CGImageCreateWithImageInRect图片裁剪问题

最新推荐文章于 2023-05-18 23:45:35 发布

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#iOS-裁剪 #CGImageCre

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在iOS开发中，使用CGImageCreateWithImageInRect进行图片裁剪时，可能会遇到实际裁剪结果与预期相反的情况。原因在于，此方法中的rect基于UIImage坐标系，其原点在左下角，不同于UIImageView。解决办法是将裁剪区域转换到正确坐标系。参考链接提供了解决方案。

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当我们直接使用CGImageCreateWithImageInRect获取图片时，会发现我们实际得到的截取图像，恰好是当rect旋转90°时，应该得到的图像：

- (UIImage *)cropImage:(UIImage*)image toRect:(CGRect)rect {
    CGImageRef imageRef = CGImageCreateWithImageInRect(image.CGImage,rect);
    UIImage *result = [UIImage imageWithCGImage:imageRef scale:image.scale orientation:image.imageOrientation];
    return result;

这是因为CGImageCreateWithImageInRect中得rect是获取自UIImage中得rect，而不是UIImageView的；而在UIImage的坐标系中，(0,0)点位于左下角，因此在裁剪区域确定时，需要转换成对应坐标系中得区域：以下是转换代码：

- (UIImage *)cropImage:(UIImage*)image toRect:(CGRect)rect {
    CGFloat (^rad)(CGFloat) = ^CGFloat(CGFloat deg) {
        return deg / 180.0f * (CGFloat) M_PI;
    };

    // determine the orientation of the image and apply a transformation to the crop rectangle to shift it to the correct position
    CGAffineTransform rectTransform;
    switch (image.imageOrientation) {
        case UIImageOrientationLeft:
            rectTransform = CGAffineTransformTranslate(CGAffineTransformMakeRotation(rad(90)), 0, -image.size.height);
            break;
        case UIImageOrientationRight:
            rectTransform = CGAffineTransformTranslate(CGAffineTransformMakeRotation(rad(-90)), -image.size.width, 0);
            break;
        case UIImageOrientationDown:
            rectTransform = CGAffineTransformTranslate(CGAffineTransformMakeRotation(rad(-180)), -image.size.width, -image.size.height);
            break;
        default:
            rectTransform = CGAffineTransformIdentity;
    };

    // adjust the transformation scale based on the image scale
    rectTransform = CGAffineTransformScale(rectTransform, image.scale, image.scale);

    // apply the transformation to the rect to create a new, shifted rect
    CGRect transformedCropSquare = CGRectApplyAffineTransform(rect, rectTransform);
    // use the rect to crop the image
    CGImageRef imageRef = CGImageCreateWithImageInRect(image.CGImage, transformedCropSquare);
    // create a new UIImage and set the scale and orientation appropriately
    UIImage *result = [UIImage imageWithCGImage:imageRef scale:image.scale orientation:image.imageOrientation];
    // memory cleanup
    CGImageRelease(imageRef);

    return result;
}