為什么我用以下代碼得到錯誤的像素顏色？

Question

我用backgroundcolor RED創建一個UIImage：

let theimage:UIImage=imageWithColor(UIColor(red: 1, green: 0, blue: 0, alpha: 1) );

func imageWithColor(color: UIColor) -> UIImage {
    let rect = CGRectMake(0.0, 0.0, 200.0, 200.0)
    UIGraphicsBeginImageContext(rect.size)
    let context = UIGraphicsGetCurrentContext()

    CGContextSetFillColorWithColor(context, color.CGColor)
    CGContextFillRect(context, rect)

    let image = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()

    return image
}

我正在檢索圖像中間的顏色，如下所示：

let h:CGFloat=theimage.size.height;
let w:CGFloat=theimage.size.width;


let test:UIColor=theimage.getPixelColor(CGPoint(x: 100, y: 100))

var rvalue:CGFloat = 0;
var gvalue:CGFloat = 0;
var bvalue:CGFloat = 0;
var alfaval:CGFloat = 0;
test.getRed(&rvalue, green: &gvalue, blue: &bvalue, alpha: &alfaval);


print("Blue Value : " + String(bvalue));
print("Red Value : " + String(rvalue));


extension UIImage {
    func getPixelColor(pos: CGPoint) -> UIColor {

        let pixelData = CGDataProviderCopyData(CGImageGetDataProvider(self.CGImage))
        let data: UnsafePointer<UInt8> = CFDataGetBytePtr(pixelData)

        let pixelInfo: Int = ((Int(self.size.width) * Int(pos.y)) + Int(pos.x)) * 4

        let r = CGFloat(data[pixelInfo]) / CGFloat(255.0)
        let g = CGFloat(data[pixelInfo+1]) / CGFloat(255.0)
        let b = CGFloat(data[pixelInfo+2]) / CGFloat(255.0)
        let a = CGFloat(data[pixelInfo+3]) / CGFloat(255.0)

        return UIColor(red: r, green: g, blue: b, alpha: a)
    }
}

結果我得到：

藍色值：1.0紅色值：0.0

為什么這個 ？ 我找不到錯誤。

Answer 1

問題不是內置的getRed函數，而是從提供者數據中的各個顏色組件構建UIColor對象的函數。 您的代碼假設提供者數據以RGBA格式存儲，但顯然不是。 它似乎是ARGB格式。 另外，我也不確定你的字節順序是否合適。

擁有圖像時，可以通過多種方式將這些圖像打包到提供者數據中。 Quartz 2D Programming Guide中顯示了一些示例：

如果您要使用針對特定格式硬編碼的getPixelColor例程，我可能會檢查alphaInfo和bitmapInfo （在Swift 4.2中）：

extension UIImage {
    func getPixelColor(point: CGPoint) -> UIColor? {
        guard let cgImage = cgImage,
            let pixelData = cgImage.dataProvider?.data
            else { return nil }

        let data: UnsafePointer<UInt8> = CFDataGetBytePtr(pixelData)

        let alphaInfo = cgImage.alphaInfo
        assert(alphaInfo == .premultipliedFirst || alphaInfo == .first || alphaInfo == .noneSkipFirst, "This routine expects alpha to be first component")

        let byteOrderInfo = cgImage.byteOrderInfo
        assert(byteOrderInfo == .order32Little || byteOrderInfo == .orderDefault, "This routine expects little-endian 32bit format")

        let bytesPerRow = cgImage.bytesPerRow
        let pixelInfo = Int(point.y) * bytesPerRow + Int(point.x) * 4;

        let a: CGFloat = CGFloat(data[pixelInfo+3]) / 255
        let r: CGFloat = CGFloat(data[pixelInfo+2]) / 255
        let g: CGFloat = CGFloat(data[pixelInfo+1]) / 255
        let b: CGFloat = CGFloat(data[pixelInfo  ]) / 255

        return UIColor(red: r, green: g, blue: b, alpha: a)
    }
}

如果你總是以編程方式為依賴於位圖信息的代碼構建這個圖像，我在創建圖像時明確指定了這些細節：

func image(with color: UIColor, size: CGSize) -> UIImage? {
    let rect = CGRect(origin: .zero, size: size)
    let colorSpace = CGColorSpaceCreateDeviceRGB()
    guard let context = CGContext(data: nil,
                                  width: Int(rect.width),
                                  height: Int(rect.height),
                                  bitsPerComponent: 8,
                                  bytesPerRow: Int(rect.width) * 4,
                                  space: colorSpace,
                                  bitmapInfo: CGBitmapInfo.byteOrder32Little.rawValue | CGImageAlphaInfo.premultipliedFirst.rawValue) else {
        return nil
    }
    context.setFillColor(color.cgColor)
    context.fill(rect)
    return context.makeImage().flatMap { UIImage(cgImage: $0) }
}

也許更好，如技術問答1509所示 ，您可能希望讓getPixelData顯式創建自己的預定格式的上下文，將圖像繪制到該上下文，現在代碼不依賴於原始圖像的格式你正在申請這個。

extension UIImage {

    func getPixelColor(point: CGPoint) -> UIColor? {
        guard let cgImage = cgImage else { return nil }

        let width = Int(size.width)
        let height = Int(size.height)
        let colorSpace = CGColorSpaceCreateDeviceRGB()

        guard let context = CGContext(data: nil,
                                      width: width,
                                      height: height,
                                      bitsPerComponent: 8,
                                      bytesPerRow: width * 4,
                                      space: colorSpace,
                                      bitmapInfo: CGBitmapInfo.byteOrder32Little.rawValue | CGImageAlphaInfo.premultipliedFirst.rawValue)
            else {
                return nil
        }

        context.draw(cgImage, in: CGRect(origin: .zero, size: size))

        guard let pixelBuffer = context.data else { return nil }

        let pointer = pixelBuffer.bindMemory(to: UInt32.self, capacity: width * height)
        let pixel = pointer[Int(point.y) * width + Int(point.x)]

        let r: CGFloat = CGFloat(red(for: pixel))   / 255
        let g: CGFloat = CGFloat(green(for: pixel)) / 255
        let b: CGFloat = CGFloat(blue(for: pixel))  / 255
        let a: CGFloat = CGFloat(alpha(for: pixel)) / 255

        return UIColor(red: r, green: g, blue: b, alpha: a)
    }

    private func alpha(for pixelData: UInt32) -> UInt8 {
        return UInt8((pixelData >> 24) & 255)
    }

    private func red(for pixelData: UInt32) -> UInt8 {
        return UInt8((pixelData >> 16) & 255)
    }

    private func green(for pixelData: UInt32) -> UInt8 {
        return UInt8((pixelData >> 8) & 255)
    }

    private func blue(for pixelData: UInt32) -> UInt8 {
        return UInt8((pixelData >> 0) & 255)
    }

    private func rgba(red: UInt8, green: UInt8, blue: UInt8, alpha: UInt8) -> UInt32 {
        return (UInt32(alpha) << 24) | (UInt32(red) << 16) | (UInt32(green) << 8) | (UInt32(blue) << 0)
    }

}

顯然，如果你要檢查一堆像素，你會想要重構這一點（將標准像素緩沖區的創建與檢查顏色的代碼分離），但希望這說明了這個想法。

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對於早期版本的Swift，請參閱此答案的先前版本 。