"unrecognized selector" when trying to access CIFilter outputImage

Question

This is the first time I'm experimenting with Core Image (on OS X, 10.7.3) and running into a brick wall. I am sure this is something stupid that I am doing and I need someone more familiar with the framework to point me to this.

Consider the following code (suppose imageURL is a valid file URL pointing to a jpg on disk):

 CIImage *inputImage = [CIImage imageWithContentsOfURL:imageURL]; CIFilter *filter = [CIFilter filterWithName:@"CIAreaAverage" keysAndValues: kCIInputImageKey, inputImage, kCIInputExtentKey, [inputImage valueForKey:@"extent"], nil]; CIImage *outputImage = (CIImage *)[filter valueForKey:@"outputImage"]; 

When you run this code, the last line starts:

 0 CoreFoundation 0x00007fff96c2efc6 __exceptionPreprocess + 198 1 libobjc.A.dylib 0x00007fff9153cd5e objc_exception_throw + 43 2 CoreFoundation 0x00007fff96cbb2ae -[NSObject doesNotRecognizeSelector:] + 190 3 CoreFoundation 0x00007fff96c1be73 ___forwarding___ + 371 4 CoreFoundation 0x00007fff96c1bc88 _CF_forwarding_prep_0 + 232 5 CoreImage 0x00007fff8f03c38d -[CIAreaAverage outputImage] + 52 6 Foundation 0x00007fff991d8384 _NSGetUsingKeyValueGetter + 62 7 Foundation 0x00007fff991d8339 -[NSObject(NSKeyValueCoding) valueForKey:] + 392 

Now, the Master Image Filter Reference clearly states that CIAreaAverage "Returns a single-pixel image containing the average color for the region of interest." Indeed, it is even more difficult when I consider the filter attributes in the debugger (before trying the call to valueForKey: :

 (lldb) po [filter attributes] (id) $3 = 0x00007fb3e3ef0e00 { CIAttributeDescription = "Calculates the average color for the specified area in an image, returning the result in a pixel."; CIAttributeFilterCategories = ( CICategoryReduction, CICategoryVideo, CICategoryStillImage, CICategoryBuiltIn ); CIAttributeFilterDisplayName = "Area Average"; CIAttributeFilterName = CIAreaAverage; CIAttributeReferenceDocumentation = "http://developer.apple.com/cgi-bin/apple_ref.cgi?apple_ref=//apple_ref/doc/filter/ci/CIAreaAverage"; inputExtent = { CIAttributeClass = CIVector; CIAttributeDefault = "[0 0 640 80]"; CIAttributeDescription = "A rectangle that specifies the subregion of the image that you want to process."; CIAttributeDisplayName = Extent; CIAttributeType = CIAttributeTypeRectangle; CIUIParameterSet = CIUISetBasic; }; inputImage = { CIAttributeClass = CIImage; CIAttributeDescription = "The image to process."; CIAttributeDisplayName = Image; CIUIParameterSet = CIUISetBasic; }; outputImage = { CIAttributeClass = CIImage; }; } 

There outputImage right there - set as type CIImage!

So what am I doing wrong? All the documents and tutorials that I have seen show that -valueForKey: is the correct way to access attributes, including outputImage .

Answer 1

I believe your extent is the culprit (oddly enough). When I change extents like CIVector *, it works.

 NSURL *imageURL = [NSURL fileURLWithPath:@"/Users/david/Desktop/video.png"]; CIImage *inputImage = [CIImage imageWithContentsOfURL:imageURL]; CIFilter *filter = [CIFilter filterWithName:@"CIAreaAverage"]; [filter setValue:inputImage forKey:kCIInputImageKey]; CGRect inputExtent = [inputImage extent]; CIVector *extent = [CIVector vectorWithX:inputExtent.origin.x Y:inputExtent.origin.y Z:inputExtent.size.width W:inputExtent.size.height]; [filter setValue:extent forKey:kCIInputExtentKey]; CIImage *outputImage = [filter valueForKey:@"outputImage"]; 

[inputImage extent] returns a CGRect, but apparently CIVector * works better.

Answer 2

Here's how I made CIAreaAverage work in an iOS app:

 CGRect inputExtent = [self.inputImage extent]; CIVector *extent = [CIVector vectorWithX:inputExtent.origin.x Y:inputExtent.origin.y Z:inputExtent.size.width W:inputExtent.size.height]; CIImage* inputAverage = [CIFilter filterWithName:@"CIAreaAverage" keysAndValues:@"inputImage", self.inputImage, @"inputExtent", extent, nil].outputImage; //CIImage* inputAverage = [self.inputImage imageByApplyingFilter:@"CIAreaMinimum" withInputParameters:@{@"inputImage" : inputImage, @"inputExtent" : extent}]; EAGLContext *myEAGLContext = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2]; NSDictionary *options = @{ kCIContextWorkingColorSpace : [NSNull null] }; CIContext *myContext = [CIContext contextWithEAGLContext:myEAGLContext options:options]; size_t rowBytes = 32 ; // ARGB has 4 components uint8_t byteBuffer[rowBytes]; // Buffer to render into [myContext render:inputAverage toBitmap:byteBuffer rowBytes:rowBytes bounds:[inputAverage extent] format:kCIFormatRGBA8 colorSpace:nil]; const uint8_t* pixel = &byteBuffer[0]; float red = pixel[0] / 255.0; float green = pixel[1] / 255.0; float blue = pixel[2] / 255.0; NSLog(@"%f, %f, %f\n", red, green, blue); return outputImage; } @end 

The result will look something like this:

 2015-05-23 15:58:20.935 CIFunHouse[2400:489913] 0.752941, 0.858824, 0.890196 2015-05-23 15:58:20.981 CIFunHouse[2400:489913] 0.752941, 0.858824, 0.890196