CGImage from NSURL works but not from UIImage

问题

I have some issues applying filters to an image. The following code works perfect when using CIImage:imageWithContentsOfURL:

NSInteger filterIndex = [(UITapGestureRecognizer *)sender view].tag;
Filter *filter = [filters objectAtIndex:filterIndex];

CIContext *context = [CIContext contextWithOptions:nil];

CIImage *image = [CIImage imageWithContentsOfURL:[NSURL URLWithString:@"http://url.to/photo.jpg"]];

[filter.filter setValue:image forKey:kCIInputImageKey];

CIImage *result = [filter.filter valueForKey:kCIOutputImageKey];
CGRect extent = [result extent];
CGImageRef cgImage = [context createCGImage:result fromRect:extent];

[self.imageView setImage:[UIImage imageWithCGImage:cgImage]];

Filter is just a model holding filter name and CIFilter:

-(id)initWithNameAndFilter:(NSString *)theName filter:(CIFilter *)theFilter;
@property (nonatomic, strong) NSString *name;
@property (nonatomic, strong) CIFilter *filter;

But when I try to load a local UIImage with the following code:

CIImage *ciImage = [UIImage imageNamed:@"test.png"].CIImage;

The filtered image only turns white (or disappears). What am I doing wrong?

回答1:

The problem is that

[UIImage imageNamed:@"test.png"].CIImage

doesn't do what you think it does. It doesn't convert a UIImage into a CIImage, or derive a CIImage from a UIImage; it merely provides a reference to the CIImage that is already the basis of this UIImage. But this UIImage does not have a CIImage basis; it is a bitmap (CGImage), not a filter output (CIImage).

That is why, as you have rightly discovered, you have use a different method, CIImage's initWithCGImage:, which does perform the conversion. The UIImage (as I just said) does have a CGImage basis, so this works.

In other words, a UIImage is (mostly) just a wrapper. It can be a wrapper for a bitmap (CGImage) or for a filter output (CIImage). And those two methods, CGImage and CIImage, are merely ways of obtaining what is wrapped; only one of them will work on any given UIImage, and most of the time it will be CGImage.

回答2:

Juraj Antas' answer in this question How to Convert UIImage to CIImage and vice versa fixed my issue:

UIImage *u = [UIImage imageNamed:@"image.jpg"];
CIImage *image = [[CIImage alloc] initWithCGImage: u.CGImage];

来源：https://stackoverflow.com/questions/22795160/cgimage-from-nsurl-works-but-not-from-uiimage