当 UIImage 被缩放时，它总是会变得模糊。如果想让它保持清晰，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;
}
首先，在缩放之前确保您对大小进行四舍五入。 在这种情况下，drawInRect：可能会模糊图像。 要四舍五入为最接近的整数值：
size.width = truncf(size.width);
size.height = truncf(size.height);
对于某些任务，您可能希望向下取整（floorf）或向上取整（ceilf）。
那么，请忽略我之前关于CILanczosScaleTransform的推荐。虽然在iOS 5.0中提供了部分Core Image功能，但是Lanczos缩放却没有。如果它变得可用，请使用它。对于在Mac OS上工作的人，它是可用的，请使用它。
但是，在vImage中有一种高质量的缩放算法。以下图片展示了使用vImageScaledImage方法与不同上下文插值选项进行比较的结果。还要注意这些选项在不同的缩放级别下表现不同。
在这张图表上，它保留了最多的线条细节：
在这张照片上，比较左下角的叶子：
在这张照片上，比较右下角的纹理：
不要在像素艺术上使用它；它会创建奇怪的缩放伪影：
虽然在一些图像上它具有有趣的四舍五入效果：
毫不奇怪，kCGImageInterpolationHigh是最慢的标准图像插值选项。在此实现中，vImageScaledImage的速度更慢。将分形图像缩小到其原始大小的一半时，它需要UIImageInterpolationHigh时间的110％。将其缩小到四分之一时，它需要时间的340％。
如果您在模拟器中运行它，则可能会有所不同；在那里，它比kCGImageInterpolationHigh快得多。也许vImage多核优化在桌面上给了它一个相对优势。
// 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://dev59.com/qHRB5IYBdhLWcg3w-8A9#1262893
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;
}
UIGraphicsBeginImageContextWithOptions(size, NO, 2.0f);
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetInterpolationQuality(context, kCGInterpolationHigh);
在绘制之前进行高质量插值。
UIGraphicsBeginImageContext(rect.size);
// Turn off interpolation to keep the scaled image from being blurred.
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetInterpolationQuality(context, kCGInterpolationHigh);
[self drawInRect:rect]; // scales image to rect
UIImage *resImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
如果你正在缩小图像，仅使用kCGInterpolationHigh绘制也不是最佳选择。它很好，但CILanczosScaleTransform略微更清晰。
编辑：从iOS 5.1开始，CILanczosScaleTransform不再可用。请查看我的其他答案以获取可以使用的高质量缩放例程。