Question

I am using some CoreImage filters to process an image. Applying the filter to my input image results in an output image called filterOutputImage of type CIImage.

I now wish to display that image, and tried doing:

self.modifiedPhoto = [UIImage imageWithCIImage:filterOutputImage];
self.photoImageView.image = self.modifiedPhoto;

The view however is blank – nothing is being displayed.

If I add logging statements that print out details about both filterOutputImage and self.modifiedPhoto, those logging statements are showing me that both those vars appear to contain legitimate image data: their size is being reported and the objects are not nil.

So after doing some Googling, I found a solution that requires going through a CGImage stage; vis:

CGImageRef outputImageRef = [context createCGImage:filterOutputImage fromRect:[filterOutputImage extent]];
self.modifiedPhoto = [UIImage imageWithCGImage:outputImageRef scale:self.originalPhoto.scale orientation:self.originalPhoto.imageOrientation];
self.photoImageView.image = self.modifiedPhoto;
CGImageRelease(outputImageRef);

This second approach works: I am getting the correct image displayed in the view.

Can someone please explain to me why my first attempt failed? What am I doing wrong with the imageWithCIImage method that is resulting in an image that seems to exist but can’t be displayed? Is it always necessary to “pass through” a CGImage stage in order to generate a UIImage from a CIImage?

Hoping someone can clear up my confusion 🙂

H.

Answer 1

I assume that self.photoImageView is a UIImageView? If so, ultimately, it is going to call -[UIImage CGImage] on the UIImage and then pass that CGImage as the contents property of a CALayer.

(See comments: my details were wrong)

Per the UIImage documentation for -[UIImage CGImage]:

If the UIImage object was initialized using a CIImage object, the
value of the property is NULL.

So the UIImageView calls -CGImage, but that results in NULL, so nothing is displayed.

I haven’t tried this, but you could try making a custom UIView and then using UIImage’s -draw... methods in -[UIView drawRect:] to draw the CIImage.