iOS Image Resize - ios

Question

I want to compress a (900 * 900) picture to (600*600) on an iPhone 6 plus.
If I use:
UIGraphicsBeginImageContextWithOptions(CGSizeMake(600, 600), NO, 0)
the generated UIImage becomes (1800 * 1800).
If I use:
UIGraphicsBeginImageContextWithOptions(CGSizeMake(200, 200), NO, 0)
or
UIGraphicsBeginImageContextWithOptions(CGSizeMake(600, 600), NO, 1)
the resulting image will be blurred, which is obviously wrong.
Is there a good way to solve this problem?
This is the code
- (UIImage*)imageByScalingForSize:(CGSize)targetSize withSourceImage:(UIImage *)sourceImage
{
UIImage *newImage = nil;
//Omit scaledWidth/scaledHeight calculation
CGFloat scaledWidth = targetSize.width;
CGFloat scaledHeight = targetSize.height;
//UIGraphicsBeginImageContext(targetSize);
UIGraphicsBeginImageContextWithOptions(targetSize, NO, 0);
CGRect thumbnailRect = CGRectZero;
thumbnailRect.origin = CGPointMake(0, 0);
thumbnailRect.size.width= scaledWidth;
thumbnailRect.size.height = scaledHeight;
[sourceImage drawInRect:thumbnailRect];
newImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return newImage;
}

Answer 1

Try this lib!
UIImage+Zoom.h
#import <UIKit/UIKit.h>
enum {
enSvResizeScale, // image scaled to fill
enSvResizeAspectFit, // image scaled to fit with fixed aspect. remainder is transparent
enSvResizeAspectFill, // image scaled to fill with fixed aspect. some portion of content may be cliped
};
typedef NSInteger SvResizeMode;
#interface UIImage (Zoom)
- (UIImage*)resizeImageToSize:(CGSize)newSize resizeMode:(SvResizeMode)resizeMode;
#end
UIImage+Zoom.m
#import "UIImage+Zoom.h"
#implementation UIImage (Zoom)
/*
* #brief resizeImage
* #param newsize the dimensions（pixel） of the output image
*/
- (UIImage*)resizeImageToSize:(CGSize)newSize resizeMode:(SvResizeMode)resizeMode
{
CGRect drawRect = [self caculateDrawRect:newSize resizeMode:resizeMode];
UIGraphicsBeginImageContext(newSize);
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextClearRect(context, CGRectMake(0, 0, newSize.width, newSize.height));
CGContextSetInterpolationQuality(context, 0.8);
[self drawInRect:drawRect blendMode:kCGBlendModeNormal alpha:1];
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return image;
}
// caculate drawrect respect to specific resize mode
- (CGRect)caculateDrawRect:(CGSize)newSize resizeMode:(SvResizeMode)resizeMode
{
CGRect drawRect = CGRectMake(0, 0, newSize.width, newSize.height);
CGFloat imageRatio = self.size.width / self.size.height;
CGFloat newSizeRatio = newSize.width / newSize.height;
switch (resizeMode) {
case enSvResizeScale:
{
// scale to fill
break;
}
case enSvResizeAspectFit: // any remain area is white
{
CGFloat newHeight = 0;
CGFloat newWidth = 0;
if (newSizeRatio >= imageRatio) { // max height is newSize.height
newHeight = newSize.height;
newWidth = newHeight * imageRatio;
}
else {
newWidth = newSize.width;
newHeight = newWidth / imageRatio;
}
drawRect.size.width = newWidth;
drawRect.size.height = newHeight;
drawRect.origin.x = newSize.width / 2 - newWidth / 2;
drawRect.origin.y = newSize.height / 2 - newHeight / 2;
break;
}
case enSvResizeAspectFill:
{
CGFloat newHeight = 0;
CGFloat newWidth = 0;
if (newSizeRatio >= imageRatio) { // max height is newSize.height
newWidth = newSize.width;
newHeight = newWidth / imageRatio;
}
else {
newHeight = newSize.height;
newWidth = newHeight * imageRatio;
}
drawRect.size.width = newWidth;
drawRect.size.height = newHeight;
drawRect.origin.x = newSize.width / 2 - newWidth / 2;
drawRect.origin.y = newSize.height / 2 - newHeight / 2;
break;
}
default:
break;
}
return drawRect;
}

Answer 2

If I use:
UIGraphicsBeginImageContextWithOptions(CGSizeMake(600, 600), NO, 0)
the generated UIImage becomes (1800 * 1800).
That is not quite true. It's true that the bitmap underlying the image is 1800 by 1800, but that is not the only thing about the image that it's important; the UIImage also has a scale and that scale is 3. Thus, the UIImage is in fact treated (with respect to the screen geometry) as 600 by 600, plus it is triple-resolution to match the triple-resolution of the Plus screen — which is exactly what you want.
(The scaling will work best, of course, if you also start with the triple-resolution version of your original image. But that's a different matter.)