NSImage和Retina显示屏的混淆

Question

我想在NSImage上添加十字架，以下是我的代码：

-(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Retina显示屏上，结果如下： 控制台显示：

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

但在视网膜显示器上（我没有使用真正的视网膜显示器，但使用了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

如果我省略这些行：

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

然而，如果我将 [NSAffineTransform scaleBy] 改为 1.0，则结果是正确的。

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

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

请问有人能给一个解释吗？hidpi模式和retina显示屏有什么区别吗？

- Irwan

Answer 1

我认为我已经找到了答案。如果将NSAffineTransform设置为NSImage的上下文，它会将坐标系统转换为像素维度，即2倍的点维度。即使像这样为空：

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。

- Irwan

Answer 2

重置转换不是错误（指的是您自己的答案）。使用hidpi使默认转换成为让大多数高级代码正常工作的转换。将其重置为标识转换会撤消此操作，以使坐标系统与像素1:1对齐。

然而，很少需要这样做。简化代码以去除图像表示和转换更改，您可以得到以下内容：

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];
...

这应该可以工作，除了该行将变为2个物理像素宽（但当然与常规屏幕上的图像宽度相似）。尝试这个更简单的特殊情况来解决：

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

- Pierre Houston