在 Swift 中获取 UIImage 的平均颜色

Question

我最近尝试将代码从 here 转换为 Swift。但是，无论图像如何，我都会不断获得白色。这是我的代码：

// Playground - noun: a place where people can play

import UIKit

extension UIImage {
    func averageColor() -> UIColor {
        var colorSpace = CGColorSpaceCreateDeviceRGB()
        var rgba: [CGFloat] = [0,0,0,0]
        var context = CGBitmapContextCreate(&rgba, 1, 1, 8, 4, colorSpace, CGBitmapInfo.fromRaw(CGImageAlphaInfo.PremultipliedLast.toRaw())!)
        rgba

        CGContextDrawImage(context, CGRectMake(0, 0, 1, 1), self.CGImage)

        if rgba[3] > 0 {
            var alpha = rgba[3] / 255
            var multiplier = alpha / 255
            return UIColor(red: rgba[0] * multiplier, green: rgba[1] * multiplier, blue: rgba[2] * multiplier, alpha: alpha)
        } else {
            return UIColor(red: rgba[0] / 255, green: rgba[1] / 255, blue: rgba[2] / 255, alpha: rgba[3] / 255)
        }
    }
}

var img = UIImage(data: NSData(contentsOfURL: NSURL(string: "http://upload.wikimedia.org/wikipedia/commons/c/c3/Aurora_as_seen_by_IMAGE.PNG")))

img.averageColor()

提前致谢。

Answer 1

iOS 9 中的 CoreImage：使用 CIAreaAverage 过滤器并传递要平均的整个图像的范围。

此外，它的速度要快得多，因为它既可以在 GPU 上运行，也可以作为高度优化的 CPU CIKernel 运行。

Answer 2

import UIKit

extension UIImage {
    func areaAverage() -> UIColor {
        var bitmap = [UInt8](count: 4, repeatedValue: 0)

        if #available(iOS 9.0, *) {
            // Get average color.
            let context = CIContext()
            let inputImage = CIImage ?? CoreImage.CIImage(CGImage: CGImage!)
            let extent = inputImage.extent
            let inputExtent = CIVector(x: extent.origin.x, y: extent.origin.y, z: extent.size.width, w: extent.size.height)
            let filter = CIFilter(name: "CIAreaAverage", withInputParameters: [kCIInputImageKey: inputImage, kCIInputExtentKey: inputExtent])!
            let outputImage = filter.outputImage!
            let outputExtent = outputImage.extent
            assert(outputExtent.size.width == 1 && outputExtent.size.height == 1)

            // Render to bitmap.
            context.render(outputImage, toBitmap: &bitmap, rowBytes: 4, bounds: CGRect(x: 0, y: 0, width: 1, height: 1), format: kCIFormatRGBA8, colorSpace: CGColorSpaceCreateDeviceRGB())
        } else {
            // Create 1x1 context that interpolates pixels when drawing to it.
            let context = CGBitmapContextCreate(&bitmap, 1, 1, 8, 4, CGColorSpaceCreateDeviceRGB(), CGBitmapInfo.ByteOrderDefault.rawValue | CGImageAlphaInfo.PremultipliedLast.rawValue)!
            let inputImage = CGImage ?? CIContext().createCGImage(CIImage!, fromRect: CIImage!.extent)

            // Render to bitmap.
            CGContextDrawImage(context, CGRect(x: 0, y: 0, width: 1, height: 1), inputImage)
        }

        // Compute result.
        let result = UIColor(red: CGFloat(bitmap[0]) / 255.0, green: CGFloat(bitmap[1]) / 255.0, blue: CGFloat(bitmap[2]) / 255.0, alpha: CGFloat(bitmap[3]) / 255.0)
        return result
    }
}

斯威夫特 3

func areaAverage() -> UIColor {
        var bitmap = [UInt8](repeating: 0, count: 4)

        if #available(iOS 9.0, *) {
            // Get average color.
            let context = CIContext()
            let inputImage: CIImage = ciImage ?? CoreImage.CIImage(cgImage: cgImage!)
            let extent = inputImage.extent
            let inputExtent = CIVector(x: extent.origin.x, y: extent.origin.y, z: extent.size.width, w: extent.size.height)
            let filter = CIFilter(name: "CIAreaAverage", withInputParameters: [kCIInputImageKey: inputImage, kCIInputExtentKey: inputExtent])!
            let outputImage = filter.outputImage!
            let outputExtent = outputImage.extent
            assert(outputExtent.size.width == 1 && outputExtent.size.height == 1)

            // Render to bitmap.
            context.render(outputImage, toBitmap: &bitmap, rowBytes: 4, bounds: CGRect(x: 0, y: 0, width: 1, height: 1), format: kCIFormatRGBA8, colorSpace: CGColorSpaceCreateDeviceRGB())
        } else {
            // Create 1x1 context that interpolates pixels when drawing to it.
            let context = CGContext(data: &bitmap, width: 1, height: 1, bitsPerComponent: 8, bytesPerRow: 4, space: CGColorSpaceCreateDeviceRGB(), bitmapInfo: CGImageAlphaInfo.premultipliedLast.rawValue)!
            let inputImage = cgImage ?? CIContext().createCGImage(ciImage!, from: ciImage!.extent)

            // Render to bitmap.
            context.draw(inputImage!, in: CGRect(x: 0, y: 0, width: 1, height: 1))
        }

        // Compute result.
        let result = UIColor(red: CGFloat(bitmap[0]) / 255.0, green: CGFloat(bitmap[1]) / 255.0, blue: CGFloat(bitmap[2]) / 255.0, alpha: CGFloat(bitmap[3]) / 255.0)
        return result
    }

Answer 3

这里有一个解决方案：

func averageColor() -> UIColor {

    let rgba = UnsafeMutablePointer<CUnsignedChar>.alloc(4)
    let colorSpace: CGColorSpaceRef = CGColorSpaceCreateDeviceRGB()
    let info = CGBitmapInfo(CGImageAlphaInfo.PremultipliedLast.rawValue)
    let context: CGContextRef = CGBitmapContextCreate(rgba, 1, 1, 8, 4, colorSpace, info)

    CGContextDrawImage(context, CGRectMake(0, 0, 1, 1), self.CGImage)

    if rgba[3] > 0 {

        let alpha: CGFloat = CGFloat(rgba[3]) / 255.0
        let multiplier: CGFloat = alpha / 255.0

        return UIColor(red: CGFloat(rgba[0]) * multiplier, green: CGFloat(rgba[1]) * multiplier, blue: CGFloat(rgba[2]) * multiplier, alpha: alpha)

    } else {

        return UIColor(red: CGFloat(rgba[0]) / 255.0, green: CGFloat(rgba[1]) / 255.0, blue: CGFloat(rgba[2]) / 255.0, alpha: CGFloat(rgba[3]) / 255.0)
    }
}

Answer 4

斯威夫特 3：

func areaAverage() -> UIColor {

    var bitmap = [UInt8](repeating: 0, count: 4)

    let context = CIContext(options: nil)
    let cgImg = context.createCGImage(CoreImage.CIImage(cgImage: self.cgImage!), from: CoreImage.CIImage(cgImage: self.cgImage!).extent)

    let inputImage = CIImage(cgImage: cgImg!)
    let extent = inputImage.extent
    let inputExtent = CIVector(x: extent.origin.x, y: extent.origin.y, z: extent.size.width, w: extent.size.height)
    let filter = CIFilter(name: "CIAreaAverage", withInputParameters: [kCIInputImageKey: inputImage, kCIInputExtentKey: inputExtent])!
    let outputImage = filter.outputImage!
    let outputExtent = outputImage.extent
    assert(outputExtent.size.width == 1 && outputExtent.size.height == 1)

    // Render to bitmap.
    context.render(outputImage, toBitmap: &bitmap, rowBytes: 4, bounds: CGRect(x: 0, y: 0, width: 1, height: 1), format: kCIFormatRGBA8, colorSpace: CGColorSpaceCreateDeviceRGB())

    // Compute result.
    let result = UIColor(red: CGFloat(bitmap[0]) / 255.0, green: CGFloat(bitmap[1]) / 255.0, blue: CGFloat(bitmap[2]) / 255.0, alpha: CGFloat(bitmap[3]) / 255.0)
    return result
}

Answer 5

您是否正确设置了上下文？如果我查看 CGBitmapContext Reference 的文档：

https://developer.apple.com/library/ios/documentation/graphicsimaging/Reference/CGBitmapContext/index.html#//apple_ref/c/func/CGBitmapContextCreate

看起来您只为将包含在 CGFloat 数组中的图像分配了足够的内存。看起来您还告诉编译器您的图像只会是一个像素一个像素的。

当您在 CGContextDrawImage 中设置 CGRect 时，看起来该大小也被确认为一个像素一个像素。

如果 Playground 只逐个像素地创建图像，那就可以解释为什么您只看到白屏。