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");