ios - UIImage在缩放时变得模糊。为什么？(iOS 5.0)

Question

UIImage在缩放的时候总是模糊，要保持清晰怎么办？

- (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;
}

Answer 1

舍入

首先，请确保在缩放之前对尺寸进行四舍五入。 drawInRect: 在这种情况下可以模糊原本可用的图像。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，您可能需要向下舍入 (floorf) 或向上舍入 (ceilf)。

CILanczosScaleTransform 不可用

然后，请忽略我之前对 CILanczosScaleTransform 的建议。虽然部分 Core Image 在 iOS 5.0 中可用，但 Lanczos 缩放不可用。如果它确实可用，请使用它。对于在 Mac OS 上工作的人来说，它是可用的，使用它。

v图像缩放

但是，vImage 中提供了一种高质量的缩放算法。 .下图显示了使用它的方法 (vImageScaledImage) 与不同上下文插值选项的比较。另请注意这些选项在不同缩放级别下的行为有何不同。

在此diagram ，它保留了最多的线条细节:

在此photograph ，比较左下方的叶子:

在此photograph ，比较右下角的纹理:

不要在 pixel art 上使用它;它会产生奇怪的缩放伪像:

虽然它在 some images 上它有有趣的舍入效果:

性能

毫不奇怪，kCGImageInterpolationHigh 是最慢的标准图像插值选项。此处实现的 vImageScaledImage 仍然较慢。为了将分形图像缩小到其原始大小的一半，它花费了 UIImageInterpolationHigh 的 110% 的时间。缩小到四分之一，花费了 340% 的时间。

如果你在模拟器中运行它，你可能会不这么认为；在那里，它可以比 kCGImageInterpolationHigh 快得多。据推测，vImage 多核优化使其在桌面上具有相对优势。

代码

// 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: http://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, 0, sourceWidth, 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,
                                                         bitsPerComponent, destBytesPerRow, colorSpace,
                                                         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;
}