从 CGImage 获取像素格式

我非常了解位图布局和像素格式主题，但在处理通过加载的 png / jpeg 图像时遇到问题NSImage– 我无法弄清楚我得到的是预期行为还是错误。

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

My kCGBitmapByteOrder32Host是小尾数，这意味着像素格式也是小尾数 - 在本例中为 BGRA。但是……根据规范，png 格式是大端字节序，这就是字节在数据中的实际排列方式 - 与位图信息告诉我的相反。

有人知道发生了什么事吗？当然，系统知道如何处理这个问题，因为 png 可以正确显示。有没有一种防弹的方法来检测 CGImage 的像素格式？完全的演示项目 https://github.com/ianbytchek/cgbitmapinfo-byte-order-bug可在 GitHub 上获取。

P. S.我正在通过复制原始像素数据CFDataGetBytePtr将缓冲区放入另一个库缓冲区中，然后对其进行处理并保存。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）可以正确显示，例如：

但是相同图像的位图信息给了我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，一切都会完美运行，但我需要自动执行此检测。

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，但我发现没有提及如何将其从那里取出。

几年后，在生产中测试我的发现后，我可以充满信心地分享它们，但希望具有理论知识的人能在这里更好地解释事情吗？刷新记忆的好地方：

• 维基百科：RGBA 色彩空间 – 表示 https://en.wikipedia.org/wiki/RGBA_color_space#Representatiom
• Apple 列表：CGBitmapContextCreate 中的字节顺序 https://web.archive.org/web/20160822071631/https://lists.apple.com/archives/quartz-dev/2009/May/msg00032.html
• Apple 列表：kCGImageAlphaPremultiplied First/Last https://web.archive.org/web/20160909120259/http://lists.apple.com/archives/carbon-dev/2006/Nov/msg00014.html

在此基础上，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，您应该始终关注色彩空间– 它直接影响原始像素数据的存储方式。CGColorSpace(name: CGColorSpace.sRGB)可能是最安全的 - 它以纯格式存储颜色，例如，如果您处理红色 RGB，它将像 (255, 0, 0) 一样存储，而设备颜色空间将为您提供类似 (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中。您需要两张带 Alpha 和不带 Alpha 的单像素红色图像，this https://i.stack.imgur.com/RdR97.png and this https://i.stack.imgur.com/150G7.png应该管用。

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, context: nil, hints: nil)!
let urlImageNoAlpha: CGImage = NSImage(byReferencing: URL(fileURLWithPath: "/Users/ianbytchek/Downloads/red-pixel-no-alpha.png")).cgImage(forProposedRect: nil, context: 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, width: 1, height: 1))
    Swift.print(context.colorSpace!, context.bitmapInfo, context.bitmapInfo.pixelFormat!, Array(components))
}

这将输出以下内容。请注意屏幕捕获的图像与从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, 73, 53, 255]
<CGColorSpace 0x7fde4e915dc0> (kCGColorSpaceICCBased; kCGColorSpaceModelRGB; Color LCD) CGBitmapInfo(rawValue: 5) rgba [235, 73, 53, 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, 1, 2, 3]
<CGColorSpace 0x7fde4d60d390> (kCGColorSpaceICCBased; kCGColorSpaceModelRGB; sRGB IEC61966-2.1) CGBitmapInfo(rawValue: 1) rgba [1, 2, 3, 255]
<CGColorSpace 0x7fde4d60d390> (kCGColorSpaceICCBased; kCGColorSpaceModelRGB; sRGB IEC61966-2.1) CGBitmapInfo(rawValue: 5) rgba [1, 2, 3, 255]