UIImage在缩放时变得模糊.为什么(iOS 5.0)

概述UI Image在缩放时总是变得模糊.如果让它保持清晰,我该怎么办？ - (UIImage *)rescaleImageToSize:(CGSize)size { CGRect rect = CGRectMake(0.0, 0.0, size.width, size.height); UIGraphicsBeginImageContext(rect.size); [self UI Image在缩放时总是变得模糊.如果让它保持清晰,我该怎么办？

- (UIImage *)rescaleImagetoSize:(CGSize)size {    CGRect rect = CGRectMake(0.0,0.0,size.wIDth,size.height);    UIGraphicsBeginImageContext(rect.size);    [self drawInRect:rect];  // scales image to rect    UIImage *resImage = UIGraphicsGetimageFromCurrentimageContext();    UIGraphicsEndImageContext();    return resImage;}

解决方法 四舍五入

首先,确保在缩放之前舍入您的大小. drawInRect：在这种情况下可以模糊其他可用的图像.舍入到最接近的整数值：

size.wIDth = truncf(size.wIDth);size.height = truncf(size.height);

对于某些任务,您可能需要舍弃(floorf)或向上舍入(ceilf).

CIlanczosScaletransform不可用

然后,不顾我以前的CIlanczosScaletransform的建议.虽然Core Image的部分内容在iOS 5.0中可用,但lanczos缩放不是.如果它有可能,使用它.对于在Mac OS上工作的人员,可以使用它.

vImage缩放

但是,在vImage中可以使用高质量的缩放算法.以下图片显示了如何使用它(vImageScaledImage)与不同的上下文插值选项进行比较.还要注意,这些选项在不同缩放级别的行为方式不一样.

在这个diagram上,它保留了最多的细节：

在这photograph,比较左下方的叶子：

在这photograph上,比较右下角的纹理：

不要在pixel art使用它它会创建奇怪的缩放工件：

虽然它在some images它有有趣的四舍五入效果：

性能

毫不奇怪,kCGImageInterpolationHigh是最慢的标准图像插值选项.这里实现的vImageScaledImage更慢.为了将分形图像缩小到其原始尺寸的一半,它花费了110％的UIImageInterpolationHigh时间.为了缩减到四分之一,它花了340％的时间.

如果您在模拟器中运行,您可能会另有想法;在那里,它可以比kCGImageInterpolationHigh快得多.大概vImage多核优化给桌面带来了一个相对的优势.

码

// Method: vImageScaledImage:(UIImage*) sourceImage withSize:(CGSize) destSize// Returns even better scaling than drawing to a context with kCGInterpolationHigh.// This employs the vImage routines in Accelerate.framework.// For more information about vImage,see https://developer.apple.com/library/mac/#documentation/performance/Conceptual/vImage/Introduction/Introduction.HTML#//apple_ref/doc/uID/TP30001001-CH201-TPXREF101// Large quantitIEs of memory are manually allocated and (hopefully) freed here.  Test your application for leaks before and after using this method.- (UIImage*) vImageScaledImage:(UIImage*) sourceImage withSize:(CGSize) destSize;{    UIImage *destimage = nil;    if (sourceImage)    {        // First,convert the UIImage to an array of bytes,in the format expected by vImage.        // Thanks: https://stackoverflow.com/a/1262893/1318452        CGImageRef sourceRef = [sourceImage CGImage];        NSUInteger sourceWIDth = CGImageGetWIDth(sourceRef);        NSUInteger sourceHeight = CGImageGetHeight(sourceRef);        CGcolorSpaceRef colorSpace = CGcolorSpaceCreateDeviceRGB();        unsigned char *sourceData = (unsigned char*) calloc(sourceHeight * sourceWIDth * 4,sizeof(unsigned char));        NSUInteger bytesPerPixel = 4;        NSUInteger sourceBytesPerRow = bytesPerPixel * sourceWIDth;        NSUInteger bitsPerComponent = 8;        CGContextRef context = CGBitmapContextCreate(sourceData,sourceWIDth,sourceHeight,bitsPerComponent,sourceBytesPerRow,colorSpace,kCGImageAlphaPremultiplIEdFirst | kCGBitmapByteOrder32Big);        CGContextDrawImage(context,CGRectMake(0,sourceHeight),sourceRef);        CGContextRelease(context);        // We Now have the source data.  Construct a pixel array        NSUInteger destWIDth = (NSUInteger) destSize.wIDth;        NSUInteger destHeight = (NSUInteger) destSize.height;        NSUInteger destBytesPerRow = bytesPerPixel * destWIDth;        unsigned char *destData = (unsigned char*) calloc(destHeight * destWIDth * 4,sizeof(unsigned char));        // Now create vImage structures for the two pixel arrays.        // Thanks: https://github.com/dhoerl/PhotoScrollerNetwork        vImage_Buffer src = {            .data = sourceData,.height = sourceHeight,.wIDth = sourceWIDth,.rowBytes = sourceBytesPerRow        };        vImage_Buffer dest = {            .data = destData,.height = destHeight,.wIDth = destWIDth,.rowBytes = destBytesPerRow        };        // Carry out the scaling.        vImage_Error err = vImageScale_ARGB8888 (                                                 &src,&dest,NulL,kvImageHighQualityresampling                                                  );        // The source bytes are no longer needed.        free(sourceData);        // Convert the destination bytes to a UIImage.        CGContextRef destContext = CGBitmapContextCreate(destData,destWIDth,destHeight,destBytesPerRow,kCGImageAlphaPremultiplIEdFirst | kCGBitmapByteOrder32Big);        CGImageRef destRef = CGBitmapContextCreateImage(destContext);        // Store the result.        destimage = [UIImage imageWithCGImage:destRef];        // Free up the remaining memory.        CGImageRelease(destRef);        CGcolorSpaceRelease(colorSpace);        CGContextRelease(destContext);        // The destination bytes are no longer needed.        free(destData);        if (err != kvImageNoError)        {            Nsstring *errorReason = [Nsstring stringWithFormat:@"vImageScale returned error code %d",err];            NSDictionary *errorInfo = [NSDictionary dictionaryWithObjectsAndKeys:                                       sourceImage,@"sourceImage",[NSValue valueWithCGSize:destSize],@"destSize",nil];            NSException *exception = [NSException exceptionWithname:@"HighQualityImageScalingFailureException" reason:errorReason userInfo:errorInfo];            @throw exception;        }    }    return destimage;}

总结

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