UIImageJPEGRepresentation返回零

Question

我使用CIFilters将图像转换为灰度，并应用一些图像处理效果。在UIImageView中显示输出是可行的；图像显示并按预期进行了修改。

然而，每次调用UIImageJPEGRepresentation似乎都不返回任何数据。从来都不起作用。使用原始彩色图像调用UIImageJPEGRepresentation很好。

这里发生了什么事？当显示图像时，为什么jpeg转换会失败？没有抛出异常(设置异常断点，它没有命中)，控制台中也没有出现任何消息。

let _cicontext = CIContext(options:nil)

// Set up grayscale and blur filters:
let grayIze = CIFilter(name: "CIColorControls")
let blur = CIFilter(name: "CIGaussianBlur")
grayIze.setValue(0, forKey:kCIInputSaturationKey)
grayIze.setValue(0.5, forKey: kCIInputBrightnessKey)
blur.setValue(4, forKey: kCIInputRadiusKey)

// Go!
let originalImage = CIImage(image: colorImageThatDefinitelyExists)
grayIze.setValue(originalImage, forKey: kCIInputImageKey)
blur.setValue(grayIze.outputImage, forKey: kCIInputImageKey)

let output = UIImage(CIImage: blur.outputImage)
let imageData: NSData? = UIImageJPEGRepresentation(output, 1.0) // Returns nil!?

Arbitur's answer 编辑:这是基于的工作代码

// Define an image context at the class level, which will only be initialized once:
static let imageContext = CIContext(options:nil)

// And here's the updated code in a function:
class func convertToGrayscale(image: UIImage)->UIImage?
{
    // Set up grayscale and blur filters:
    let filter1_grayIze = CIFilter(name: "CIColorControls")
    let filter2_blur = CIFilter(name: "CIGaussianBlur")
    filter1_grayIze.setValue(0, forKey:kCIInputSaturationKey)
    filter1_grayIze.setValue(0.5, forKey: kCIInputBrightnessKey)
    filter2_blur.setValue(4, forKey: kCIInputRadiusKey)

    // Go!
    let originalImage = CIImage(image: image)
    filter1_grayIze.setValue(originalImage, forKey: kCIInputImageKey)
    filter2_blur.setValue(filter1_grayIze.outputImage, forKey: kCIInputImageKey)
    let outputCIImage = filter2_blur.outputImage

    let temp:CGImageRef = imageContext.createCGImage(outputCIImage, fromRect: outputCIImage.extent())
    let ret = UIImage(CGImage: temp)
    return ret
}

// And finally, the function call:
    if let grayImage = ProfileImage.convertToGrayscale(colorImage)
    {
        let imageData: NSData? = UIImageJPEGRepresentation(grayImage, 1.0)
    }

Answer 1

我以前遇到过CIImage的问题，为了解决这个问题，我用CIImage做了一个CGImage，然后用CGImage做了一个UIImage。

Answer 2

UIImageJPEGRepresentation似乎使用了UIImage的CGImage属性。问题是，当您用一个UIImage初始化CIImage时，该属性就是nil。

我的解决方案是在UIImageJPEGRepresentation调用之前添加以下块：

上一次更新Swift 5.1

if image.cgImage == nil {
    guard 
        let ciImage = image.ciImage, 
        let cgImage = CIContext(options: nil).createCGImage(ciImage, from: ciImage.extent) 
    else { 
         return nil 
    }

    image = UIImage(cgImage: cgImage)
}

Answer 3

丹尼尔的回答很有效。下面他的答案转换为在Swift4中使用。

Swift4

    if image?.cgImage == nil {
        guard let ciImage = image?.ciImage, let cgImage = CIContext(options: nil).createCGImage(ciImage, from: ciImage.extent) else { return }

        image = UIImage(cgImage: cgImage)
    }