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 应用程序中的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，但任何时候比例因子不是 1.0，那么图像质量就会下降——远远超过预期的像素损失.

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.

我尝试了其他几种缩放技术，但它们似乎都围绕着 CGContext 的东西展开，而且似乎都在做同样的事情.

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很快被管道中的其他人丢弃(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UIImageresize问题，这篇文章 提供了许多处理 UIImage 对象的方法.UIImage 有一些方向问题需要修复.这个和另一篇文章会解决这个问题.

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！