裁剪图像时添加了细白线(Objective-C OSX) [英] Thin white lines being added when cropping an image (Objective-C OSX)

问题描述

我正在剪切大图像并将其保存为许多不同的图像.我首先在iOS中实现了它，并且工作正常，但是当我尝试将代码移植到OSX时，图像的顶部和右侧出现一条细白线(1像素).该线不是纯白色或实线(请参见下面的示例).

I am cutting up a large image and saving it into many different images. I first implemented this in iOS and it is working fine, but when I try and port the code to OSX, a thin white line (1 pixel) appears on the top and right of the image. The line is not pure white, or solid (see sample below).

这是制作一个子图像的iOS代码，其作用类似于冠军:

Here is the iOS code to make one sub-image, that works like a champ:

-(void)testMethod:(int)page forRect:(CGRect)rect{
    NSString *filePath = @"imageName";

    NSData *data = [HeavyResourceManager dataForPath:filePath];//this just gets the image as NSData
    UIImage *image = [UIImage imageWithData:data];

    CGImageRef imageRef = CGImageCreateWithImageInRect([image CGImage], rect);//crop in the rect

    UIImage *result = [UIImage imageWithCGImage:imageRef scale:0 orientation:image.imageOrientation];
    CGImageRelease(imageRef);

    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectoryPath = [paths objectAtIndex:0];

    [UIImageJPEGRepresentation(result, 1.0) writeToFile:[documentsDirectoryPath stringByAppendingPathComponent::@"output.jpg"] atomically:YES];
}

这是OSX中的移植代码，该代码导致添加白线:

Here is the ported code in OSX that causes the white lines to be added:

NSImage *source = [[[NSImage alloc]initWithContentsOfFile:imagePath] autorelease];
//init the image
NSImage *target = [[[NSImage alloc]initWithSize:panelRect.size] autorelease];
//start drawing
[target lockFocus];
[source drawInRect:NSMakeRect(0,0,panelRect.size.width,panelRect.size.height)
          fromRect:NSMakeRect(panelRect.origin.x , source.size.height - panelRect.origin.y - panelRect.size.height, panelRect.size.width, panelRect.size.height)
         operation:NSCompositeCopy
          fraction:1.0];
[target unlockFocus];

//create a NSBitmapImageRep
NSBitmapImageRep *bmpImageRep = [[[NSBitmapImageRep alloc]initWithData:[target TIFFRepresentation]] autorelease];
//write to tiff
[[target TIFFRepresentation] writeToFile:@"outputImage.tiff" atomically:NO];
[target addRepresentation:bmpImageRep];     
NSDictionary *imageProps = [NSDictionary dictionaryWithObject:[NSNumber numberWithFloat:1.0] forKey:NSImageCompressionFactor];

//get the data from the representation
NSData *data = [bmpImageRep representationUsingType: NSJPEGFileType
                                             properties: imageProps];

//write the data to a file
[data writeToFile: @"outputImage.jpg" atomically:NO];

data = [bmpImageRep representationUsingType: NSPNGFileType properties: imageProps];

//write the data to png
[data writeToFile: @"outputImage.png" atomically:NO];

上面的代码将图像保存为三种不同的格式，以检查问题是否不在特定格式的保存过程中.似乎不是因为所有格式都存在相同的问题.

The above code saves the image to three different formats to check if the problem was not in the save process of a specific format. It does not seem to be because all the formats have the same problem.

这是图像右上角的放大(4x)版本:

Here is a blown up (4x) version of top right hand corner of the images:

(OSX，请注意白线的顶部和左侧.这里看起来像是模糊的，因为图像被炸毁了)

(OSX, note the white line top and left. It looks like a blur here, because the image is blown up)

(iOS，请注意没有白线)

(iOS, note there are no white lines)

如果有人可以告诉我为什么会这样，我会很高兴.也许这与质量差异有关(OSX版本的质量似乎较低-尽管您可能没有注意到)?也许有一种完全不同的方式来做到这一点?

If someone could tell me why this might be happening, I would be very happy. Perhaps it has something to do with the quality difference (the OSX version seems lower quality - though you can't notice)? Perhaps there is a completely different way to do this?

作为参考，这是未缩放的osx图像:

For reference, here is the unscaled osx image:

更新:多亏了 Daij-Djan ，我得以停止了抗锯齿方法:

Update: Thanks to Daij-Djan, I was able to stop the drawInRect method from antialiasing:

    //start drawing on target
    [target lockFocus];
    [NSGraphicsContext saveGraphicsState];
    [[NSGraphicsContext currentContext]
            setImageInterpolation:NSImageInterpolationNone];
    [[NSGraphicsContext currentContext] setShouldAntialias:NO];

    //draw the portion of the source image on target image
    [source drawInRect:NSMakeRect(0,0,panelRect.size.width,panelRect.size.height)
              fromRect:NSMakeRect(panelRect.origin.x , source.size.height - panelRect.origin.y - panelRect.size.height, panelRect.size.width, panelRect.size.height)
             operation:NSCompositeDestinationAtop
              fraction:1.0];

    [NSGraphicsContext restoreGraphicsState];
    //end drawing
    [target unlockFocus];

---

更新:将插值"更改为NSImageInterpolationNone，因为这样可以更好地表示.高插值进行细微调整，在放大文本时值得注意.删除插值可阻止像素跳动，但是颜色仍然略有不同(灰色为164到155).能够像在iOS中一样裁剪图像真是太好了...

---

Update: Changed 'Interpolation' to NSImageInterpolationNone, as this gives a better representation. The high interpolation makes minor adjustments, which is noticeable when zooming in on text. Removing interpolation stops pixels from jumping around, but still, there is a little difference in the color (164 to 155 for a grey color). Would be great to be able to just cut up an image like I can in iOS...

推荐答案

它看起来像是抗锯齿...您必须舍入在剪切/缩放图像时计算出的浮点值.

it looks like antialiasing... you gotta round the float values you calculate when cutting/scaling the image.

在浮点值上使用froundf()

use froundf() on the float values

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