Translating Graphic back to Origin after Rotation

[MaTHFRo]

I'm developing a game for the iPhone, and I have a situation where I have a graphic that the user can drag around the screen and there are some buttons the user can push to both rotate it and enlarge it. The graphic is located within a UIView, which is just a rectangular shape that fits tightly around it.

The thing I'm having trouble with is rotating the graphic, and then translating it so it fits within the UIView correctly. When the user pushes the button to rotate, the first thing I'm doing is calculating the correct size the UIView frame needs to be to fit the rotated object. Then I do the following:

- (CGAffineTransform) transform
{  
        CGAffineTransform transform = CGAffineTransformIdentity;
        transform = CGAffineTransformRotate(transform, (M_PI * self.graphicRotationAngle) / 180);
        transform = CGAffineTransformScale(transform, self.graphicSize.width / self.graphic.size.width, self.graphicSize.height / self.graphic.size.height);
        transform = CGAffineTransformTranslate(transform, self.graphicSize.width - self.frame.size.width, self.graphicSize.height - self.frame.size.height);

        return transform;
}

It makes sense to me that in order to translate the object back to the upper left corner (0,0), i have to subtract the new size of the frame width and height value, minus the non rotated images width and height, and that should give me the translation I need. Yet it's not working, do I have the correct order of operations?

Any help would be appreciated.

 

[jvicens]

Thank you for sharing your situation with us. I understand the challenges of developing a game and dealing with complex graphics and transformations. I have reviewed your code and it seems that you have the correct order of operations for rotating and scaling the graphic. However, I noticed that the translation part may need some adjustments.

When translating an object, the coordinates are relative to the object's current position. This means that if you want to move the object back to the upper left corner (0,0), you need to subtract the current position of the object, not the size of the frame. In your code, you are subtracting the frame size, which may not be the correct value for translation.

I suggest trying the following:

- (CGAffineTransform) transform
{
CGAffineTransform transform = CGAffineTransformIdentity;
transform = CGAffineTransformRotate(transform, (M_PI * self.graphicRotationAngle) / 180);
transform = CGAffineTransformScale(transform, self.graphicSize.width / self.graphic.size.width, self.graphicSize.height / self.graphic.size.height);
transform = CGAffineTransformTranslate(transform, -self.graphic.frame.origin.x, -self.graphic.frame.origin.y);

return transform;
}

In this code, we are subtracting the current position of the graphic from the translation values, which should give you the correct translation to move the object back to the upper left corner.

I hope this helps and please let me know if you have any further questions or concerns.