radian)) + (baseY * cos(radian));        
		float n5Y  = (baseX * sin(radian)) + (baseH * cos(radian));
		float n9Y  = (baseW * sin(radian)) + (baseH * cos(radian));
		float n13Y = (baseW * sin(radian)) + (baseY * cos(radian));         

		objPosData[0]  	= n0X  + x;
		objPosData[4]  	= n4X  + x;
		objPosData[8]  	= n8X  + x + w;
		objPosData[12] 	= n12X + x + w;

		objPosData[1]  	= n1Y  + y;
		objPosData[5]  	= n5Y  + y + h;
		objPosData[9]  	= n9Y  + y + h;
		objPosData[13] 	= n13Y + y
	
	
    }
Edited by C_Worm on
Simon Anciaux
#22349
I believe you don't need to add the width and height at the end: 
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objPosData[0]  	= n0X  + x;
objPosData[4]  	= n4X  + x;
objPosData[8]  	= n8X  + x;
objPosData[12] 	= n12X + x;

objPosData[1]  	= n1Y  + y;
objPosData[5]  	= n5Y  + y;
objPosData[9]  	= n9Y  + y;
objPosData[13] 	= n13Y + y;
C_Worm
#22350
Final Solution was this: 

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	void rotate(float radian)
	{

		float baseX = 0 - w/2;
		float baseY = 0 - h/2;
		float baseW = baseX + w;
		float baseH = baseY + h;

		float n0X  = (baseX * cos(radian)) - (baseY * sin(radian));    
		float n4X  = (baseX * cos(radian)) - (baseH * sin(radian));
		float n8X  = (baseW * cos(radian)) - (baseH * sin(radian));
		float n12X = (baseW * cos(radian)) - (baseY * sin(radian));     

		float n1Y  = (baseX * sin(radian)) + (baseY * cos(radian));        
		float n5Y  = (baseX * sin(radian)) + (baseH * cos(radian));
		float n9Y  = (baseW * sin(radian)) + (baseH * cos(radian));
		float n13Y = (baseW * sin(radian)) + (baseY * cos(radian));         

		objPosData[0]  	= n0X   + x + (w/2);	
		objPosData[4]  	= n4X   + x + (w/2);	
		objPosData[8]  	= n8X   + x + (w/2);	
		objPosData[12] 	= n12X  + x + (w/2);

		objPosData[1]  	= n1Y   + y + (h/2);	
		objPosData[5]  	= n5Y   + y + (h/2);	
		objPosData[9]  	= n9Y   + y + (h/2);	
		objPosData[13] 	= n13Y  + y + (h/2);
	}
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