Re: rotating an object around the center of a view
Re: rotating an object around the center of a view
- Subject: Re: rotating an object around the center of a view
- From: Erik Buck <email@hidden>
- Date: Sun, 4 Mar 2007 11:05:58 -0500
To rotate about some point, translate to that point, rotate, and then
translate back. The approach is not Cocoa specific and is covered in
any introductory graphics text book.
Here is some Cocoa code as a sample
/* MyRotateView */
#import <Cocoa/Cocoa.h>
@interface MyRotateView : NSView
{
float rotationAngleDeg;
}
- (IBAction)takeRotationAngleDegFrom:(id)sender;
@end
#import "MyRotateView.h"
@implementation MyRotateView
- (void)drawRect:(NSRect)rect
{
NSPoint centerOfView = NSMakePoint(NSMidX([self
bounds]), NSMidY([self bounds]));
float inscribedCircleRadius = 0.5f * MIN([self
bounds].size.width,
[self bounds].size.height);
NSRect pivotPointIndicatorRect = NSMakeRect
(centerOfView.x - 10.0f,
centerOfView.y - 5.0f, 20.0f, 10.0f);
NSAffineTransform *translateTransform = [NSAffineTransform
transform];
{ // draw something not rotated
// erase past drawing
[[NSColor whiteColor] set];
NSRectFill([self bounds]);
NSBezierPath *path = [NSBezierPath bezierPath];
[[NSColor grayColor] set];
[path moveToPoint:NSMakePoint(centerOfView.x -
inscribedCircleRadius, centerOfView.y)];
[path lineToPoint:NSMakePoint(centerOfView.x +
inscribedCircleRadius, centerOfView.y)];
[path moveToPoint:NSMakePoint(centerOfView.x, centerOfView.y -
inscribedCircleRadius)];
[path lineToPoint:NSMakePoint(centerOfView.x, centerOfView.y +
inscribedCircleRadius)];
[path stroke];
}
[NSGraphicsContext saveGraphicsState]; // Save the state
[translateTransform translateXBy:centerOfView.x
yBy:centerOfView.y]; // Trans to center of rotation
[translateTransform
rotateByDegrees:rotationAngleDeg]; // Rotate
[translateTransform translateXBy:-centerOfView.x yBy:-
centerOfView.y];// Translate back
[translateTransform concat];
{ // draw something rotated
NSBezierPath *path = [NSBezierPath
bezierPathWithOvalInRect:pivotPointIndicatorRect];
[[NSColor greenColor] set];
[path moveToPoint:NSMakePoint(centerOfView.x - 20.0f,
centerOfView.y)];
[path lineToPoint:NSMakePoint(centerOfView.x +
inscribedCircleRadius, centerOfView.y)];
[path stroke];
}
[NSGraphicsContext restoreGraphicsState]; // Restore the state
{ // draw something not rotated
NSBezierPath *path = [NSBezierPath
bezierPathWithOvalInRect:NSMakeRect(centerOfView.x -
inscribedCircleRadius, centerOfView.y - inscribedCircleRadius,
2.0f * inscribedCircleRadius, 2.0f * inscribedCircleRadius)];
[[NSColor blackColor] set];
[path stroke];
}
}
- (IBAction)takeRotationAngleDegFrom:(id)sender
{
rotationAngleDeg = [sender floatValue];
[self setNeedsDisplay:YES];
}
@end
There is a sample called TransparentTetris that does this too at
http://www.cocoaprogramming.net/Downloads.html
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