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What is wrong with my CIAreaHistogram?

I would like to use the histogram CIAreaHistogram and CIHistogramFilter from Core Image Filter in my app. The implementation is relatively simple. Starting from a UIImage you have to create a CIImage and pass it to the filter queue. Some parts of the source code are shown below.

The Problem

When I compare the calculated and displayed histograms with histograms from 'Photoshop' or 'Photos', I found out that they do not match. The wrong histogram is the lower histogram in the picture. To determine the cause of the problem, I first developed a subroutine calculateHistogram without using these filters. The result of the calculation is displayed in the HistogramView (UIView). The calculation is correct and corresponds to the expectations (upper histogram in the picture).

In the next step, I wanted to find out which of the two filters caused the problem. For this I converted the pixel data of my calculation into a CIImage (width = 256, height = 1) and transferred this CIImage as input to the CIHistogramFilter. As can be seen in the middle histogram of the image, this histogram is also correct.

Based on these considerations, I suspect that something is wrong with the calculation in CIAreaHistogram. On the other hand, it is also possible that I have made a mistake in using the filters. Does anyone have an explanation for this problem?

Photos showing original and equalized images.

Example

Example of calculating and displaying a histogram using the Core Image Filters.

	let sourceImage = UIImage(named: name )
	let sourceCGImage = sourceImage!.cgImage!
	let sourceCIImage = CIImage(cgImage: sourceCGImage)
	
	let imageHist1 = areaHistogramFilter(sourceCIImage, rect: sourceCIImage.extent)
	
	let histImage1 = histogramDisplayFilter(imageHist1!, height: 200, highLimit: 1.0, lowLimit: 0.0)
	let cgImage1 = context.createCGImage(histImage1!, from: histImage1!.extent)
	let uiImage1 = UIImage(cgImage: cgImage1!)
	
	// Show the histogram in UIImageView
	
	histogramView1.image = uiImage1

Subroutines for the Core Image Filters

func areaHistogramFilter(_ input: CIImage, rect: CGRect, count: Int = 256, scale: Float = 15) -> CIImage?
{
	let filter = CIFilter(name:"CIAreaHistogram")
	filter?.setValue(input, forKey: kCIInputImageKey)
	filter?.setValue(CIVector(cgRect: rect), forKey: "inputExtent")
	filter?.setValue(count, forKey: "inputCount")
	filter?.setValue(scale, forKey: "inputScale")
	return filter?.outputImage
}

func histogramDisplayFilter(_ input: CIImage, height: Float = 100, highLimit: Float = 1.0, lowLimit: Float = 0.0) -> CIImage?
{
	let filter = CIFilter(name:"CIHistogramDisplayFilter")
	filter?.setValue(input,     forKey: kCIInputImageKey)
	filter?.setValue(height,    forKey: "inputHeight")
	filter?.setValue(highLimit, forKey: "inputHighLimit")
	filter?.setValue(lowLimit,  forKey: "inputLowLimit")
	return filter?.outputImage
}