UIImage的高质量缩放 [英] High Quality Scaling of UIImage

问题描述

我需要缩放来自iPhone应用程序中视图层的图像的分辨率。显而易见的方法是在UIGraphicsBeginImageContextWithOptions中指定一个比例因子，但是只要比例因子不是1.0，那么图像的质量就会变成底池 - 远远超过像素丢失的预期。

I need to scale the resolution of an image coming from a view layer in an iPhone application. The obvious way is to specify a scale factor in UIGraphicsBeginImageContextWithOptions, but any time the scale factor is not 1.0 then quality of the image goes to pot -- far more than would be expected from the loss of pixels.

我已经尝试了其他几种缩放技术，但它们似乎都围绕着CGContext的东西而且似乎都做了同样的事情。

I've tried several other scaling techniques, but they all seem to revolve around CGContext stuff and all appear to do the same thing.

简单地改变图像大小（不改变点分辨率）是不够的，主要是因为该信息似乎被管道中的其他人很快丢弃（图像将被转换为JPG并通过电子邮件发送）。

Simply changing image "size" (without changing the dot resolution) isn't sufficient, mostly because that info seems to be discarded very quickly by other hands in the pipeline (the image will be converted to a JPG and emailed).

有没有其他方法可以在iPhone上缩放图像？

Is there any other way to scale an image on iPhone?

推荐答案

关于 UIImage 调整大小问题，这篇文章提供许多方法来处理UIImage对象。 UIImage 有一些方向问题需要修复。 这和另一篇文章将解决它。

About UIImage resize problem, this post give many ways to handle UIImage object. The UIImage has some orientation problems need to be fixed. This and Another post will address it.

-(UIImage*)resizedImageToSize:(CGSize)dstSize
{
    CGImageRef imgRef = self.CGImage;
    // the below values are regardless of orientation : for UIImages from Camera, width>height (landscape)
    CGSize  srcSize = CGSizeMake(CGImageGetWidth(imgRef), CGImageGetHeight(imgRef)); // not equivalent to self.size (which is dependant on the imageOrientation)!

    /* Don't resize if we already meet the required destination size. */
    if (CGSizeEqualToSize(srcSize, dstSize)) {
        return self;
    }

    CGFloat scaleRatio = dstSize.width / srcSize.width;

    // Handle orientation problem of UIImage
    UIImageOrientation orient = self.imageOrientation;
    CGAffineTransform transform = CGAffineTransformIdentity;
    switch(orient) {

        case UIImageOrientationUp: //EXIF = 1
            transform = CGAffineTransformIdentity;
            break;

        case UIImageOrientationUpMirrored: //EXIF = 2
            transform = CGAffineTransformMakeTranslation(srcSize.width, 0.0);
            transform = CGAffineTransformScale(transform, -1.0, 1.0);
            break;

        case UIImageOrientationDown: //EXIF = 3
            transform = CGAffineTransformMakeTranslation(srcSize.width, srcSize.height);
            transform = CGAffineTransformRotate(transform, M_PI);
            break;

        case UIImageOrientationDownMirrored: //EXIF = 4
            transform = CGAffineTransformMakeTranslation(0.0, srcSize.height);
            transform = CGAffineTransformScale(transform, 1.0, -1.0);
            break;

        case UIImageOrientationLeftMirrored: //EXIF = 5
            dstSize = CGSizeMake(dstSize.height, dstSize.width);
            transform = CGAffineTransformMakeTranslation(srcSize.height, srcSize.width);
            transform = CGAffineTransformScale(transform, -1.0, 1.0);
            transform = CGAffineTransformRotate(transform, 3.0 * M_PI_2);
            break;  

        case UIImageOrientationLeft: //EXIF = 6  
            dstSize = CGSizeMake(dstSize.height, dstSize.width);
            transform = CGAffineTransformMakeTranslation(0.0, srcSize.width);
            transform = CGAffineTransformRotate(transform, 3.0 * M_PI_2);
            break;  

        case UIImageOrientationRightMirrored: //EXIF = 7  
            dstSize = CGSizeMake(dstSize.height, dstSize.width);
            transform = CGAffineTransformMakeScale(-1.0, 1.0);
            transform = CGAffineTransformRotate(transform, M_PI_2);
            break;  

        case UIImageOrientationRight: //EXIF = 8  
            dstSize = CGSizeMake(dstSize.height, dstSize.width);
            transform = CGAffineTransformMakeTranslation(srcSize.height, 0.0);
            transform = CGAffineTransformRotate(transform, M_PI_2);
            break;  

        default:  
            [NSException raise:NSInternalInconsistencyException format:@"Invalid image orientation"];  

    }  

    /////////////////////////////////////////////////////////////////////////////
    // The actual resize: draw the image on a new context, applying a transform matrix
    UIGraphicsBeginImageContextWithOptions(dstSize, NO, self.scale);

    CGContextRef context = UIGraphicsGetCurrentContext();

       if (!context) {
           return nil;
       }

    if (orient == UIImageOrientationRight || orient == UIImageOrientationLeft) {
        CGContextScaleCTM(context, -scaleRatio, scaleRatio);
        CGContextTranslateCTM(context, -srcSize.height, 0);
    } else {  
        CGContextScaleCTM(context, scaleRatio, -scaleRatio);
        CGContextTranslateCTM(context, 0, -srcSize.height);
    }

    CGContextConcatCTM(context, transform);

    // we use srcSize (and not dstSize) as the size to specify is in user space (and we use the CTM to apply a scaleRatio)
    CGContextDrawImage(UIGraphicsGetCurrentContext(), CGRectMake(0, 0, srcSize.width, srcSize.height), imgRef);
    UIImage* resizedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return resizedImage;
}

这篇关于UIImage的高质量缩放的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！