# convolution3X3() Applies a convolution 3 x 3 filter to the `RGBA` components of an image. ``` class func convolution3X3() -> any CIFilter & CIConvolution ``` ## Return Value The modified image. ## Discussion This method applies a 3 x 3 convolution to the `RGBA` components of an image. The effect uses a 3 x 3 area surrounding an input pixel, the pixel itself, and those within a distance of 1 pixel horizontally and vertically. The effect repeats this for every pixel within the image. The work area is then combined with the weight property vector to produce the processed image. This filter differs from the [`convolutionRGB3X3()`](/documentation/CoreImage/CIFilter-swift.class/convolutionRGB3X3()), which only processes the `RGB` color components. The convolution 3 x 3 filter uses the following properties: - `bias`: A `float` representing the value that’s added to each output pixel as a . - `weights`: A [`CIVector`](/documentation/CoreImage/CIVector) representing the convolution kernel. - `inputImage`: An image with the type [`CIImage`](/documentation/CoreImage/CIImage). > Note: > When using a nonzero `bias` value, the output image has an infinite extent. You should crop the output image before attempting to render it. The following code creates a filter that sharpens the input image: ```swift func convolution3X3(inputImage: CIImage) -> CIImage? { let convolutionFilter = CIFilter.convolution3X3() convolutionFilter.inputImage = inputImage let kernel = CIVector(values: [ 0, -2, 0, -2, 9, -2, 0, -2, 0 ], count: 9) convolutionFilter.weights = kernel convolutionFilter.bias = 0.0 return convolutionFilter.outputImage! } ```  --- 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)