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。此图表明我们的IHDR的长度为13(0x0000000D)个字节。
我迅速编写了一个代码,以读取相同的png文件并打印字节,该字节不会给我一个IHDR,其长度等于PNG签名后的块的前4个字节中的13个字节。控制台中代码的输出为
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
我在这里错过了一些东西吗,或者我阅读的规范已经过时了?
前8个字节是实际的PNG签名字节。 IHDR字节是我无法获得预期的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,但是长度和块类型应与我的阅读相同)。
直接读取png文件的代码如下:
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
})
}()
}
答案 0 :(得分:0)
如MartinR所指出的那样,PNG文件上有一个扩展名,称为CgBI。
普通PNG文件的结构是PNG签名后跟IHDR块。
以下是普通PNG文件的十六进制表示形式的字节示例(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文件具有以下结构:PNG签名后跟CgBI块,然后是IHDR块。
当我说扩展名时,请不要将其混淆为“ filename.png,filename.cgbi”。它实际上是应该构造PNG文件的方式的扩展。
以下是具有CgBI扩展名的PNG文件的十六进制表示形式的字节示例(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=======
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文件在所有图像查看器上呈现,但扩展名CgBI可能会或可能不会在所有图像查看器上呈现,这取决于它们为此类文件提供的支持。
MacOS预览版可以显示此类图像,而iOS上的UIImageView也可以显示我的图像示例集中具有扩展名CNG的PNG文件。