---
title: CIKernelROICallback
framework: coreimage
role: symbol
role_heading: Type Alias
path: coreimage/cikernelroicallback
---

# CIKernelROICallback

The signature for a block that computes the region of interest (ROI) for a given area of destination image pixels. Core Image calls this block when applying the kernel. You specify this block when using the apply(extent:roiCallback:arguments:) method.

## Declaration

```swift
typealias CIKernelROICallback = (Int32, CGRect) -> CGRect
```

## Discussion

Discussion The block takes the following parameters: The block returns a CGRect structure describing the region of interest for the specified rectangle. When applying a filter kernel, the region of interest is the area of source image pixels that must be processed to produce a given area of destination image pixels. (For a more detailed definition, see The Region of Interest.) For example, a kernel that applies a blur effect in a ten-pixel radius must sample source image pixels ten pixels away in each direction from every output pixel. Thus, its region of interest is a rectangle ten pixels larger on each side than the destination rectangle: CIKernelROICallback callback = ^(int index, CGRect rect) {     return CGRectInset(rect, -10, -10); }; If your kernel does not need the image at index to produce output in the rectangle rect, your block should return CGRectNull.

## See Also

### Applying a Kernel to Filter an Image

- [apply(extent:roiCallback:arguments:)](coreimage/cikernel/apply(extent:roicallback:arguments:).md)