Performance Improvements of CIFilter Chain?

Question

In a photo app (no video), I have a number of built-in and custom Metal CIFilters chained together in a class like so (I've left out the lines to set filter parameters, other than the input image):

var colorControlsFilter = CIFilter(name: "CIColorControls")!
var highlightShadowFilter = CIFilter(name: "CIHighlightShadowAdjust")!

func filter(image data: Data) -> UIImage
{
    var outputImage: CIImage?

    let rawFilter = CIFilter(imageData: imageData, options: nil)
    outputImage = rawFilter?.outputImage

    colorControlsFilter.setValue(outputImage, forKey: kCIInputImageKey)
    outputImage = colorControlsFilter.setValue.outputImage

    highlightShadowFilter.setValue(outputImage, forKey: kCIInputImageKey)
    outputImage = highlightShadowFilter.setValue.outputImage

    ...
    ...

    if let ciImage = outputImage
    {
        return renderImage(ciImage: ciImage)
    }
}

func renderImage(ciImage: CIImage) -> UIImage?
{
    var outputImage: UIImage?
    let size = ciImage.extent.size

    UIGraphicsBeginImageContext(size)
    if let context = UIGraphicsGetCurrentContext()
    {
        context.interpolationQuality = .high
        context.setShouldAntialias(true)

        let inputImage = UIImage(ciImage: ciImage)
        inputImage.draw(in: CGRect(x: 0, y: 0, width: size.width, height: size.height))

        outputImage = UIGraphicsGetImageFromCurrentImageContext()

        UIGraphicsEndImageContext()
    }

    return outputImage
}

Processing takes about a second.

Is this way of linking together output to input of the filters the most efficient? Or more generally: What performance optimisations could I do?

Answer 1

You should use a CIContext to render the image:

var context = CIContext() // create this once and re-use it for each image

func render(image ciImage: CIImage) -> UIImage? {
    let cgImage = context.createCGImage(ciImage, from: ciImage.extent)
    return cgImage.map(UIImage.init)
}

It's important to create the CIContext only once since it's expensive to create because it's holding and caching all (Metal) resources needed for rendering the image.