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NSImage and Retina Display Failure

I want to add crosses on NSImage, here is my code:

-(NSSize)convertPixelSizeToPointSize:(NSSize)px
{
    CGFloat displayScale = [[NSScreen mainScreen] backingScaleFactor];
    NSSize res;
    res.width = px.width / displayScale;
    res.height = px.height / displayScale;
    return res;
}

-(void)awakeFromNib
{
    CGFloat scale = [[NSScreen mainScreen] backingScaleFactor];

    NSLog(@"backingScaleFactor : %f",scale);

    NSImage *img = [[[NSImage alloc]initWithContentsOfFile:@"/Users/support/Pictures/cat.JPG"] autorelease];


    NSBitmapImageRep *imgRep = [NSBitmapImageRep imageRepWithData:[img TIFFRepresentation]];
    NSSize imgPixelSize = NSMakeSize([imgRep pixelsWide],[imgRep pixelsHigh]);
    NSSize imgPointSize = [self convertPixelSizeToPointSize:imgPixelSize];
    [img setSize:imgPointSize];

    NSLog(@"imgPixelSize.width: %f , imgPixelSize.height:%f",imgPixelSize.width,imgPixelSize.height);
    NSLog(@"imgPointSize.width: %f , imgPointSize.height:%f",imgPointSize.width,imgPointSize.height);

    [img lockFocus];
    NSAffineTransform *trans = [[[NSAffineTransform alloc] init] autorelease];
    [trans scaleBy:1.0 / scale];
    [trans set];

    NSBezierPath *path = [NSBezierPath bezierPath];     
    [[NSColor redColor] setStroke];
    [path moveToPoint:NSMakePoint(0.0, 0.0)];
    [path lineToPoint:NSMakePoint(imgPixelSize.width, imgPixelSize.height)];
    [path moveToPoint:NSMakePoint(0.0, imgPixelSize.height)];
    [path lineToPoint:NSMakePoint(imgPixelSize.width, 0.0)];

    [path setLineWidth:1];
    [path stroke];
    [img unlockFocus];

    [imageView setImage:img];

    imgRep = [NSBitmapImageRep imageRepWithData:[img TIFFRepresentation]];
    NSData *imageData = [imgRep representationUsingType:NSJPEGFileType properties:nil];
    [imageData writeToFile:@"/Users/support/Pictures/11-5.JPG" atomically:NO];
}

display without retina result: enter image description here

and the console is displayed:

2012-07-06 00:53:09.889 RetinaTest[8074:403] backingScaleFactor : 1.000000
2012-07-06 00:53:09.901 RetinaTest[8074:403] imgPixelSize.width: 515.000000 , imgPixelSize.height:600.000000
2012-07-06 00:53:09.902 RetinaTest[8074:403] imgPointSize.width: 515.000000 , imgPointSize.height:600.000000

but on the retina screen (I did not use a real retina display, but hidpi mode): enter image description here

Prefixes:

2012-07-06 00:56:05.071 RetinaTest[8113:403] backingScaleFactor : 2.000000
2012-07-06 00:56:05.083 RetinaTest[8113:403] imgPixelSize.width: 515.000000 , imgPixelSize.height:600.000000
2012-07-06 00:56:05.084 RetinaTest[8113:403] imgPointSize.width: 257.500000 , imgPointSize.height:300.000000

If I omit these lines:

NSAffineTransform *trans = [[[NSAffineTransform alloc] init] autorelease];
            [trans scaleBy:1.0 / scale];
            [trans set];

enter image description here

However, if I change [NSAffineTransform scaleBy] to 1.0, the result will be correct

NSAffineTransform *trans = [[[NSAffineTransform alloc] init] autorelease];
        [trans scaleBy:1.0];
        [trans set];

enter image description here

Console:

2012-07-06 01:01:03.420 RetinaTest[8126:403] backingScaleFactor : 2.000000
2012-07-06 01:01:03.431 RetinaTest[8126:403] imgPixelSize.width: 515.000000 , imgPixelSize.height:600.000000
2012-07-06 01:01:03.432 RetinaTest[8126:403] imgPointSize.width: 257.500000 , imgPointSize.height:300.000000

Can someone give an explanation please? Is hidpi mode different from retina display?

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2 answers

I think I found the answer. If NSAffineTransform is configured for an NSImage context, it converts the coordinate system to a pixel size that is 2 x dots in size. Even if it is empty as follows:

NSAffineTransform *trans = [[[NSAffineTransform alloc] init] autorelease];
[trans set];

, , .

+4

( ). hidpi , . , 1:1 .

. :

NSImage *img = [[[NSImage alloc]initWithContentsOfFile:@"/Users/support/Pictures/cat.JPG"] autorelease];
NSSize size = img.size;

NSBezierPath *path = [NSBezierPath bezierPath];     
[[NSColor redColor] setStroke];
[path moveToPoint:NSMakePoint(0.0, 0.0)];
[path lineToPoint:NSMakePoint(size.width, size.height)];
[path moveToPoint:NSMakePoint(0.0, size.height)];
[path lineToPoint:NSMakePoint(size.width, 0.0)];

[path setLineWidth:1];
[path stroke];
[img unlockFocus];

[imageView setImage:img];
...

, , 2 (, , ). , :

[path setLineWidth:(img.scale > 1) ? 0.5 : 1.0];
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