Xcode 如何以正确的方式从NSImage（视网膜问题）保存PNG文件？

我正试图将每个图像保存在一个数组中，作为一个.PNG文件（也是正确的大小，由于retina mac dpi问题，没有放大），但似乎找不到解决方案。目前的解决方案似乎都不适合我。我试过每一个，但它们都会在retina.etc中将72x72文件保存为144x144

更具体地说，我正在寻找NSImage类别（是的，我在Mac环境中工作）

我试图让用户选择一个目录来保存它们，并从数组中执行图像保存，如下所示：

- (IBAction)saveImages:(id)sender {
    // Prepare Images that are checked and put them in an array
    [self prepareImages];

    if ([preparedImages count] == 0) {
        NSLog(@"We have no preparedImages to save!");
        NSAlert *alert = [[NSAlert alloc] init];
        [alert setAlertStyle:NSInformationalAlertStyle];
        [alert setMessageText:NSLocalizedString(@"Error", @"Save Images Error Text")];
        [alert setInformativeText:NSLocalizedString(@"You have not selected any images to create.", @"Save Images Error Informative Text")];

        [alert beginSheetModalForWindow:self.window
                          modalDelegate:self
                        didEndSelector:@selector(testDatabaseConnectionDidEnd:returnCode:
                                                   contextInfo:)
                            contextInfo:nil];
        return;
    } else {
        NSLog(@"We have prepared %lu images.", (unsigned long)[preparedImages count]);
    }

    // Save Dialog
    // Create a File Open Dialog class.
    //NSOpenPanel* openDlg = [NSOpenPanel openPanel];
    NSSavePanel *panel = [NSSavePanel savePanel];

    // Set array of file types
    NSArray *fileTypesArray;
    fileTypesArray = [NSArray arrayWithObjects:@"jpg", @"gif", @"png", nil];

    // Enable options in the dialog.
    //[openDlg setCanChooseFiles:YES];
    //[openDlg setAllowedFileTypes:fileTypesArray];
    //[openDlg setAllowsMultipleSelection:TRUE];
    [panel setNameFieldStringValue:@"Images.png"];
    [panel setDirectoryURL:directoryPath];


    // Display the dialog box.  If the OK pressed,
    // process the files.
    [panel beginWithCompletionHandler:^(NSInteger result) {

        if (result == NSFileHandlingPanelOKButton) {
            NSLog(@"OK Button!");
            // create a file manager and grab the save panel's returned URL
            NSFileManager *manager = [NSFileManager defaultManager];
            directoryPath = [panel URL];
            [[self directoryLabel] setStringValue:[NSString stringWithFormat:@"%@", directoryPath]];

            // then copy a previous file to the new location

            // copy item at URL was self.myURL
            // copy images that are created from array to this path


            for (NSImage *image in preparedImages) {
#warning Fix Copy Item At URL to copy image from preparedImages array to save each one
                NSString *imageName = image.name;
                NSString *imagePath = [[directoryPath absoluteString] stringByAppendingPathComponent:imageName];

                //[manager copyItemAtURL:nil toURL:directoryPath error:nil];
                NSLog(@"Trying to write IMAGE: %@ to URL: %@", imageName, imagePath);
                //[image writePNGToURL:[NSURL URLWithString:imagePath] outputSizeInPixels:image.size error:nil];
                [self saveImage:image atPath:imagePath];
            }
            //[manager copyItemAtURL:nil toURL:directoryPath error:nil];


        }
    }];

    [preparedImages removeAllObjects];

    return;

}

一个用户试图使用此NSImage类别回答他的问题，但它没有为我生成任何文件或PNG

@interface NSImage (SSWPNGAdditions)

- (BOOL)writePNGToURL:(NSURL*)URL outputSizeInPixels:(NSSize)outputSizePx error:(NSError*__autoreleasing*)error;

@end

@implementation NSImage (SSWPNGAdditions)

- (BOOL)writePNGToURL:(NSURL*)URL outputSizeInPixels:(NSSize)outputSizePx error:(NSError*__autoreleasing*)error
{
    BOOL result = YES;
    NSImage* scalingImage = [NSImage imageWithSize:[self size] flipped:[self isFlipped] drawingHandler:^BOOL(NSRect dstRect) {
        [self drawAtPoint:NSMakePoint(0.0, 0.0) fromRect:dstRect operation:NSCompositeSourceOver fraction:1.0];
        return YES;
    }];
    NSRect proposedRect = NSMakeRect(0.0, 0.0, outputSizePx.width, outputSizePx.height);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB);
    CGContextRef cgContext = CGBitmapContextCreate(NULL, proposedRect.size.width, proposedRect.size.height, 8, 4*proposedRect.size.width, colorSpace, kCGImageAlphaPremultipliedLast);
    CGColorSpaceRelease(colorSpace);
    NSGraphicsContext* context = [NSGraphicsContext graphicsContextWithGraphicsPort:cgContext flipped:NO];
    CGContextRelease(cgContext);
    CGImageRef cgImage = [scalingImage CGImageForProposedRect:&proposedRect context:context hints:nil];
    CGImageDestinationRef destination = CGImageDestinationCreateWithURL((__bridge CFURLRef)(URL), kUTTypePNG, 1, NULL);
    CGImageDestinationAddImage(destination, cgImage, nil);
    if(!CGImageDestinationFinalize(destination))
    {
        NSDictionary* details = @{NSLocalizedDescriptionKey:@"Error writing PNG image"};
        [details setValue:@"ran out of money" forKey:NSLocalizedDescriptionKey];
        *error = [NSError errorWithDomain:@"SSWPNGAdditionsErrorDomain" code:10 userInfo:details];
        result = NO;
    }
    CFRelease(destination);
    return result;
}

@end

---

我对原件中提供的答案也有疑问。通过进一步阅读，我找到了Erica Sadun写的一篇关于调试没有视网膜显示器的视网膜显示器代码的文章。她创建所需大小的位图，然后将当前图形上下文（基于显示/受视网膜影响）替换为与新位图关联的通用上下文。然后，她将原始图像渲染到位图中（使用通用图形上下文）

我拿了她的代码，在NSImage上快速分类，似乎可以帮我完成这项工作。打完电话

NSBitmapImageRep *myRep = [myImage unscaledBitmapImageRep];

无论开始使用何种类型的物理显示，都应具有正确（原始）尺寸的位图。从这一点上，您可以调用未缩放位图上的

representationUsingType:properties

，以获得希望写出的任何格式

这是我的分类（标题省略）。注意-您可能需要公开位图初始值设定项的颜色空间部分。这是适用于我的特定案例的值

-(NSBitmapImageRep *)unscaledBitmapImageRep {

    NSBitmapImageRep *rep = [[NSBitmapImageRep alloc]
                               initWithBitmapDataPlanes:NULL
                                             pixelsWide:self.size.width
                                             pixelsHigh:self.size.height
                                          bitsPerSample:8
                                        samplesPerPixel:4
                                               hasAlpha:YES
                                               isPlanar:NO
                                         colorSpaceName:NSDeviceRGBColorSpace
                                            bytesPerRow:0
                                           bitsPerPixel:0];
    rep.size = self.size;

   [NSGraphicsContext saveGraphicsState];
   [NSGraphicsContext setCurrentContext:
            [NSGraphicsContext graphicsContextWithBitmapImageRep:rep]];

    [self drawAtPoint:NSMakePoint(0, 0) 
             fromRect:NSZeroRect 
            operation:NSCompositeSourceOver 
             fraction:1.0];

    [NSGraphicsContext restoreGraphicsState];
    return rep;
}

---

泰德-非常感谢你的代码-我为此苦恼了好几天！它帮助我从NSImage中写入一个文件，同时将分辨率保持在72DPI，尽管我的Mac上安装了视网膜显示器。为了帮助其他希望将NSImage保存到分辨率为72 DPI的特定像素大小和类型（PNG、JPG等）的文件中的人，下面是适用于我的代码。我发现在调用unscaledBitmapImageRep之前，您需要设置图像的大小

-(void)saveImage:(NSImage *)image
     AsImageType:(NSBitmapImageFileType)imageType
         forSize:(NSSize)targetSize
          atPath:(NSString *)path
{
    image.size = targetSize;

    NSBitmapImageRep * rep = [image unscaledBitmapImageRep:targetSize];

    // Write the target image out to a file
    NSDictionary *imageProps = [NSDictionary dictionaryWithObject:[NSNumber numberWithFloat:1.0] forKey:NSImageCompressionFactor];
    NSData *targetData = [rep representationUsingType:imageType properties:imageProps];
    [targetData writeToFile:path atomically: NO];

    return;
}

我还包括了下面类别标题和.m文件的源代码

NSImage+Scaling.h文件：

#import <Cocoa/Cocoa.h>
#import <QuartzCore/QuartzCore.h>

@interface NSImage (Scaling)

-(NSBitmapImageRep *)unscaledBitmapImageRep;

@end

---

谢谢泰德&雪桨手

对于不熟悉Cocoa和使用Swift 4的任何人，您可以从编辑历史记录中查看Swift 2和Swift 3版本：

import Cocoa

func unscaledBitmapImageRep(forImage image: NSImage) -> NSBitmapImageRep {
    guard let rep = NSBitmapImageRep(
        bitmapDataPlanes: nil,
        pixelsWide: Int(image.size.width),
        pixelsHigh: Int(image.size.height),
        bitsPerSample: 8,
        samplesPerPixel: 4,
        hasAlpha: true,
        isPlanar: false,
        colorSpaceName: .deviceRGB,
        bytesPerRow: 0,
        bitsPerPixel: 0
        ) else {
            preconditionFailure()
    }

    NSGraphicsContext.saveGraphicsState()
    NSGraphicsContext.current = NSGraphicsContext(bitmapImageRep: rep)
    image.draw(at: .zero, from: .zero, operation: .sourceOver, fraction: 1.0)
    NSGraphicsContext.restoreGraphicsState()

    return rep
}

func writeImage(
    image: NSImage,
    usingType type: NSBitmapImageRep.FileType,
    withSizeInPixels size: NSSize?,
    to url: URL) throws {
    if let size = size {
        image.size = size
    }
    let rep = unscaledBitmapImageRep(forImage: image)

    guard let data = rep.representation(using: type, properties: [.compressionFactor: 1.0]) else {
        preconditionFailure()
    }

    try data.write(to: url)
}

---

我在将NSImage对象保存到PNG或JPG文件时遇到了同样的困难，我终于明白了为什么

首先，上面显示的代码摘录效果良好：

import Cocoa

func unscaledBitmapImageRep(forImage image: NSImage) -> NSBitmapImageRep {
    guard let rep = NSBitmapImageRep(
        bitmapDataPlanes: nil,
        pixelsWide: Int(image.size.width),
        pixelsHigh: Int(image.size.height),
        bitsPerSample: 8,
        samplesPerPixel: 4,
        hasAlpha: true,
        isPlanar: false,
        colorSpaceName: .deviceRGB,
        bytesPerRow: 0,
        bitsPerPixel: 0
    ) else {
        preconditionFailure()
    }

    NSGraphicsContext.saveGraphicsState()
    NSGraphicsContext.current = NSGraphicsContext(bitmapImageRep: rep)
    image.draw(at: .zero, from: .zero, operation: .sourceOver, fraction: 1.0)
    NSGraphicsContext.restoreGraphicsState()

    return rep
}

func writeImage(
    image: NSImage,
    usingType type: NSBitmapImageRep.FileType,
    withSizeInPixels size: NSSize?,
    to url: URL) throws {
    if let size = size {
        image.size = size
    }
    let rep = unscaledBitmapImageRep(forImage: image)

    guard let data = rep.representation(using: type, properties:[.compressionFactor: 1.0]) else {
    preconditionFailure()
    }

    try data.write(to: url)
}

…然而，由于我使用的是沙盒的Mac应用程序，正如您所知，这是Apple App Store发行版的一项要求，因此我注意到，在测试我的初步代码时，我必须小心选择目标目录

如果我通过以下方式使用文件URL：

let fileManager = FileManager.default
let documentsURL = fileManager.urls(for: .documentDirectory, in: .userDomainMask).first!
let documentPath = documentsURL.path
let filePath = documentsURL.appendingPathComponent("TestImage.png")

文件路径=file:///Users/Andrew/Library/Containers/Objects-and-Such.ColourSpace/Data/Documents/TestImage.png

…适用于沙盒应用程序，如果我选择了桌面：

文件路径=file:///Users/Andrew/Library/Containers/Objects-and-Such.ColourSpace/Data/Desktop/TestImage.png

我希望这有帮助

let fileManager = FileManager.default
let documentsURL = fileManager.urls(for: .documentDirectory, in: .userDomainMask).first!
let documentPath = documentsURL.path
let filePath = documentsURL.appendingPathComponent("TestImage.png")