objective-c - 如何在 Cocoa 应用程序中设置导出的 JPEG 图像的每英寸像素数？

Question

我有一个 Cocoa Mac 图像编辑应用程序，可以让用户导出 JPEG 图像。我当前正在使用以下代码将这些图像导出为 JPEG 文件:

//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];

但是，我希望用户能够提供此 JPEG 图像的每英寸像素数(就像在 Photoshop 的导出选项中一样)。我需要在上面的代码中修改什么来调整导出的 JPEG 的该值？

Answer 1

我找不到使用 NSImage API 执行此操作的方法，但 CGImage 可以通过设置 kCGImagePropertyDPIHeight/Width 来实现。

我还设置了 kCGImageDestinationLossyCompressionQuality，我认为它与 NSImageCompressionFactor 相同。

//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");