【发布时间】：2019-07-17 15:35:07
【问题描述】：
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，我们有了 IHDR 块。该图像表明我们有长度为 13(0x0000000D) 字节的 IHDR。
我在 swift 中编写了一个代码来读取相同的 png 文件并打印从 PNG 签名后块的前 4 个字节开始的长度等于 13 个字节的 IHDR 字节。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
我在这里遗漏了什么或者我阅读的规范已经过时了吗？
前 8 个字节实际上是 PNG 签名字节。 IHDR 字节是我没有得到预期长度和块类型的地方（IHDR 的宽度、高度和其他字节对于不同的文件是可变的，但长度和块类型应该与我的阅读相同）。
读取png文件的代码很简单，如下：
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
})
}()
}
【问题讨论】：
-
我用 PNG 文件尝试了您的代码，并按照您的预期打印了
IHDR length bytes: 00 00 00 0D。也许您的文件已损坏？ -
@MartinR 它可能已损坏，但随后其他图像查看器（例如 MacOS 上的预览）打开文件并显示图像。我觉得很奇怪。
-
@MartinR 我总是从不同的 png 文件中读取长度字节为 0x 00 00 00 04。这可能是我的机器特有的问题吗？
-
"43 67 42 49" 是 "CgBI"，因此您的文件显然不是常规的 PNG 文件，而是（Apple 专有的）CgBI 文件。
-
@MartinR 非常感谢您为我指明了正确的方向。这个问题提供了更多信息：stackoverflow.com/questions/34473678/…
标签： swift png nsfilehandle