swift – 从CGImage获取像素格式

我理解位图布局和像素格式主题很好,但在使用通过NS Image加载的png / jpeg图像时遇到问题 – 我无法弄清楚我得到的是预期的行为还是错误.

let nsImage:NSImage = NSImage(byReferencingURL: …)
let cgImage:CGImage = nsImage.CGImageForProposedRect(nil,context: nil,hints: nil)!
let bitmapInfo:CGBitmapInfo = CGImageGetBitmapInfo(cgImage)
Swift.print(bitmapInfo.contains(CGBitmapInfo.ByteOrderDefault)) // True

我的kCGBitmapByteOrder32Host是小端,这意味着像素格式也是小端 – 在这种情况下是BGRA.但是…… png格式是规范的大端,这就是字节实际排列在数据中的方式 – 与位图信息告诉我的方式相反.

有谁知道发生了什么事？当然,系统知道如何处理这个问题,因为pngs正确显示.是否有检测CGImage像素格式的防弹方式？ GitHub提供完整的demo project.

P. S.我正在通过CFDataGetBytePtr缓冲区将原始像素数据复制到另一个库缓冲区中,然后进行处理和保存.为此,我需要明确指定像素格式.我正在处理的实际图像(我检查过的任何png / jpeg文件)显示正确,例如：

但是相同图像的位图信息给出了不正确的字节序信息,导致位图被处理为BGRA像素格式而不是实际的RGBA,当我处理它时,结果如下所示：

生成的图像演示了红色和蓝色像素之间的颜色交换,如果明确指定RGBA像素格式,一切都很完美,但我需要将此检测自动化.

P. P. S.文档简要提到CGColorSpace是定义像素格式/字节顺序的另一个重要变量,但我没有提到如何将它从那里拿出来.

几年后,在测试我的生产结果后,我可以很自信地分享它们,但希望有理论知识的人能在这里更好地解释一下吗？刷新记忆的好地方：

> Wikipedia: RGBA color space – Representation
> Apple Lists: Byte Order in CGBitmapContextCreate
> Apple Lists: kCGImageAlphaPremultiplied First/Last

基于此,您可以使用以下扩展：

public enum PixelFormat
{
    case abgr
    case argb
    case bgra
    case rgba
}

extension CGBitmapInfo
{
    public static var byteOrder16Host: CGBitmapInfo {
        return CFByteOrderGetCurrent() == Int(CFByteOrderLittleEndian.rawValue) ? .byteOrder16Little : .byteOrder16Big
    }

    public static var byteOrder32Host: CGBitmapInfo {
        return CFByteOrderGetCurrent() == Int(CFByteOrderLittleEndian.rawValue) ? .byteOrder32Little : .byteOrder32Big
    }
}

extension CGBitmapInfo
{
    public var pixelFormat: PixelFormat? {

        // AlphaFirst – the alpha channel is next to the red channel,argb and bgra are both alpha first formats.
        // AlphaLast – the alpha channel is next to the blue channel,rgba and abgr are both alpha last formats.
        // LittleEndian – blue comes before red,bgra and abgr are little endian formats.
        // Little endian ordered pixels are BGR (BGRX,XBGR,BGRA,ABGR,BGR).
        // BigEndian – red comes before blue,argb and rgba are big endian formats.
        // Big endian ordered pixels are RGB (XRGB,RGBX,ARGB,RGBA,RGB).

        let alphaInfo: CGImageAlphaInfo? = CGImageAlphaInfo(rawValue: self.rawValue & type(of: self).alphaInfoMask.rawValue)
        let alphaFirst: Bool = alphaInfo == .premultipliedFirst || alphaInfo == .first || alphaInfo == .noneSkipFirst
        let alphaLast: Bool = alphaInfo == .premultipliedLast || alphaInfo == .last || alphaInfo == .noneSkipLast
        let endianLittle: Bool = self.contains(.byteOrder32Little)

        // This is slippery… while byte order host returns little endian,default bytes are stored in big endian
        // format. Here we just assume if no byte order is given,then simple RGB is used,aka big endian,though…

        if alphaFirst && endianLittle {
            return .bgra
        } else if alphaFirst {
            return .argb
        } else if alphaLast && endianLittle {
            return .abgr
        } else if alphaLast {
            return .rgba
        } else {
            return nil
        }
    }
}

请注意,您应始终注意色彩空间 – 它会直接影响原始像素数据的存储方式. CGColorSpace(名称：CGColorSpace.sRGB)可能是最安全的 – 它以普通格式存储颜色,例如,如果处理红色RGB,它将被存储就像那样(255,0),而设备颜色空间将给出你喜欢(235,73,53).

要在实践中看到这一点,请将上方和下方放入游乐场.你需要两个带有alpha和不带的单像素红色图像,this和this应该可以工作.

import AppKit
import CoreGraphics

extension CFData
{
    public var pixelComponents: [UInt8] {
        let buffer: UnsafeMutablePointer<UInt8> = UnsafeMutablePointer.allocate(capacity: 4)
        defer { buffer.deallocate(capacity: 4) }
        CFDataGetBytes(self,CFRange(location: 0,length: CFDataGetLength(self)),buffer)
        return Array(UnsafeBufferPointer(start: buffer,count: 4))
    }
}

let color: NSColor = .red
Thread.sleep(forTimeInterval: 2)

// Must flip coordinates to capture what we want…
let screen: NSScreen = NSScreen.screens.first(where: { $0.frame.contains(NSEvent.mouseLocation) })!
let rect: CGRect = CGRect(origin: CGPoint(x: NSEvent.mouseLocation.x - 10,y: screen.frame.height - NSEvent.mouseLocation.y),size: CGSize(width: 1,height: 1))

Swift.print("Will capture image with \(rect) frame.")

let screenImage: CGImage = CGWindowListCreateImage(rect,[],kCGNullWindowID,[])!
let urlImageWithAlpha: CGImage = NSImage(byReferencing: URL(fileURLWithPath: "/Users/ianbytchek/Downloads/red-pixel-with-alpha.png")).cgImage(forProposedRect: nil,hints: nil)!
let urlImageNoAlpha: CGImage = NSImage(byReferencing: URL(fileURLWithPath: "/Users/ianbytchek/Downloads/red-pixel-no-alpha.png")).cgImage(forProposedRect: nil,hints: nil)!

Swift.print(screenImage.colorSpace!,screenImage.bitmapInfo,screenImage.bitmapInfo.pixelFormat!,screenImage.dataProvider!.data!.pixelComponents)
Swift.print(urlImageWithAlpha.colorSpace!,urlImageWithAlpha.bitmapInfo,urlImageWithAlpha.bitmapInfo.pixelFormat!,urlImageWithAlpha.dataProvider!.data!.pixelComponents)
Swift.print(urlImageNoAlpha.colorSpace!,urlImageNoAlpha.bitmapInfo,urlImageNoAlpha.bitmapInfo.pixelFormat!,urlImageNoAlpha.dataProvider!.data!.pixelComponents)

let formats: [CGBitmapInfo.RawValue] = [
    CGImageAlphaInfo.premultipliedFirst.rawValue,CGImageAlphaInfo.noneSkipFirst.rawValue,CGImageAlphaInfo.premultipliedLast.rawValue,CGImageAlphaInfo.noneSkipLast.rawValue,]

for format in formats {

    // This "paints" and prints out components in the order they are stored in data.

    let context: CGContext = CGContext(data: nil,width: 1,height: 1,bitsPerComponent: 8,bytesPerRow: 32,space: CGColorSpace(name: CGColorSpace.sRGB)!,bitmapInfo: format)!
    let components: UnsafeBufferPointer<UInt8> = UnsafeBufferPointer(start: context.data!.assumingMemoryBound(to: UInt8.self),count: 4)

    context.setFillColor(red: 1 / 0xFF,green: 2 / 0xFF,blue: 3 / 0xFF,alpha: 1)
    context.fill(CGRect(x: 0,y: 0,height: 1))
    Swift.print(context.colorSpace!,context.bitmapInfo,context.bitmapInfo.pixelFormat!,Array(components))
}

这将输出以下内容.注意屏幕捕获的图像与磁盘加载的图像有何不同.

Will capture image with (285.7734375,294.5,1.0,1.0) frame.
<CGColorSpace 0x7fde4e9103e0> (kCGColorSpaceICCBased; kCGColorSpaceModelRGB; iMac) CGBitmapInfo(rawValue: 8194) bgra [27,13,252,255]
<CGColorSpace 0x7fde4d703b20> (kCGColorSpaceICCBased; kCGColorSpaceModelRGB; Color LCD) CGBitmapInfo(rawValue: 3) rgba [235,53,255]
<CGColorSpace 0x7fde4e915dc0> (kCGColorSpaceICCBased; kCGColorSpaceModelRGB; Color LCD) CGBitmapInfo(rawValue: 5) rgba [235,255]
<CGColorSpace 0x7fde4d60d390> (kCGColorSpaceICCBased; kCGColorSpaceModelRGB; sRGB IEC61966-2.1) CGBitmapInfo(rawValue: 2) argb [255,1,2,3]
<CGColorSpace 0x7fde4d60d390> (kCGColorSpaceICCBased; kCGColorSpaceModelRGB; sRGB IEC61966-2.1) CGBitmapInfo(rawValue: 6) argb [255,3]
<CGColorSpace 0x7fde4d60d390> (kCGColorSpaceICCBased; kCGColorSpaceModelRGB; sRGB IEC61966-2.1) CGBitmapInfo(rawValue: 1) rgba [1,3,255]
<CGColorSpace 0x7fde4d60d390> (kCGColorSpaceICCBased; kCGColorSpaceModelRGB; sRGB IEC61966-2.1) CGBitmapInfo(rawValue: 5) rgba [1,255]