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Re: Resizing an NSImage (and getting good-looking results!)
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Re: Resizing an NSImage (and getting good-looking results!)


  • Subject: Re: Resizing an NSImage (and getting good-looking results!)
  • From: glenn andreas <email@hidden>
  • Date: Tue, 25 Jan 2005 12:20:27 -0600


On Jan 25, 2005, at 9:31 AM, Toby Atkin-Wright wrote:

I want to load a JPEG, reduce it in size, and convert the new image to a JPEG ready for upload to a web site. The following code works, but the results look unpleasantly rough:

// load and prepare image
NSImage* theImage = [[NSImage alloc] initWithContentsOfFile:theFilename];
[theImage setScalesWhenResized:YES];


// resize the image
[theImage lockFocus];
[[NSGraphicsContext currentContext] setImageInterpolation:NSImageInterpolationHigh]; // does this do anything?
[theImage setSize:theNewSize]; // theNewSize is an NSSize struct
[theImage unlockFocus];

// convert it back to a JPEG
NSData* theImageDataTemp = [theImage TIFFRepresentation];
NSBitmapImageRep* theImageRepresentation = [NSBitmapImageRep imageRepWithData:theImageDataTemp];
NSDictionary* theProperties = [NSDictionary dictionaryWithObject:[NSNumber numberWithFloat:0.9] forKey:NSImageCompressionFactor];
NSData* theImageData = [theImageRepresentation representationUsingType:NSJPEGFileType properties:theProperties];



I guess its not doing any anti-aliasing when resizing the image. I was hoping that calling setImageInterpolation:NSImageInterpolationHigh on the graphics context would help, but it doesn't seem to make any difference.


What should I be doing?


To get the image resized & resampled, doing a simple setSize of theImage isn't going to do the trick - you'll need to explicitly redraw & resample it at the new size yourself.


I've gotten good results with code like this:

// resize the image
NSImage *resizedImage = [[NSImage alloc] initWithSize: theNewSize];
[resizedImage lockFocus];
[NSGraphicsContext saveGraphicsState];
[[NSGraphicsContext currentContext] setImageInterpolation:NSImageInterpolationHigh];
[image drawInRect:NSMakeRect(0.0,0.0,theNewSize.width, theNewSize.height) fromRect:NSMakeRect(0.0,0.0,[image size].width, [image size].height) operation:NSCompositeCopy fraction:1.0];
[NSGraphicsContext restoreGraphicsState];
[resizedImage unlockFocus];



(and then you grab the data from the resized image, and don't forget to release it when done)



Glenn Andreas                      email@hidden  <http://www.gandreas.com/> oh my! Mad, Bad, and Dangerous to Know

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References: 
 >Resizing an NSImage (and getting good-looking results!) (From: Toby Atkin-Wright <email@hidden>)

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