如何修复Core Image的CILanczosScaleTransform过滤器边框伪像？ [英] How can I fix a Core Image's CILanczosScaleTransform filter border artifact?

问题描述

我想为iOS实现图像缩小算法。读完Core Images的 CILanczosScaleTransform 非常适合它之后，我通过以下方式实现了它：

I want to implement an image downscaling algorithm for iOS. After reading that Core Images's CILanczosScaleTransform was a great fit for it, I implemented it the following way:

public func resizeImage(_ image: UIImage, targetWidth: CGFloat) -> UIImage? {
    assert(targetWidth > 0.0)

    let scale = Double(targetWidth) / Double(image.size.width)

    guard let ciImage = CIImage(image: image) else {
        fatalError("Couldn't create CIImage from image in input")
    }

    guard let filter = CIFilter(name: "CILanczosScaleTransform") else {
        fatalError("The filter CILanczosScaleTransform is unavailable on this device.")
    }

    filter.setValue(ciImage, forKey: kCIInputImageKey)
    filter.setValue(scale, forKey: kCIInputScaleKey)

    guard let result = filter.outputImage else {
        fatalError("No output on filter.")
    }

    guard let cgImage = context.createCGImage(result, from: result.extent) else {
        fatalError("Couldn't create CG Image")
    }

    return UIImage(cgImage: cgImage)
}

它的效果很好，但是我得到的是经典的边框伪像该算法的像素邻域基础。我在Apple的文档中找不到与此相关的任何内容。 是否有比渲染更大图像然后裁剪边框来解决此问题更聪明的东西？

It works well but I get a classic border artifact probably due to the pixel-neighborhood base of the algorithm. I couldn't find anything in Apple's doc about this. Is there something smarter than rendering a bigger image and then crop the border to solve this issue?

推荐答案

您可以使用 imageByClampingToExtent 。

调用此方法...通过从原始图像的边缘重复
像素颜色来创建无限范围的图像。

Calling this method ... creates an image of infinite extent by repeating pixel colors from the edges of the original image.

您可以像这样使用它：

...
guard let ciImage = CIImage(image: image)?.clampedToExtent() else {
    fatalError("Couldn't create CIImage from image in input")
}

在此处查看更多信息： Apple Docted forextent

See more information here: Apple Doc for clampedtoextent

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