effective way to generate thumbnail from uiimage - objective-c

Question

I have an UICollectionView and each item shows a square image.
Each image is big file (> 3MB) and every time app try to jump into this view. It will delay 2-3 seconds.
I try to create a thumbnail from big file then apply to each collection item. But it seems not saving time.
Is there any effective way ?
Follow is the method I use to create thumbnail
-(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;
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;
}

Answer 1

There's a built in way to create thumbnails from compressed image files: directly using ImageIO. It is lightning fast compared to rendering the image into a smaller bitmap context.
Here's a method that does the job:
+ (UIImage *)thumbnailWithContentsOfURL:(NSURL *)URL maxPixelSize:(CGFloat)maxPixelSize
{
CGImageSourceRef imageSource = CGImageSourceCreateWithURL((__bridge CFURLRef)URL, NULL);
NSAssert(imageSource != NULL, #"cannot create image source");
NSDictionary *imageOptions = #{
(NSString const *)kCGImageSourceCreateThumbnailFromImageIfAbsent : (NSNumber const *)kCFBooleanTrue,
(NSString const *)kCGImageSourceThumbnailMaxPixelSize : #(maxPixelSize),
(NSString const *)kCGImageSourceCreateThumbnailWithTransform : (NSNumber const *)kCFBooleanTrue
};
CGImageRef thumbnail = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, (__bridge CFDictionaryRef)imageOptions);
CFRelease(imageSource);
UIImage *result = [[UIImage alloc] initWithCGImage:thumbnail];
CGImageRelease(thumbnail);
return result;
}