# apply: Produces a [`CIImage`](/documentation/CoreImage/CIImage) object by applying a kernel function. ``` - (CIImage *) apply:(CIKernel *) k; ``` ## Parameters `k` A [`CIKernel`](/documentation/CoreImage/CIKernel) object that contains a kernel function. ## Discussion If you are implementing a custom filter, this method needs to be called from within the [`outputImage`](/documentation/CoreImage/CIFilter-swift.class/outputImage) method in order to apply your kernel function to the [`CIImage`](/documentation/CoreImage/CIImage) object. For example, if the kernel function has this signature: ```objc kernel vec4 brightenEffect (sampler src, float k) ``` You would supply two arguments after the `k` argument to the `apply:k, ...` method. In this case, the first argument must be a sampler and the second a floating-point value. For more information on kernels, see [Core Image Kernel Language Reference](https://developer.apple.com/library/archive/documentation/GraphicsImaging/Reference/CIKernelLangRef/Introduction/Introduction.html#//apple_ref/doc/uid/TP40004397). --- 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)