# roi(forInput:arguments:outputRect:) Override this class method to implement your processor’s ROI callback. ``` class func roi(forInput inputIndex: Int32, arguments: [String : Any]?, outputRect: CGRect) -> CGRect ``` ## Parameters `inputIndex` the index that tells you which processor input for which to return the ROI rectangle. `arguments` the arguments dictionary that was passed to [`apply(withExtent:inputs:arguments:)`](/documentation/CoreImage/CIImageProcessorKernel/apply(withExtent:inputs:arguments:)). `outputRect` the output `CGRect` that processor will be asked to output. ## Return Value The `CGRect` of the `inputIndex`th input that is required for the above `outputRect` ## Discussion This will be called one or more times per render to determine what portion of the input images are needed to render a given ‘outputRect’ of the output. This will not be called if processor has no input images. The default implementation would return outputRect. > Important: this is a class method so that you cannot use or capture any state by accident. > All the parameters that affect the output results must be passed to > ``doc://com.apple.coreimage/documentation/CoreImage/CIImageProcessorKernel/apply(withExtent:inputs:arguments:)``. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)