NSImage和视网膜显示混淆

Question

我想在NSImage上添加十字，下面是我的代码：

-(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];
}

在非视网膜显示器上，结果是：

​

并显示控制台：

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

但在视网膜显示屏上(我没有使用真正的视网膜显示屏，而是hidpi模式)：

​

控制台：

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

如果我省略这几行：

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

​

​

但是，如果我将NSAffineTransform scaleBy更改为1.0，结果是正确的

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

​

​

控制台：

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

有谁能解释一下吗？hidpi模式与视网膜显示是否不同？

Answer 1

我想我已经找到答案了。如果NSAffineTransform设置为NSImage的上下文，它会将坐标系转换为像素维度，即2 x点维度。即使它像这样是空的：

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

我不知道这是一个bug，还是它的工作方式。

Answer 2

重置转换是没有问题的(参考您自己的答案)。使用hidpi可以使默认转换成为让大多数高级代码按原样正常工作的转换。重置为标识变换将撤消此操作，以便将坐标系1:1与像素对齐。

不过，人们应该很少需要这样做。简化您的代码以删除图像表示和转换更改会得到以下结果：

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];
...

这应该是可行的，除非线条将改为2个物理像素宽(但当然与常规屏幕上的图像的宽度相似)。尝试这个更简单的特殊情况来解决这个问题：

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