cocoa - CILanczosScaleTransform 后出现奇怪的现象

Question

我正在尝试使用 Core Image 和 Lanczos Scale Transform 过滤器进行图像缩放。当我进行放大时，这很好。但在缩小并保存为 JPEG 时，我发现一类图像会产生奇怪的噪声伪影和行为。

对于 inputScale 倍数为 0.5:0.5、0.25、0.125 等，总是没问题的。对于其他 inputScale 值，它已损坏。

当我保存为 TIFF、PNG、JPEG2000 或在屏幕上绘图时 - 没问题。当我保存为 JPG 或 BMP 时 - 它损坏了。

我上传了示例图片:(original, 4272x2848, 3.4Mb)[http://www.zoomfoot.com/get/package/517e2423-7795-4239-a166-03d507ec51d8]

( scaled with noise, 1920x1280, 2.2Mb )[http://www.zoomfoot.com/get/package/6eb64d33-3a30-4e8d-9953-67ce1e7d7ef9]

它在“日落”图像上的重现性也相当好。

我尝试了更多缩放图像的方法。使用 CIAffineTransform 并使用 NSImage 本身进行绘图。两者都提供没有任何噪音的结果。

这是我用于缩放和保存的代码:

================== 兰佐斯 =============

<br/>
- (NSImage *) scaleLanczosImage:(NSString *)path
       Width:(int) desiredWidth
      Height:(int) desiredHeight
{
 CIImage *image=[CIImage imageWithContentsOfURL:
       [NSURL fileURLWithPath:path]];<p></p>

<p>CIFilter *scaleFilter = [CIFilter filterWithName:@"CILanczosScaleTransform"];
 int originalHeight=[image extent].size.height;
 int originalWidth=[image extent].size.width;</p>

<p>float yScale=(float)desiredHeight / (float)originalHeight; 
 float xScale=(float)desiredWidth / (float)originalWidth; 
 float scale=fminf(yScale, xScale);</p>

<p>[scaleFilter setValue:[NSNumber numberWithFloat:scale]
    forKey:@"inputScale"]; 
 [scaleFilter setValue:[NSNumber numberWithFloat:1.0]
    forKey:@"inputAspectRatio"];
 [scaleFilter setValue: image
    forKey:@"inputImage"];</p>

<p>image = [scaleFilter valueForKey:@"outputImage"]; </p>

<p>NSBitmapImageRep* rep = [[NSBitmapImageRep alloc] initWithCIImage:image];
 NSMutableDictionary *options=[NSMutableDictionary dictionaryWithObject:[NSDecimalNumber numberWithFloat:1.0] 
               forKey:NSImageCompressionFactor];</p>

<p>[options setValue:[NSNumber numberWithBool:YES] forKey:NSImageProgressive];
 NSData* jpegData = [rep representationUsingType:NSJPEGFileType
           properties:options];</p>

<p>[jpegData writeToFile:@"/Users/dmitry/tmp/img_4389-800x600.jpg" atomically:YES];
}  </p>

<h1></h1>

================= NSImage =============

<br/>
- (void) scaleNSImage:(NSString *)path
    Width:(int) desiredWidth
      Height:(int) desiredHeight
{
 NSImage *sourceImage = [[NSImage alloc] initWithContentsOfFile:path];
 NSImage *resizedImage = [[NSImage alloc] initWithSize: 
        NSMakeSize(desiredWidth, desiredHeight)];<p></p>

<p>NSSize originalSize = [sourceImage size];</p>

<p>[resizedImage lockFocus];
 [sourceImage drawInRect: NSMakeRect(0, 0, desiredWidth, desiredHeight) 
       fromRect: NSMakeRect(0, 0, originalSize.width, originalSize.height) 
      operation: NSCompositeSourceOver fraction: 1.0];</p>

<p>[resizedImage unlockFocus];</p>

<p>NSMutableDictionary *options=[NSMutableDictionary dictionaryWithObject:[NSDecimalNumber numberWithFloat:1.0] 
                 forKey:NSImageCompressionFactor];</p>

<p>[options setValue:[NSNumber numberWithBool:YES] forKey:NSImageProgressive];</p>

<p>NSBitmapImageRep *rep  = [[NSBitmapImageRep alloc] 
            initWithData:[resizedImage TIFFRepresentation]];</p>

<p>NSData* jpegData = [rep representationUsingType:NSJPEGFileType
           properties:options];</p>

<p>[jpegData writeToFile:@"~/nsimg_4389-800x600.jpg" atomically:YES];</p>

<p>} </p>

<h1></h1>

如果有人可以在这里解释或提出一些建议，我真的很感激。

谢谢。

Answer 1

使用 CIImage 进行缩放有一些怪癖。会this丹·伍德 (Dan Wood) 的帖子 有帮助吗？