Question

I know i have to translate the coordinate system, when drawing. The thing is that when i use:

CGContextTranslateCTM(_context, 0.0, _size.height);
CGContextScaleCTM(_context, 1.0, -1.0);

My rect of the image is flipped, and the image is ‘non-flipped’, if i dont use the above my image is flipped, and the rect is non-flipped.

Here’s my code:

CGRectMake(60, 100, image.size.width, image.size.height);
CGContextSaveGState(_context);
UIGraphicsBeginImageContext(image.size);
CGContextClearRect(_context, placeholderRect);
CGContextDrawImage(_context, placeholderRect, image.CGImage);
UIGraphicsEndImageContext();
CGContextRestoreGState(_context);

How do i maintain my rect (non-flipped), while the image is flipped?

Thanks

Answer 1

The trick may lie in this UIImage method

+ (UIImage *)imageWithCGImage:(CGImageRef)imageRef scale:(CGFloat)scale orientation:(UIImageOrientation)orientation

Where orientation is one of these 🙂

typedef enum {
   UIImageOrientationUp,
   UIImageOrientationDown,   // 180 deg rotation
   UIImageOrientationLeft,   // 90 deg CCW
   UIImageOrientationRight,   // 90 deg CW
   UIImageOrientationUpMirrored,    // as above but image mirrored along other axis. horizontal flip
   UIImageOrientationDownMirrored,  // horizontal flip
   UIImageOrientationLeftMirrored,  // vertical flip
   UIImageOrientationRightMirrored, // vertical flip
} UIImageOrientation;

Edit: Update

I just did this simple code sample in a UIView subclass overriding drawrect and it worked perfectly. It doesn’t use the UIImage method but works all the same.

UIImage *image = [UIImage imageNamed:@"image.png"];

CGContextRef _context = UIGraphicsGetCurrentContext();

CGContextSaveGState(_context);

CGContextDrawImage(_context, CGRectMake(100, 20, image.size.width, image.size.height), image.CGImage);

CGContextScaleCTM(_context, -1.0f, 1.0f);

CGContextDrawImage(_context, CGRectMake(-300, 20, image.size.width, image.size.height), image.CGImage);

CGContextRestoreGState(_context);