UIImage在缩放时变得模糊.为什么(iOS 5.0)

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;
}