how to write custom cifilter

To write a custom CIFilter, you can follow these general steps:

1. Subclass CIFilter: Create a new class that subclasses CIFilter.
2. Define Input Properties: Define the input properties for your filter, such as images or parameters.
3. Override outputImage: Override the outputImage method to implement the filter's logic.
4. Register the Filter: Register your custom filter with Core Image.

Here's a basic example in Swift:

import CoreImage

class CustomFilter: CIFilter {
    var inputImage: CIImage?
    var inputIntensity: NSNumber?

    override var outputImage: CIImage? {
        guard let inputImage = inputImage else { return nil }
        
        // Example: Apply a Gaussian Blur filter
        let filter = CIFilter(name: "CIGaussianBlur")
        filter?.setValue(inputImage, forKey: kCIInputImageKey)
        filter?.setValue(inputIntensity, forKey: kCIInputRadiusKey)
        
        return filter?.outputImage
    }
}

// Register the filter
CIFilter.registerName("CustomFilter", constructor: CustomFilter(), classAttributes: [
    kCIAttributeFilterDisplayName: "Custom Filter"
])

For more detailed information on how to work with CI filters, you can refer to the session Use HDR for dynamic image experiences in your app from WWDC 2024, which covers various aspects of working with Core Image and HDR content.