setting UIImageView content mode after applying a CIFIlter

Question

Thanks for looking.

Here's my code

 CIImage *result = _vignette.outputImage;
self.mainImageView.image = nil;
//self.mainImageView.contentMode = UIViewContentModeScaleAspectFit;
self.mainImageView.image = [UIImage imageWithCIImage:result];
self.mainImageView.contentMode = UIViewContentModeScaleAspectFit;

in here _vignette is correctly set up filter and image effect is applying to the image correctly.

I'm using a source image with resolution 500x375. My imageView has almost iPhone screen's resolution. So to avoid stretching I'm using AspectFit.

But after applying effect when I'm assigning the result image back to my imageView it streches. No matter which UIViewContentMode I use. It doesn't work. It seems it always applies ScaleToFill regardless the filter I've given.

Any idea why is this happening? Any suggestion is highly appreciated.

Answer 1

(1) Aspect Fit does stretch the image - to fit. If you don't want the image stretched at all, use Center (for example).

(2) imageWithCIImage gives you a very weird beast, a UIImage not based on CGImage, and so not susceptible to the normal rules of layer display. It is really nothing but a thin wrapper around CIImage, which is not what you want. You must convert (render) the CIFilter output thru CGImage to UIImage, thus giving you a UIImage that actually has some bits (CGImage, a bitmap). My discussion here gives you code that demonstrates:

http://www.apeth.com/iOSBook/ch15.html#_cifilter_and_ciimage

In other words, at some point you must call CIContext createCGImage:fromRect: to generate a CGImageRef from the output of your CIFilter, and pass that on into a UIImage. Until you do that, you don't have the output of your filter operations as a real UIImage.

Alternatively, you can draw the image from imageWithCIImage into a graphics context. For example, you can draw it into an image graphics context and then use that image.

What you can't do is display the image from imageWithCIImage directly. That's because it isn't an image! It has no underlying bitmap (CGImage). There's no there there. All it is is a set of CIFilter instructions for deriving the image.