How do I set the pixels per inch for an exported JPEG image in a Cocoa app? - objective-c

Question

I have a Cocoa Mac image editing app which lets users export JPEG images. I'm currently using the following code to export these images as JPEG files:
//this is user specified
NSInteger resolution;
NSImage* savedImage = [[NSImage alloc] initWithSize:NSMakeSize(600, 600)];
[savedImage lockFocus];
//draw here
[savedImage unlockFocus];
NSBitmapImageRep* savedImageBitmapRep = [NSBitmapImageRep imageRepWithData:[savedImage TIFFRepresentationUsingCompression:NSTIFFCompressionNone factor:1.0]];
NSDictionary* properties = [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithFloat:1.0], NSImageCompressionFactor, nil];
//holds the jpeg file
NSData * imageData = nil;
imageData = [savedImageBitmapRep representationUsingType:NSJPEGFileType properties:properties];
However, I would like for the user to be able to provide the pixels per inch for this JPEG image (like you can in Photoshop's export options). What would I need to modify in the above code to adjust this value for the exported JPEG?

Answer 1

I couldn't find a way to do it with the NSImage APIs but CGImage can by setting kCGImagePropertyDPIHeight/Width.
I also set kCGImageDestinationLossyCompressionQuality which I think is the same as NSImageCompressionFactor.
//this is user specified
NSInteger resolution = 100;
NSImage* savedImage = [[NSImage alloc] initWithSize:NSMakeSize(600, 600)];
[savedImage lockFocus];
//draw here
[savedImage unlockFocus];
NSBitmapImageRep* savedImageBitmapRep = [NSBitmapImageRep imageRepWithData:[savedImage TIFFRepresentationUsingCompression:NSTIFFCompressionNone factor:1.0]];
NSDictionary* properties = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithFloat:1.0], kCGImageDestinationLossyCompressionQuality,
[NSNumber numberWithInteger:resolution], kCGImagePropertyDPIHeight,
[NSNumber numberWithInteger:resolution], kCGImagePropertyDPIWidth,
nil];
NSMutableData* imageData = [NSMutableData data];
CGImageDestinationRef imageDest = CGImageDestinationCreateWithData((CFMutableDataRef) imageData, kUTTypeJPEG, 1, NULL);
CGImageDestinationAddImage(imageDest, [savedImageBitmapRep CGImage], (CFDictionaryRef) properties);
CGImageDestinationFinalize(imageDest);
// Do something with imageData
if (![imageData writeToFile:[#"~/Desktop/test.jpg" stringByExpandingTildeInPath] atomically:NO])
NSLog(#"Failed to write imageData");

Answer 2

For NSImage or NSImageRep you do not set the resolution directly but set the size instead.
For size, numberOfPixels and resolution the following equation holds:
size = numberOfPixels * 72.0 / resolution
size is a length and is expressed in dots with the unit inch/72.
(size and resolution are floats). You can see that for an image with dpi=72 size and numberOfPixels are numerally the same (but the meaning is very different).
After creating an NSBitmapImageRep the size with the desired resolution can be set:
NSBitmapImageRep* savedImageBitmapRep = . . . ; // create the new rep
NSSize newSize;
newSize.width = [savedImageBitmapRep pixelsWide] * 72.0 / resolution; // x-resolution
newSize.height = [savedImageBitmapRep pixelsHigh] * 72.0 / resolution; // y-resolution
[savedImageBitmapRep setSize:newSize];
// save the rep
Two remarks: do you really need the lockFocus / unlockFocus way? The preferred way to build a new NSBitmapImageRep is to use NSGraphicsContext. see : http://www.mail-archive.com/cocoa-dev#lists.apple.com/msg74857.html
And: to use TIFFRepresentation for an NSBitmapImageRep is very time and space consuming. Since 10.6 another
way exists and costs nothing, because lockFocus and unlockFocus create an object of class NSCGImageSnapshotRep which under the hood is a CGImage. (In OS versions before 10.6 it was an NSCachedImageRep.) The following does it:
[anImage lockFocus];
// draw something
[anImage unlockFocus];
// now anImage contains an NSCGImageSnapshotRep
CGImageRef cg = [anImage CGImageForProposedRect:NULL context:nil hints:nil];
NSBitmapImageRep *newRep = [[NSBitmapImageRep alloc] initWithCGImage:cg];
// set the resolution
// here you may NSLog anImage, cg and newRep
// save the newRep
// release the newRep if needed