swift - 无法读取 PNG 文件的 IHDR block

Question

我已经阅读了 PNG 文件规范，并了解到在 PNG 签名的前 8 个字节之后，我们有 IHDR block 。该图像表明我们的 IHDR 长度为 13(0x0000000D) 字节。

我已经用 swift 编写了一段代码来读取相同的 png 文件并打印字节，但这些字节不会为我提供长度等于 PNG 签名后 block 的前 4 个字节的 13 个字节的 IHDR。控制台中代码的输出是

PNG Signature Bytes: 89 50 4E 47 0D 0A 1A 0A 
File offset: 8
IHDR length bytes: 00 00 00 04 
File offset: 12
IHDR Chunktype bytes: 43 67 42 49 
File offset: 16
IHDR Data byte: 50 00 20 02 
File offset: 20
pngImageWidth: 1342185474
pngImageWidth: 20480
pngImageHeight: 8194

我是否在这里遗漏了某些内容，或者我阅读的规范已过时？

前 8 个字节实际上是 PNG 签名字节。 IHDR 字节是我没有获得预期长度和 block 类型的地方(IHDR 的宽度、高度和其他字节对于不同的文件是可变的，但长度和 block 类型应该与我的阅读相同)。

读取 png 文件的代码很简单，如下所示:

enum PNGFileAnatomyConstants {
    static let pngSignatureLength = 8
    static let ihdrLength = 4
    static let chunkTypeLength = 4
    static let chunkCRCLength = 4
    static let imageWidthLength = 4
    static let imageHeigthLength = 4
}

func anatomyOfPNGFile() {
    let bundle = Bundle.main

    guard let pngFileUrl = bundle.url(forResource: "PNGFileSignature", withExtension: "png") else { fatalError() }

    do {
        // Signature start------------------------------------------------------------------------------------------

        let readFileHandle = try FileHandle(forReadingFrom: pngFileUrl)
        defer {
            readFileHandle.closeFile()
        }
        let pngSignatureData = readFileHandle.readData(ofLength: PNGFileAnatomyConstants.pngSignatureLength)
        let signatureString  = pngSignatureData.hexEncodedString(options: [Data.HexEncodingOptions.upperCase])
        if signatureString != "89 50 4E 47 0D 0A 1A 0A " {
            fatalError(" Not a png")
        }
        print("PNG Signature Bytes: \(signatureString)")
        print("File offset: \(readFileHandle.offsetInFile)")
        // Signature ebd------------------------------------------------------------------------------------------




        // IHDR Length start------------------------------------------------------------------------------------------
        let ihdrLengthDataBigEndian = readFileHandle.readData(ofLength: PNGFileAnatomyConstants.ihdrLength)

        let ihdrLength: UInt32
        if PlatformEndianess.isLittleEndian {
            ihdrLength = Data(ihdrLengthDataBigEndian.reversed()).withUnsafeBytes({ (unsafePointer: UnsafePointer<UInt32>) -> UInt32 in
                return unsafePointer.pointee
            })
        } else {
            ihdrLength = ihdrLengthDataBigEndian.withUnsafeBytes({ (unsafePointer: UnsafePointer<UInt32>) -> UInt32 in
                return unsafePointer.pointee
            })
        }

        let ihdrLengthDataBigEndianString = ihdrLengthDataBigEndian.hexEncodedString(options: [.upperCase])
        print("IHDR length bytes: \(ihdrLengthDataBigEndianString)")
        print("File offset: \(readFileHandle.offsetInFile)")
        // IHDR Length end------------------------------------------------------------------------------------------





        // IHDR chunk type start------------------------------------------------------------------------------------------
        let ihdrChunkTypeData = readFileHandle.readData(ofLength: PNGFileAnatomyConstants.chunkTypeLength)
        let ihdrChunkTypeDataString  = ihdrChunkTypeData.hexEncodedString(options: [Data.HexEncodingOptions.upperCase])
        print("IHDR Chunktype bytes: \(ihdrChunkTypeDataString)")
        print("File offset: \(readFileHandle.offsetInFile)")
        // IHDR chunk type end------------------------------------------------------------------------------------------



        // IHDR data byte start------------------------------------------------------------------------------------------
        let ihdrData = readFileHandle.readData(ofLength: Int(ihdrLength))
        let ihdrDataString = ihdrData.hexEncodedString(options: [.upperCase])
        print("IHDR Data byte: \(ihdrDataString)")
        print("File offset: \(readFileHandle.offsetInFile)")
        // IHDR data byte end------------------------------------------------------------------------------------------

        do {
            let pngImageWidth: UInt32
            if PlatformEndianess.isLittleEndian {
                pngImageWidth = Data(ihdrData.reversed()).withUnsafeBytes({ (unsafePointer: UnsafePointer<UInt32>) -> UInt32 in
                    return unsafePointer.pointee
                })
            } else {
                pngImageWidth = ihdrData.withUnsafeBytes({ (unsafePointer: UnsafePointer<UInt32>) -> UInt32 in
                    return unsafePointer.pointee
                })
            }

            print("pngImageWidth: \(pngImageWidth)")
        }

        do {
            let pngImageWidth: UInt16
            let widthData = Data(bytes: [ihdrData[0], ihdrData[1]])
            if PlatformEndianess.isLittleEndian {

                pngImageWidth = Data(widthData.reversed()).withUnsafeBytes({ (unsafePointer: UnsafePointer<UInt16>) -> UInt16 in
                    return unsafePointer.pointee
                })
            } else {
                pngImageWidth = widthData.withUnsafeBytes({ (unsafePointer: UnsafePointer<UInt16>) -> UInt16 in
                    return unsafePointer.pointee
                })
            }

            print("pngImageWidth: \(pngImageWidth)")//20480

            let pngImageHeight: UInt16
            let heightData = Data(bytes: [ihdrData[2], ihdrData[3]])
            if PlatformEndianess.isLittleEndian {

                pngImageHeight = Data(heightData.reversed()).withUnsafeBytes({ (unsafePointer: UnsafePointer<UInt16>) -> UInt16 in
                    return unsafePointer.pointee
                })
            } else {
                pngImageHeight = heightData.withUnsafeBytes({ (unsafePointer: UnsafePointer<UInt16>) -> UInt16 in
                    return unsafePointer.pointee
                })
            }

            print("pngImageHeight: \(pngImageHeight)")//20480
        }

    } catch {
        fatalError(error.localizedDescription)
    }
}

extension Data {
    struct HexEncodingOptions: OptionSet {
        let rawValue: Int
        static let upperCase = HexEncodingOptions(rawValue: 1 << 0)
    }

    func hexEncodedString(options: HexEncodingOptions = []) -> String {
        let hexDigits = Array((options.contains(.upperCase) ? "0123456789ABCDEF " : "0123456789abcdef ").utf16)
        var chars: [unichar] = []
        chars.reserveCapacity(3 * count)
        for byte in self {
            chars.append(hexDigits[Int(byte / 16)])
            chars.append(hexDigits[Int(byte % 16)])
            chars.append(hexDigits.last!)
        }
        return String(utf16CodeUnits: chars, count: chars.count)
    }
}

class PlatformEndianess {
    static var isLittleEndian: Bool = {
        var integer: UInt16 = 0x0001
        return withUnsafeBytes(of: &integer, { (rawBufferPointer) -> Bool in
            return rawBufferPointer.first == 0x01
        })
    }()
}

Answer 1

正如 MartinR 所指出的PNG 文件存在一个名为 CgBI 的扩展名。

普通 PNG 文件的结构是 PNG 签名后跟 IHDR block 。

下面是普通 PNG 文件的十六进制表示形式的字节示例( xx 是具有变量值的占位符字节):

PNG Signature(8 bytes): 89 50 4E 47 0D 0A 1A 0A
=======Chunk start=======
IHDR Chunk:
    IHDR chunk length(4 bytes): 00 00 00 0D
    IHDR chunk type(Identifies chunk type to be IHDR): 49 48 44 52
    Image width in pixels(variable 4): xx xx xx xx
    Image height in pixels(variable 4): xx xx xx xx
    Flags in the chunk(variable 5 bytes): xx xx xx xx xx
    CRC checksum(variable 4 bytes): xx xx xx xx 
=======Chunk end=======

具有 CgBI 扩展名的 PNG 文件具有这样的结构:PNG 签名后跟 CgBI block ，然后是 IHDR block 。

当我说扩展名时，请不要将其与“filename.png、filename.cgbi”混淆。它实际上是 PNG 文件结构方式的扩展。

下面是带有 CgBI 扩展名的 PNG 文件的十六进制表示字节示例(xx 是具有变量值的占位符字节):

PNG Signature(8 bytes): 89 50 4E 47 0D 0A 1A 0A
=======Chunk start=======
CgBI Chunk:
    CgBI chunk length(4 bytes): 00 00 00 04
    CgBI chunk type(Identifies chunk type to be CgBI): 43 67 42 49
    CgBI info flags(4 bytes): xx xx xx xx
    CRC checksum(variable 4 bytes): xx xx xx xx 
=======Chunk end=======
=======Chunk start=======
IHDR Chunk:
    IHDR chunk length(4 bytes): 00 00 00 0D
    IHDR chunk type(Identifies chunk type to be IHDR): 49 48 44 52
    Image width in pixels(variable 4): xx xx xx xx
    Image height in pixels(variable 4): xx xx xx xx
    Flags in the chunk(variable 5 bytes): xx xx xx xx xx
    CRC checksum(variable 4 bytes): xx xx xx xx 
=======Chunk end=======

虽然 PNG 文件可以在所有图像查看器上呈现，但扩展名 CgBI 可能会也可能不会在所有图像查看器上呈现，具体取决于它们为此类文件提供的支持。

MacOS 预览可以显示此类图像，iOS 上的 UIImageView 也可以显示我的示例图像集中的文件(带有 CgBI 扩展名的 PNG)。