Input Power value for CIGammaAdjust filter in Core Image

Question

I want to perform auto gamma correction using the Core Image filter CIGammaAdjust. CIGammaAdjust requires "inputPower" value to be set as in the following code:

CIFilter* gammaFilter = [CIFilter filterWithName:@"CIGammaAdjust"];
[gammaFilter setValue:resultImage forKey:@"inputImage"];
[gammaFilter setValue:???  forKey: @"inputPower"];
resultImage = [gammaFilter valueForKey: @"outputImage"];

The question is, how should I calculate the value for Input Power? I have the gamma value of the image calculated. How do I convert the Image gamma value to input power? I tried using just the reciprocal of gamma value but that doesn't seem to give the right result.

Answer 1

https://developer.apple.com/library/mac/documentation/graphicsimaging/reference/CoreImageFilterReference/Reference/reference.html#//apple_ref/doc/uid/TP30000136-DontLinkElementID_69

This is what the mac dev library says, "...

inputPower: An NSNumber object whose attribute type is CIAttributeTypeScalar and whose display name is Power.

Default value: 0.75

Discussion: This filter is typically used to compensate for nonlinear effects of displays. Adjusting the gamma effectively changes the slope of the transition between black and white. It uses the following formula:

pow(s.rgb, vec3(power))

So to answer your question, try ratios. The max of inputPower is I believe 3.00, and the minimum is probably 0 to .1. I'd take your gamma value and divid it by the max possible gamma value. Then multiply that found value by the max inputPower value (2.9 to 3).

Hope this helps!

Answer 2

Swift: 5.6, iOS: 15.5

    if let currentFilter = CIFilter(name: "CIGammaAdjust") {
        let inputImage = CIImage(image: imgSRreSize)
        currentFilter.setValue(inputImage, forKey: kCIInputImageKey)
        currentFilter.setValue(2, forKey: "inputPower")

        if let output = currentFilter.outputImage {
            if let cgimg = context.createCGImage(output, from: output.extent) {
                let processedImage = UIImage(cgImage: cgimg)
                let finalImg = processedImage
                //Use finalImage

            }
        } else {
            print("failed")

        }
        
    }