Burger::GIFEncoder Class Reference

GIF style LZW compressor. More...

Collaboration diagram for Burger::GIFEncoder:

Public Member Functions
	GIFEncoder ()
	Init the encoder to an inert state.
uint_t	GetClearCode (void) const
	Return the LZW Clear code.
uint_t	GetEOFCode (void) const
	Return the LZW End of File code.
void	Init (OutputMemoryStream *pOutput, uint_t uDepth)
	Given a palette depth, create a GIF LZW encoder.
void	ClearHash (void)
	Erase the GIF LZW hash.
uint_t	LookupHash (uint_t uInput)
	Given a code, create a hash index.
void	AddHashEntry (uint_t uKey, uint_t uCode)
	Insert a new code into the hash.
void	WriteByte (uint_t uInput)
	Write a byte of data to the GIF byte stream.
void	WriteCode (uint_t uCode)
	Given an LZW code, encode it into the bit stream.
void	WritePixels (const uint8_t *pInput, uintptr_t uInputLength)
	Encode a scan line of pixels.
void	Flush (void)
	Encoding is complete, flush the buffer.

Static Public Member Functions
static GIFEncoder *	New (void)
	Allocate and initialize a GIFEncoder.
static uint_t	GetHashKey (uint_t uInput)
	Create a hash key.
static uint_t	GetKey (uint_t uInput)
	Return the LZW key from a hash.
static uint_t	PutKey (uint_t uInput)
	Prepare a LZW key to form a key.
static uint_t	GetCode (uint_t uInput)
	Return the LZW code from a hash.
static uint_t	PutCode (uint_t uInput)
	Prepare a LZW code to form a key.

Private Types
enum	eEncodingState { ENCODE_INPROGRESS , ENCODE_FINAL }

Private Attributes
OutputMemoryStream *	m_pOutput
	Pointer to the stream to store the compressed data.
uint_t	m_uColorDepth
	Number of bits of color data (8 is the default)
uint_t	m_uRunningCode
	Current LZW code.
uint_t	m_uRunningBits
	Number of valid bits in the current LZW code.
uint_t	m_uMaximumRunningCode
	If the current LZW matches this valid, increase bit size.
uint_t	m_uCode
	Value being encoded.
uint_t	m_uShiftState
	Number of bits in the bit bucket.
uint_t	m_uShiftData
	Bit bucket.
eEncodingState	m_eEncodingState
	Flag for determining if a flush is needed on destruction.
uint_t	m_uBytesInBuffer
	Number of LZW bytes cached before a data flush.
uint8_t	m_Buffer [256]
	Buffer of encoded LZW bytes.
uint_t	m_HashTable [cHashSize]
	Hash for quick match in the LZW buffer for encoding strings.

Static Private Attributes
static const uint_t	cHashSize = 1U<<13U
	8192 entry (13 bit) hash table
static const uint_t	cHashKeyMask = cHashSize-1U
	Mask for the hash size.
static const uint_t	cLZBits = 12U
	Number of bits for an LZW token.
static const uint_t	cLZLastCode = (1U<<cLZBits)-1U
	Largest allowable LZW token.
static const uint_t	cStartingCode = (1U<<cLZBits)
	Impossible LZW token for alerting encoding hasn't started.

Detailed Description

GIF style LZW compressor.

---

This class will compress pixel data into using GIF LZW format

See also

FileGIF or GIFDecoder

Member Enumeration Documentation

eEncodingState

enum Burger::GIFEncoder::eEncodingState

private

Enumerator
ENCODE_INPROGRESS	Encoding is in progress.
ENCODE_FINAL	Encoding is complete, don't output to the stream anymore.

Constructor & Destructor Documentation

GIFEncoder()

Burger::GIFEncoder::GIFEncoder

(

)

Init the encoder to an inert state.

---

Member Function Documentation

AddHashEntry()

void Burger::GIFEncoder::AddHashEntry	(	uint_t	uKey,
		uint_t	uCode )

Insert a new code into the hash.

---

Given a hash key, look for an empty entry in the hash and insert the LZW token into it.

Parameters

uKey	Hash key (Index into the hash)
uCode	LZW code

ClearHash()

void Burger::GIFEncoder::ClearHash

(

void

)

Erase the GIF LZW hash.

---

Flush()

void Burger::GIFEncoder::Flush

(

void

)

Encoding is complete, flush the buffer.

---

At the conclusion of compression, call this function to perform cleanup and flush out any cached data

GetClearCode()

uint_t Burger::GIFEncoder::GetClearCode ( void ) const

inline

Return the LZW Clear code.

---

Based on the bit depth, return the code to clear the LZW cache

Returns

LZW clear code

GetCode()

uint_t Burger::GIFEncoder::GetCode ( uint_t uInput )

inlinestatic

Return the LZW code from a hash.

---

Parameters

uInput

Hash value

Returns

LZW code

GetEOFCode()

uint_t Burger::GIFEncoder::GetEOFCode ( void ) const

inline

Return the LZW End of File code.

---

Based on the bit depth, return the code to end decompression of LZW data

Returns

LZW EOF code

GetHashKey()

uint_t Burger::GIFEncoder::GetHashKey ( uint_t uInput )

inlinestatic

Create a hash key.

---

Given a 16 bit value of the current and previous pixel, return a hash value to index into the hash table

Parameters

uInput

Value with LZW code shifted left by 8 bits and the pixel in the lower 8 bits

Returns

Hash of the key

GetKey()

uint_t Burger::GIFEncoder::GetKey ( uint_t uInput )

inlinestatic

Return the LZW key from a hash.

---

Parameters

uInput

Hash value

Returns

LZW Key

Init()

void Burger::GIFEncoder::Init	(	Burger::OutputMemoryStream *	pOutput,
		uint_t	uDepth )

Given a palette depth, create a GIF LZW encoder.

---

Call this function before any encoding is performed.

Parameters

pOutput	Pointer to a valid OutputMemoryStream to store the compressed data
uDepth	Depth of the pixel data in bits (2-8)

LookupHash()

uint_t Burger::GIFEncoder::LookupHash

(

uint_t

uInput

)

Given a code, create a hash index.

---

Scan the hash table for a LZW token or return BURGER_MAXUINT if there is no matching entry.

Parameters

uInput

Encoded pixel and previous pixel

Returns

BURGER_MAXUINT on error or a valid 12 bit hash index

New()

Burger::GIFEncoder *BURGER_API Burger::GIFEncoder::New ( void )

static

Allocate and initialize a GIFEncoder.

---

Returns

A pointer to a default GIFEncoder class or NULL if out of memory

See also

Delete(const T *)

PutCode()

uint_t Burger::GIFEncoder::PutCode ( uint_t uInput )

inlinestatic

Prepare a LZW code to form a key.

---

Parameters

uInput

LZW code

Returns

Code shifted to a position for a key

PutKey()

uint_t Burger::GIFEncoder::PutKey ( uint_t uInput )

inlinestatic

Prepare a LZW key to form a key.

---

Parameters

uInput

LZW token

Returns

Token shifted to a position for a key

WriteByte()

void Burger::GIFEncoder::WriteByte

(

uint_t

uInput

)

Write a byte of data to the GIF byte stream.

---

Cache a byte of output data and send it out as a "pascal" string with a length followed by a stream of data. Only write to the stream when 255 bytes have been accumulated

Parameters

uInput

8 bit data to write to the byte stream

WriteCode()

void Burger::GIFEncoder::WriteCode

(

uint_t

uCode

)

Given an LZW code, encode it into the bit stream.

---

Write out an LZW code using bit packing.

Parameters

uCode

Bit pattern for LZW code

WritePixels()

void Burger::GIFEncoder::WritePixels	(	const uint8_t *	pInput,
		uintptr_t	uInputLength )

Encode a scan line of pixels.

---

Given a scan line of pixels, compress using LZW. Assume the pixels are bytes.

Parameters

pInput	Pointer to an array of pixels for a single scan line
uInputLength	Number of pixels to encode

Member Data Documentation

cHashKeyMask

const uint_t Burger::GIFEncoder::cHashKeyMask = cHashSize-1U

staticprivate

Mask for the hash size.

cHashSize

const uint_t Burger::GIFEncoder::cHashSize = 1U<<13U

staticprivate

8192 entry (13 bit) hash table

cLZBits

const uint_t Burger::GIFEncoder::cLZBits = 12U

staticprivate

Number of bits for an LZW token.

cLZLastCode

const uint_t Burger::GIFEncoder::cLZLastCode = (1U<<cLZBits)-1U

staticprivate

Largest allowable LZW token.

cStartingCode

const uint_t Burger::GIFEncoder::cStartingCode = (1U<<cLZBits)

staticprivate

Impossible LZW token for alerting encoding hasn't started.

m_Buffer

uint8_t Burger::GIFEncoder::m_Buffer[256]

private

Buffer of encoded LZW bytes.

m_eEncodingState

eEncodingState Burger::GIFEncoder::m_eEncodingState

private

Flag for determining if a flush is needed on destruction.

m_HashTable

uint_t Burger::GIFEncoder::m_HashTable[cHashSize]

private

Hash for quick match in the LZW buffer for encoding strings.

m_pOutput

OutputMemoryStream* Burger::GIFEncoder::m_pOutput

private

Pointer to the stream to store the compressed data.

m_uBytesInBuffer

uint_t Burger::GIFEncoder::m_uBytesInBuffer

private

Number of LZW bytes cached before a data flush.

m_uCode

uint_t Burger::GIFEncoder::m_uCode

private

Value being encoded.

m_uColorDepth

uint_t Burger::GIFEncoder::m_uColorDepth

private

Number of bits of color data (8 is the default)

m_uMaximumRunningCode

uint_t Burger::GIFEncoder::m_uMaximumRunningCode

private

If the current LZW matches this valid, increase bit size.

m_uRunningBits

uint_t Burger::GIFEncoder::m_uRunningBits

private

Number of valid bits in the current LZW code.

m_uRunningCode

uint_t Burger::GIFEncoder::m_uRunningCode

private

Current LZW code.

m_uShiftData

uint_t Burger::GIFEncoder::m_uShiftData

private

Bit bucket.

m_uShiftState

uint_t Burger::GIFEncoder::m_uShiftState

private

Number of bits in the bit bucket.