无法读取PNG文件的IHDR块

Question

我已经阅读了PNG文件规范并学习到，在PNG签名的前8个字节之后，我们有IHDR块。该图像指出我们有13（0x0000000D）字节长度的IHDR。{{link1：}}

我用Swift编写了一段代码来读取同一个PNG文件，并打印出从PNG签名后第4个字节开始不给我一个长度为13字节的IHDR的字节块。代码在控制台的输出结果是

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

我错过了什么，还是我读的规范已经过时了？

实际上，前8个字节是PNG签名字节。我不理解IHDR字节中期望的长度和块类型（IHDR的宽度、高度和其他字节因不同文件而异，但根据我的阅读，长度和块类型应该相同）。

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
        })
    }()
}

- Rohan Bhale

---

@MartinR 可能是文件已损坏，但其他图像查看器（例如 MacOS 上的预览）可以打开该文件并显示图像。这对我来说很奇怪。 - Rohan Bhale

"43 67 42 49" 是 "CgBI"，因此您的文件显然不是普通的PNG文件，而是（苹果专有的）CgBI文件。 - Martin R

@MartinR，您能否添加一个答案，提到苹果有自己的PNG扩展名叫做CgBI。如果像我这样的人遇到这样的文件并且无法理解它们，这将非常有帮助。 - Rohan Bhale

一切不是已经在 https://stackoverflow.com/q/34473678 中说过了吗？我们可以将其视为重复内容并关闭。但我会留给你决定，欢迎您在此添加一个（自我）答案。 - Martin R

@MartinR 看起来信息传达方面有重复。链接中的提问者担心服务器为什么不接受它。而在我的情况下，我的应用程序实际上正在读取png文件。最终似乎我和提问者都不知道CgBI格式。我还需要读取此文件的字节。 - Rohan Bhale

Answer 1

如MartinR所指出的，PNG文件存在一个名为CgBI的扩展。

普通的PNG文件有这样的结构: PNG签名后跟着IHDR块。

以下是普通PNG文件的字节示例( xx是具有可变值的占位符字节):

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文件具有一种结构，其中PNG签名后跟CgBI块，然后是IHDR块。

当我说扩展名时，请不要将其与“filename.png，filename.cgbi”等混淆。实际上，它是PNG文件所应遵循的结构的扩展。

以下是带有CgBI扩展名的PNG文件的十六进制字节表示的示例（xx是具有可变值的占位符字节）：

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=======

尽管PNG文件可以在所有图片查看器上渲染，但是CgBI扩展文件可能或可能不会在所有图片查看器上渲染，这取决于它们对此类文件的支持程度。

MacOS预览可以显示这样的图像，并且iOS上的UIImageView也能够显示我的一组示例图像（带有CgBI扩展名的PNG文件）。

- Rohan Bhale