NextTrang NextTrang - 1 month ago 10
Objective-C Question

Save image after rotate it. But picture has lower resolution after saving

I am trying rotate an image from IKImageView. But after rotating and saving it, I got a lower resolution picture.

Below is code which I am using for rotating

-(NSImage *)imageRotated:(float)degrees{
degrees = fmod(degrees, 360.);
if(0 == degrees){
return self;
NSSize size = [self size];
NSSize maxSize;
if(90. == degrees || 270. == degrees || -90== degrees || -270. == degrees){
maxSize = NSMakeSize(size.height, size.width);
}else if (180. == degrees || -180. == degrees){
maxSize = size;
maxSize = NSMakeSize(20+MAX(size.width, size.height), 20+MAX(size.width, size.height));

NSAffineTransform *rot = [NSAffineTransform transform];
[rot rotateByDegrees:degrees];
NSAffineTransform *center = [ NSAffineTransform transform];
[center translateXBy:maxSize.width/2 yBy:maxSize.height/2];
[rot appendTransform:center];
NSImage *image = [[NSImage alloc]init];
image = [NSImage imageWithSize:maxSize flipped:NO drawingHandler:^BOOL(NSRect desRect){
[rot concat];
NSRect rect = NSMakeRect(0, 0, size.width, size.height);
NSPoint corner = NSMakePoint(-size.width/2, -size.height/2);
[self drawAtPoint:corner fromRect:rect operation:NSCompositeCopy fraction:1.0];
return YES;
return image;

then, I converted image to bitmap by below code

- (NSBitmapImageRep *)bitmapImageRepresentation{
int width = [self size].width;
int height = [self size].height;

if(width <1 || height < 1)
return nil;

NSBitmapImageRep *rep = [[NSBitmapImageRep alloc]

NSGraphicsContext *ctx = [NSGraphicsContext graphicsContextWithBitmapImageRep:rep];
[NSGraphicsContext saveGraphicsState];
[NSGraphicsContext setCurrentContext:ctx];
[self drawAtPoint:NSZeroPoint fromRect:NSZeroRect operation:NSCompositeCopy fraction:1.0];
[ctx flushGraphics];
[NSGraphicsContext restoreGraphicsState];
return rep;

and I used below code to convert to NSData, then write to file

- (NSData*)toNSData{
NSImage *imgTemp = [[NSImage alloc]initWithSize:NSMakeSize(self.size.width*2, self.size.height*2)];
imgTemp = self;
NSBitmapImageRep *bmprep = [imgTemp bitmapImageRepresentation];
NSDictionary *imageProps = [NSDictionary dictionaryWithObject:[NSNumber numberWithFloat:1.0] forKey:NSImageCurrentFrame];
NSData *pngData = [[NSData alloc]init];
pngData = [bmprep representationUsingType:NSPNGFileType properties:imageProps];
return pngData;

Could you show me the reason why picture after saving have lower resolution?.

I am using MacBook Pro (Retina, 13-inch, Mid 2014)

Thank you very much in advance :)

Answer Source

I think you are rotating the image twice.

Inside the method rotateLeft

//First rotation
//Rotate Picture
        Img = [self imageRotatedByDegrees:Img and:90];

//Second rotation
[_imageView rotateImageLeft:sender];

So I believe you will need to block the second rotation and you will get rid of your issue.