Opencv中jpeg編碼完整流程分析
阿新 • • 發佈：2019-02-04
本文分析了Opencv中jpeg的編碼流程，希望能夠在加速jpeg編碼效率上獲得一些啟發
從Java層開始，Opencv 2.4.13中imencode函式封裝在了Highgui類中，而3.0.0以後，Highgui類被取締，相關編解碼操作放在了ImgCodecs這個類裡面估計是為了Java和C++保持統一風格吧。這裡直接呼叫即可
Highgui.imencode(String ext, Mat img, MatOfByte buf)
Highgui.imencode(String ext, Mat img, MatOfByte buf, MatOfInt params)以上可以通過MatOfInt可指定編碼時相關引數
開始看到這個時，感覺很奇怪，為啥要用MatOfInt來指定引數呢？？？不用著急，我們接著往下看
以上兩個函式分別對應的native方法如下
不難發現，Opencv還是遵循著以往的JNI風格，所有物件都只傳遞物件起始地址，並且物件可以通過該地址恢復物件所有資訊，即存在以該地址為引數的建構函式// C++: bool imencode(String ext, Mat img, vector_uchar& buf, vector_int params = std::vector<int>()) private static native boolean imencode_0(String ext, long img_nativeObj, long buf_mat_nativeObj, long params_mat_nativeObj); private static native boolean imencode_1(String ext, long img_nativeObj, long buf_mat_nativeObj);
我們接著往下，看看JNI層的程式碼，這裡以3.1.0為例，程式碼位於OPENCV_HOME/release/modules/java/imgcodecs.cpp中
JNIEXPORT jboolean JNICALL Java_org_opencv_imgcodecs_Imgcodecs_imencode_10 (JNIEnv*, jclass, jstring, jlong, jlong, jlong); JNIEXPORT jboolean JNICALL Java_org_opencv_imgcodecs_Imgcodecs_imencode_10 (JNIEnv* env, jclass , jstring ext, jlong img_nativeObj, jlong buf_mat_nativeObj, jlong params_mat_nativeObj) { static const char method_name[] = "imgcodecs::imencode_10()"; try { LOGD("%s", method_name); std::vector<uchar> buf; Mat& buf_mat = *((Mat*)buf_mat_nativeObj); std::vector<int> params; Mat& params_mat = *((Mat*)params_mat_nativeObj); Mat_to_vector_int( params_mat, params ); const char* utf_ext = env->GetStringUTFChars(ext, 0); String n_ext( utf_ext ? utf_ext : "" ); env->ReleaseStringUTFChars(ext, utf_ext); Mat& img = *((Mat*)img_nativeObj); bool _retval_ = cv::imencode( n_ext, img, buf, params ); vector_uchar_to_Mat( buf, buf_mat ); return _retval_; } catch(const std::exception &e) { throwJavaException(env, &e, method_name); } catch (...) { throwJavaException(env, 0, method_name); } return 0; } JNIEXPORT jboolean JNICALL Java_org_opencv_imgcodecs_Imgcodecs_imencode_11 (JNIEnv*, jclass, jstring, jlong, jlong); JNIEXPORT jboolean JNICALL Java_org_opencv_imgcodecs_Imgcodecs_imencode_11 (JNIEnv* env, jclass , jstring ext, jlong img_nativeObj, jlong buf_mat_nativeObj) { static const char method_name[] = "imgcodecs::imencode_11()"; try { LOGD("%s", method_name); std::vector<uchar> buf; Mat& buf_mat = *((Mat*)buf_mat_nativeObj); const char* utf_ext = env->GetStringUTFChars(ext, 0); String n_ext( utf_ext ? utf_ext : "" ); env->ReleaseStringUTFChars(ext, utf_ext); Mat& img = *((Mat*)img_nativeObj); <span style="color:#FF0000;">bool _retval_ = cv::imencode( n_ext, img, buf );</span> vector_uchar_to_Mat( buf, buf_mat ); return _retval_; } catch(const std::exception &e) { throwJavaException(env, &e, method_name); } catch (...) { throwJavaException(env, 0, method_name); } return 0; }
以帶參編譯的方法為例，也就是imencode10()，首先從地址分別構造出原Java中的MatOfByte和ParamsMat兩個Mat的資料結構，分別用於儲存encode完的資料和相應傳入的編碼引數，接下來param_mat轉換成了一個int的vector，我們跟進看下發現
void Mat_to_vector_int(Mat& mat, std::vector<int>& v_int)
{
v_int.clear();
CHECK_MAT(mat.type()==CV_32SC1 && mat.cols==1);
v_int = (std::vector<int>) mat;
}轉換過程相當簡單。。。其實這個原來Java中MatOfInt的類，儲存的其實就是一系列的32為signed int的值而已。。。這些data是可以直接進行型別轉換的
在Java層我們僅需要這樣構建params引數即可
MatOfInt params = new MatOfInt(Imgcodecs.CV_IMWRITE_JPEG_QUALITY, 90);
對應的構造方法原型為
public MatOfInt(int...a)
我們接著回到正題，拿到編譯引數以後，真正的編碼呼叫的是cv::imencode，這個函式的實現是在imgcodes/src下面的loadsave.cpp中
bool imencode( const String& ext, InputArray _image,
std::vector<uchar>& buf, const std::vector<int>& params )
{
Mat image = _image.getMat();
int channels = image.channels();
CV_Assert( channels == 1 || channels == 3 || channels == 4 );
ImageEncoder encoder = findEncoder( ext );
if( !encoder )
CV_Error( CV_StsError, "could not find encoder for the specified extension" );
if( !encoder->isFormatSupported(image.depth()) )
{
CV_Assert( encoder->isFormatSupported(CV_8U) );
Mat temp;
image.convertTo(temp, CV_8U);
image = temp;
}
bool code;
if( encoder->setDestination(buf) )
{
code = encoder->write(image, params);
encoder->throwOnEror();
CV_Assert( code );
}
else
{
String filename = tempfile();
code = encoder->setDestination(filename);
CV_Assert( code );
code = encoder->write(image, params);
encoder->throwOnEror();
CV_Assert( code );
FILE* f = fopen( filename.c_str(), "rb" );
CV_Assert(f != 0);
fseek( f, 0, SEEK_END );
long pos = ftell(f);
buf.resize((size_t)pos);
fseek( f, 0, SEEK_SET );
buf.resize(fread( &buf[0], 1, buf.size(), f ));
fclose(f);
remove(filename.c_str());
}
return code;
}
整個imencode的流程如下，先檢查影象的通道數，然後檢查檔案型別判斷是否支援，每個畫素點的單通道是否是8bit，接下來setDescription的邏輯也非常簡單
bool BaseImageEncoder::setDestination( std::vector<uchar>& buf )
{
if( !m_buf_supported )
return false;
m_buf = &buf;
m_buf->clear();
m_filename = String();
return true;
}如上，就是清空一下快取區，接下來就進入了對應encoder的write函式，對應JpegEncoder的原始碼如下
bool JpegEncoder::write( const Mat& img, const std::vector<int>& params )
{
m_last_error.clear();
struct fileWrapper
{
FILE* f;
fileWrapper() : f(0) {}
~fileWrapper() { if(f) fclose(f); }
};
volatile bool result = false;
fileWrapper fw;
int width = img.cols, height = img.rows;
std::vector<uchar> out_buf(1 << 12);
AutoBuffer<uchar> _buffer;
uchar* buffer;
struct jpeg_compress_struct cinfo;
JpegErrorMgr jerr;
JpegDestination dest;
jpeg_create_compress(&cinfo);
cinfo.err = jpeg_std_error(&jerr.pub);
jerr.pub.error_exit = error_exit;
if( !m_buf )
{
fw.f = fopen( m_filename.c_str(), "wb" );
if( !fw.f )
goto _exit_;
jpeg_stdio_dest( &cinfo, fw.f );
}
else
{
dest.dst = m_buf;
dest.buf = &out_buf;
jpeg_buffer_dest( &cinfo, &dest );
dest.pub.next_output_byte = &out_buf[0];
dest.pub.free_in_buffer = out_buf.size();
}
if( setjmp( jerr.setjmp_buffer ) == 0 )
{
cinfo.image_width = width;
cinfo.image_height = height;
int _channels = img.channels();
int channels = _channels > 1 ? 3 : 1;
cinfo.input_components = channels;
cinfo.in_color_space = channels > 1 ? JCS_RGB : JCS_GRAYSCALE;
int quality = 95;
int progressive = 0;
int optimize = 0;
int rst_interval = 0;
int luma_quality = -1;
int chroma_quality = -1;
for( size_t i = 0; i < params.size(); i += 2 )
{
if( params[i] == CV_IMWRITE_JPEG_QUALITY )
{
quality = params[i+1];
quality = MIN(MAX(quality, 0), 100);
}
if( params[i] == CV_IMWRITE_JPEG_PROGRESSIVE )
{
progressive = params[i+1];
}
if( params[i] == CV_IMWRITE_JPEG_OPTIMIZE )
{
optimize = params[i+1];
}
if( params[i] == CV_IMWRITE_JPEG_LUMA_QUALITY )
{
if (params[i+1] >= 0)
{
luma_quality = MIN(MAX(params[i+1], 0), 100);
quality = luma_quality;
if (chroma_quality < 0)
{
chroma_quality = luma_quality;
}
}
}
if( params[i] == CV_IMWRITE_JPEG_CHROMA_QUALITY )
{
if (params[i+1] >= 0)
{
chroma_quality = MIN(MAX(params[i+1], 0), 100);
}
}
if( params[i] == CV_IMWRITE_JPEG_RST_INTERVAL )
{
rst_interval = params[i+1];
rst_interval = MIN(MAX(rst_interval, 0), 65535L);
}
}
jpeg_set_defaults( &cinfo );
cinfo.restart_interval = rst_interval;
jpeg_set_quality( &cinfo, quality,
TRUE /* limit to baseline-JPEG values */ );
if( progressive )
jpeg_simple_progression( &cinfo );
if( optimize )
cinfo.optimize_coding = TRUE;
#if JPEG_LIB_VERSION >= 70
if (luma_quality >= 0 && chroma_quality >= 0)
{
cinfo.q_scale_factor[0] = jpeg_quality_scaling(luma_quality);
cinfo.q_scale_factor[1] = jpeg_quality_scaling(chroma_quality);
if ( luma_quality != chroma_quality )
{
/* disable subsampling - ref. Libjpeg.txt */
cinfo.comp_info[0].v_samp_factor = 1;
cinfo.comp_info[0].h_samp_factor = 1;
cinfo.comp_info[1].v_samp_factor = 1;
cinfo.comp_info[1].h_samp_factor = 1;
}
jpeg_default_qtables( &cinfo, TRUE );
}
#endif // #if JPEG_LIB_VERSION >= 70
jpeg_start_compress( &cinfo, TRUE );
if( channels > 1 )
_buffer.allocate(width*channels);
buffer = _buffer;
for( int y = 0; y < height; y++ )
{
uchar *data = img.data + img.step*y, *ptr = data;
if( _channels == 3 )
{
icvCvt_BGR2RGB_8u_C3R( data, 0, buffer, 0, cvSize(width,1) );
ptr = buffer;
}
else if( _channels == 4 )
{
icvCvt_BGRA2BGR_8u_C4C3R( data, 0, buffer, 0, cvSize(width,1), 2 );
ptr = buffer;
}
jpeg_write_scanlines( &cinfo, &ptr, 1 );
}
jpeg_finish_compress( &cinfo );
result = true;
}
_exit_:
if(!result)
{
char jmsg_buf[JMSG_LENGTH_MAX];
jerr.pub.format_message((j_common_ptr)&cinfo, jmsg_buf);
m_last_error = jmsg_buf;
}
jpeg_destroy_compress( &cinfo );
return result;
}這裡jpeg編碼的實現使用了libjpeg，我們一步一步的分析
首先是一些變數的宣告和初始化，接著通過判斷m_buf是否為NULL，m_buf就是等待寫入編碼後資料的緩衝區起始地址，如果為NULL，則通過檔案讀入，這與之前的程式碼保持一致，對於jpegEncoder而言，m_buf非NULL。接下來用到了setjmp（見我上一篇部落格），這裡第一次返回總是0。接下來是一些引數的初始化以及指定引數的檢測。我們直接跳過這些步驟，進入關鍵的部分
有幾個關鍵函式
1，jpeg_create_compress(&cinfo);
2，jpeg_start_compress( &cinfo, TRUE );
3，icvCvt_BGR2RGB_8u_C3R( data, 0, buffer, 0, cvSize(width,1) ); 或 icvCvt_BGRA2BGR_8u_C4C3R( data, 0, buffer, 0, cvSize(width,1), 2 );
4，jpeg_write_scanlines( &cinfo, &ptr, 1 );
5，jpeg_finish_compress( &cinfo );
這裡預設大家瞭解Jpeg編碼的原理了，如果需要補充，可以看下以下博文，除了編碼原理外還有一些可能必要的知識
接下來先介紹一下libjpeg中一些重要的資料結構，這裡只給出較為詳細的介紹，如果有問題可以對照libjpeg原始碼按照流程自行閱讀
1，cinfo是一個jpeg_compress_struct，其包含的型別為
struct jpeg_compress_struct {
jpeg_common_fields; /* Fields shared with jpeg_decompress_struct */
/* Destination for compressed data */
struct jpeg_destination_mgr * dest;
/* Description of source image --- these fields must be filled in by
* outer application before starting compression. in_color_space must
* be correct before you can even call jpeg_set_defaults().
*/
JDIMENSION image_width; /* input image width */
JDIMENSION image_height; /* input image height */
int input_components; /* # of color components in input image */
J_COLOR_SPACE in_color_space; /* colorspace of input image */
double input_gamma; /* image gamma of input image */
/* Compression parameters --- these fields must be set before calling
* jpeg_start_compress(). We recommend calling jpeg_set_defaults() to
* initialize everything to reasonable defaults, then changing anything
* the application specifically wants to change. That way you won't get
* burnt when new parameters are added. Also note that there are several
* helper routines to simplify changing parameters.
*/
unsigned int scale_num, scale_denom; /* fraction by which to scale image */
JDIMENSION jpeg_width; /* scaled JPEG image width */
JDIMENSION jpeg_height; /* scaled JPEG image height */
/* Dimensions of actual JPEG image that will be written to file,
* derived from input dimensions by scaling factors above.
* These fields are computed by jpeg_start_compress().
* You can also use jpeg_calc_jpeg_dimensions() to determine these values
* in advance of calling jpeg_start_compress().
*/
int data_precision; /* bits of precision in image data */
int num_components; /* # of color components in JPEG image */
J_COLOR_SPACE jpeg_color_space; /* colorspace of JPEG image */
jpeg_component_info * comp_info;
/* comp_info[i] describes component that appears i'th in SOF */
JQUANT_TBL * quant_tbl_ptrs[NUM_QUANT_TBLS];
int q_scale_factor[NUM_QUANT_TBLS];
/* ptrs to coefficient quantization tables, or NULL if not defined,
* and corresponding scale factors (percentage, initialized 100).
*/
JHUFF_TBL * dc_huff_tbl_ptrs[NUM_HUFF_TBLS];
JHUFF_TBL * ac_huff_tbl_ptrs[NUM_HUFF_TBLS];
/* ptrs to Huffman coding tables, or NULL if not defined */
UINT8 arith_dc_L[NUM_ARITH_TBLS]; /* L values for DC arith-coding tables */
UINT8 arith_dc_U[NUM_ARITH_TBLS]; /* U values for DC arith-coding tables */
UINT8 arith_ac_K[NUM_ARITH_TBLS]; /* Kx values for AC arith-coding tables */
int num_scans; /* # of entries in scan_info array */
const jpeg_scan_info * scan_info; /* script for multi-scan file, or NULL */
/* The default value of scan_info is NULL, which causes a single-scan
* sequential JPEG file to be emitted. To create a multi-scan file,
* set num_scans and scan_info to point to an array of scan definitions.
*/
boolean raw_data_in; /* TRUE=caller supplies downsampled data */
boolean arith_code; /* TRUE=arithmetic coding, FALSE=Huffman */
boolean optimize_coding; /* TRUE=optimize entropy encoding parms */
boolean CCIR601_sampling; /* TRUE=first samples are cosited */
boolean do_fancy_downsampling; /* TRUE=apply fancy downsampling */
int smoothing_factor; /* 1..100, or 0 for no input smoothing */
J_DCT_METHOD dct_method; /* DCT algorithm selector */
/* The restart interval can be specified in absolute MCUs by setting
* restart_interval, or in MCU rows by setting restart_in_rows
* (in which case the correct restart_interval will be figured
* for each scan).
*/
unsigned int restart_interval; /* MCUs per restart, or 0 for no restart */
int restart_in_rows; /* if > 0, MCU rows per restart interval */
/* Parameters controlling emission of special markers. */
boolean write_JFIF_header; /* should a JFIF marker be written? */
UINT8 JFIF_major_version; /* What to write for the JFIF version number */
UINT8 JFIF_minor_version;
/* These three values are not used by the JPEG code, merely copied */
/* into the JFIF APP0 marker. density_unit can be 0 for unknown, */
/* 1 for dots/inch, or 2 for dots/cm. Note that the pixel aspect */
/* ratio is defined by X_density/Y_density even when density_unit=0. */
UINT8 density_unit; /* JFIF code for pixel size units */
UINT16 X_density; /* Horizontal pixel density */
UINT16 Y_density; /* Vertical pixel density */
boolean write_Adobe_marker; /* should an Adobe marker be written? */
J_COLOR_TRANSFORM color_transform;
/* Color transform identifier, writes LSE marker if nonzero */
/* State variable: index of next scanline to be written to
* jpeg_write_scanlines(). Application may use this to control its
* processing loop, e.g., "while (next_scanline < image_height)".
*/
JDIMENSION next_scanline; /* 0 .. image_height-1 */
/* Remaining fields are known throughout compressor, but generally
* should not be touched by a surrounding application.
*/
/*
* These fields are computed during compression startup
*/
boolean progressive_mode; /* TRUE if scan script uses progressive mode */
int max_h_samp_factor; /* largest h_samp_factor */
int max_v_samp_factor; /* largest v_samp_factor */
int min_DCT_h_scaled_size; /* smallest DCT_h_scaled_size of any component */
int min_DCT_v_scaled_size; /* smallest DCT_v_scaled_size of any component */
JDIMENSION total_iMCU_rows; /* # of iMCU rows to be input to coef ctlr */
/* The coefficient controller receives data in units of MCU rows as defined
* for fully interleaved scans (whether the JPEG file is interleaved or not).
* There are v_samp_factor * DCTSIZE sample rows of each component in an
* "iMCU" (interleaved MCU) row.
*/
/*
* These fields are valid during any one scan.
* They describe the components and MCUs actually appearing in the scan.
*/
int comps_in_scan; /* # of JPEG components in this scan */
jpeg_component_info * cur_comp_info[MAX_COMPS_IN_SCAN];
/* *cur_comp_info[i] describes component that appears i'th in SOS */
JDIMENSION MCUs_per_row; /* # of MCUs across the image */
JDIMENSION MCU_rows_in_scan; /* # of MCU rows in the image */
int blocks_in_MCU; /* # of DCT blocks per MCU */
int MCU_membership[C_MAX_BLOCKS_IN_MCU];
/* MCU_membership[i] is index in cur_comp_info of component owning */
/* i'th block in an MCU */
int Ss, Se, Ah, Al; /* progressive JPEG parameters for scan */
int block_size; /* the basic DCT block size: 1..16 */
const int * natural_order; /* natural-order position array */
int lim_Se; /* min( Se, DCTSIZE2-1 ) */
/*
* Links to compression subobjects (methods and private variables of modules)
*/
struct jpeg_comp_master * master;
struct jpeg_c_main_controller * main;
struct jpeg_c_prep_controller * prep;
struct jpeg_c_coef_controller * coef;
struct jpeg_marker_writer * marker;
struct jpeg_color_converter * cconvert;
struct jpeg_downsampler * downsample;
struct jpeg_forward_dct * fdct;
struct jpeg_entropy_encoder * entropy;
jpeg_scan_info * script_space; /* workspace for jpeg_simple_progression */
int script_space_size;
};2，jpeg_common_struct
typedef struct jpeg_common_struct * j_common_ptr;
/* Routines that are to be used by both halves of the library are declared
* to receive a pointer to this structure. There are no actual instances of
* jpeg_common_struct, only of jpeg_compress_struct and jpeg_decompress_struct.
*/
struct jpeg_common_struct {
jpeg_common_fields; /* Fields common to both master struct types */
/* Additional fields follow in an actual jpeg_compress_struct or
* jpeg_decompress_struct. All three structs must agree on these
* initial fields! (This would be a lot cleaner in C++.)
*/
};
#define jpeg_common_fields \
struct jpeg_error_mgr * err; /* Error handler module */\
struct jpeg_memory_mgr * mem; /* Memory manager module */\
struct jpeg_progress_mgr * progress; /* Progress monitor, or NULL if none */\
void * client_data; /* Available for use by application */\
boolean is_decompressor; /* So common code can tell which is which */\
int global_state /* For checking call sequence validity */
接下來是函式型別方面的巨集定義，以便大家接下來看程式碼不至於太迷茫
/* a function called through method pointers: */
#define METHODDEF(type) static type
/* a function used only in its module: */
#define LOCAL(type) static type
/* a function referenced thru EXTERNs: */
#define GLOBAL(type) type
/* a reference to a GLOBAL function: */
#define EXTERN(type) extern type初看libjpeg程式碼，覺得真的是滿眼的巨集定義，這也越發顯示了grep命令的強大之處，linux下面讀原始碼神器啊
我們直接進入jpeg_create_compress中看下邏輯
<pre name="code" class="cpp">#define jpeg_create_compress(cinfo) \
jpeg_CreateCompress((cinfo), JPEG_LIB_VERSION, \
(size_t) sizeof(struct jpeg_compress_struct))
#define jpeg_CreateCompress jCreaCompressEXTERN(void) jpeg_CreateCompress JPP((j_compress_ptr cinfo, int version, size_t structsize));
<pre name="code" class="cpp">/*
* Initialization of a JPEG compression object.
* The error manager must already be set up (in case memory manager fails).
*/
GLOBAL(void)
jpeg_CreateCompress (j_compress_ptr cinfo, int version, size_t structsize)
{
int i;
/* Guard against version mismatches between library and caller. */
cinfo->mem = NULL; /* so jpeg_destroy knows mem mgr not called */
if (version != JPEG_LIB_VERSION)
ERREXIT2(cinfo, JERR_BAD_LIB_VERSION, JPEG_LIB_VERSION, version);
if (structsize != SIZEOF(struct jpeg_compress_struct))
ERREXIT2(cinfo, JERR_BAD_STRUCT_SIZE,
(int) SIZEOF(struct jpeg_compress_struct), (int) structsize);
/* For debugging purposes, we zero the whole master structure.
* But the application has already set the err pointer, and may have set
* client_data, so we have to save and restore those fields.
* Note: if application hasn't set client_data, tools like Purify may
* complain here.
*/
{
struct jpeg_error_mgr * err = cinfo->err;
void * client_data = cinfo->client_data; /* ignore Purify complaint here */
MEMZERO(cinfo, SIZEOF(struct jpeg_compress_struct));
cinfo->err = err;
cinfo->client_data = client_data;
}
cinfo->is_decompressor = FALSE;
/* Initialize a memory manager instance for this object */
jinit_memory_mgr((j_common_ptr) cinfo);
/* Zero out pointers to permanent structures. */
cinfo->progress = NULL;
cinfo->dest = NULL;
cinfo->comp_info = NULL;
for (i = 0; i < NUM_QUANT_TBLS; i++) {
cinfo->quant_tbl_ptrs[i] = NULL;
cinfo->q_scale_factor[i] = 100;
}
for (i = 0; i < NUM_HUFF_TBLS; i++) {
cinfo->dc_huff_tbl_ptrs[i] = NULL;
cinfo->ac_huff_tbl_ptrs[i] = NULL;
}
/* Must do it here for emit_dqt in case jpeg_write_tables is used */
cinfo->block_size = DCTSIZE;
cinfo->natural_order = jpeg_natural_order;
cinfo->lim_Se = DCTSIZE2-1;
cinfo->script_space = NULL;
cinfo->input_gamma = 1.0; /* in case application forgets */
/* OK, I'm ready */
cinfo->global_state = CSTATE_START;
}jpeg_create_compress先檢查呼叫方和使用庫中jpeg的版本，如果版本一致則呼叫jinit_memory_mgr進行記憶體的初始化，再接著是各種引數的初始化，大家可以對照jpeg_compress_struct中各個引數的註釋理解下這個邏輯
GLOBAL(void)
jpeg_start_compress (j_compress_ptr cinfo, boolean write_all_tables)
{
if (cinfo->global_state != CSTATE_START)
ERREXIT1(cinfo, JERR_BAD_STATE, cinfo->global_state);
if (write_all_tables)
jpeg_suppress_tables(cinfo, FALSE); /* mark all tables to be written */
/* (Re)initialize error mgr and destination modules */
(*cinfo->err->reset_error_mgr) ((j_common_ptr) cinfo);
(*cinfo->dest->init_destination) (cinfo);
/* Perform master selection of active modules */
jinit_compress_master(cinfo);
/* Set up for the first pass */
(*cinfo->master->prepare_for_pass) (cinfo);
/* Ready for application to drive first pass through jpeg_write_scanlines
* or jpeg_write_raw_data.
*/
cinfo->next_scanline = 0;
cinfo->global_state = (cinfo->raw_data_in ? CSTATE_RAW_OK : CSTATE_SCANNING);
}jpeg_start_compress中的邏輯也相當簡單，初始化error manager與編碼結果儲存模組，初始化編碼master，ready for jpeg_write_scanlines blabla，值得一提的是jinit_compress_master
GLOBAL(void)
jinit_compress_master (j_compress_ptr cinfo)
{
/* Initialize master control (includes parameter checking/processing) */
jinit_c_master_control(cinfo, FALSE /* full compression */);
/* Preprocessing */
if (! cinfo->raw_data_in) {
jinit_color_converter(cinfo);
jinit_downsampler(cinfo);
jinit_c_prep_controller(cinfo, FALSE /* never need full buffer here */);
}
/* Forward DCT */
jinit_forward_dct(cinfo);
/* Entropy encoding: either Huffman or arithmetic coding. */
if (cinfo->arith_code)
jinit_arith_encoder(cinfo);
else {
jinit_huff_encoder(cinfo);
}
/* Need a full-image coefficient buffer in any multi-pass mode. */
jinit_c_coef_controller(cinfo,
(boolean) (cinfo->num_scans > 1 || cinfo->optimize_coding));
jinit_c_main_controller(cinfo, FALSE /* never need full buffer here */);
jinit_marker_writer(cinfo);
/* We can now tell the memory manager to allocate virtual arrays. */
(*cinfo->mem->realize_virt_arrays) ((j_common_ptr) cinfo);
/* Write the datastream header (SOI) immediately.
* Frame and scan headers are postponed till later.
* This lets application insert special markers after the SOI.
*/
(*cinfo->marker->write_file_header) (cinfo);
}這裡包括jpeg整個編碼流程中重要的資料結構的初始化，註釋非常清楚，這裡不詳細介紹
/*
* Write some scanlines of data to the JPEG compressor.
*
* The return value will be the number of lines actually written.
* This should be less than the supplied num_lines only in case that
* the data destination module has requested suspension of the compressor,
* or if more than image_height scanlines are passed in.
*
* Note: we warn about excess calls to jpeg_write_scanlines() since
* this likely signals an application programmer error. However,
* excess scanlines passed in the last valid call are *silently* ignored,
* so that the application need not adjust num_lines for end-of-image
* when using a multiple-scanline buffer.
*/
GLOBAL(JDIMENSION)
jpeg_write_scanlines (j_compress_ptr cinfo, JSAMPARRAY scanlines,
JDIMENSION num_lines)
{
JDIMENSION row_ctr, rows_left;
if (cinfo->global_state != CSTATE_SCANNING)
ERREXIT1(cinfo, JERR_BAD_STATE, cinfo->global_state);
if (cinfo->next_scanline >= cinfo->image_height)
WARNMS(cinfo, JWRN_TOO_MUCH_DATA);
/* Call progress monitor hook if present */
if (cinfo->progress != NULL) {
cinfo->progress->pass_counter = (long) cinfo->next_scanline;
cinfo->progress->pass_limit = (long) cinfo->image_height;
(*cinfo->progress->progress_monitor) ((j_common_ptr) cinfo);
}
/* Give master control module another chance if this is first call to
* jpeg_write_scanlines. This lets output of the frame/scan headers be
* delayed so that application can write COM, etc, markers between
* jpeg_start_compress and jpeg_write_scanlines.
*/
if (cinfo->master->call_pass_startup)
(*cinfo->master->pass_startup) (cinfo);
/* Ignore any extra scanlines at bottom of image. */
rows_left = cinfo->image_height - cinfo->next_scanline;
if (num_lines > rows_left)
num_lines = rows_left;
row_ctr = 0;
(*cinfo->main->process_data) (cinfo, scanlines, &row_ctr, num_lines);
cinfo->next_scanline += row_ctr;
return row_ctr;
}z在jpeg_write_scanlines中，首先仍然是預處理和初始化的邏輯，主要需要注意的是process_data中的實現，在jpeg_start_compress --> jinit_compress_master --> jinit_c_main_controller 中 cinfo->main->start_pass = start_pass_main，而start_pass_main中則將cinfo->main->pub.process_data = process_data_simple_main;
即呼叫process_data相當於呼叫了process_data_simple_main
/*
* Process some data.
* This routine handles the simple pass-through mode,
* where we have only a strip buffer.
*/
METHODDEF(void)
process_data_simple_main (j_compress_ptr cinfo,
JSAMPARRAY input_buf, JDIMENSION *in_row_ctr,
JDIMENSION in_rows_avail)
{
my_main_ptr mainp = (my_main_ptr) cinfo->main;
while (mainp->cur_iMCU_row < cinfo->total_iMCU_rows) {
/* Read input data if we haven't filled the main buffer yet */
if (mainp->rowgroup_ctr < (JDIMENSION) cinfo->min_DCT_v_scaled_size)
(*cinfo->prep->pre_process_data) (cinfo,
input_buf, in_row_ctr, in_rows_avail,
mainp->buffer, &mainp->rowgroup_ctr,
(JDIMENSION) cinfo->min_DCT_v_scaled_size);
/* If we don't have a full iMCU row buffered, return to application for
* more data. Note that preprocessor will always pad to fill the iMCU row
* at the bottom of the image.
*/
if (mainp->rowgroup_ctr != (JDIMENSION) cinfo->min_DCT_v_scaled_size)
return;
/* Send the completed row to the compressor */
if (! (*cinfo->coef->compress_data) (cinfo, mainp->buffer)) {
/* If compressor did not consume the whole row, then we must need to
* suspend processing and return to the application. In this situation
* we pretend we didn't yet consume the last input row; otherwise, if
* it happened to be the last row of the image, the application would
* think we were done.
*/
if (! mainp->suspended) {
(*in_row_ctr)--;
mainp->suspended = TRUE;
}
return;
}
/* We did finish the row. Undo our little suspension hack if a previous
* call suspended; then mark the main buffer empty.
*/
if (mainp->suspended) {
(*in_row_ctr)++;
mainp->suspended = FALSE;
}
mainp->rowgroup_ctr = 0;
mainp->cur_iMCU_row++;
}
}process_data中首先則是一個數據的預處理邏輯pre_process_data，在jpeg_start_compress --> jinit_compress_master -->jinit_c_prep_controller中，prep->pub.pre_process_data = pre_process_data;
/*
* Process some data in the simple no-context case.
*
* Preprocessor output data is counted in "row groups". A row group
* is defined to be v_samp_factor sample rows of each component.
* Downsampling will produce this much data from each max_v_samp_factor
* input rows.
*/
METHODDEF(void)
pre_process_data (j_compress_ptr cinfo,
JSAMPARRAY input_buf, JDIMENSION *in_row_ctr,
JDIMENSION in_rows_avail,
JSAMPIMAGE output_buf, JDIMENSION *out_row_group_ctr,
JDIMENSION out_row_groups_avail)
{
my_prep_ptr prep = (my_prep_ptr) cinfo->prep;
int numrows, ci;
JDIMENSION inrows;
jpeg_component_info * compptr;
while (*in_row_ctr < in_rows_avail &&
*out_row_group_ctr < out_row_groups_avail) {
/* Do color conversion to fill the conversion buffer. */
inrows = in_rows_avail - *in_row_ctr;
numrows = cinfo->max_v_samp_factor - prep->next_buf_row;
numrows = (int) MIN((JDIMENSION) numrows, inrows);
(*cinfo->cconvert->color_convert) (cinfo, input_buf + *in_row_ctr,
prep->color_buf,
(JDIMENSION) prep->next_buf_row,
numrows);
*in_row_ctr += numrows;
prep->next_buf_row += numrows;
prep->rows_to_go -= numrows;
/* If at bottom of image, pad to fill the conversion buffer. */
if (prep->rows_to_go == 0 &&
prep->next_buf_row < cinfo->max_v_samp_factor) {
for (ci = 0; ci < cinfo->num_components; ci++) {
expand_bottom_edge(prep->color_buf[ci], cinfo->image_width,
prep->next_buf_row, cinfo->max_v_samp_factor);
}
prep->next_buf_row = cinfo->max_v_samp_factor;
}
/* If we've filled the conversion buffer, empty it. */
if (prep->next_buf_row == cinfo->max_v_samp_factor) {
(*cinfo->downsample->downsample) (cinfo,
prep->color_buf, (JDIMENSION) 0,
output_buf, *out_row_group_ctr);
prep->next_buf_row = 0;
(*out_row_group_ctr)++;
}
/* If at bottom of image, pad the output to a full iMCU height.
* Note we assume the caller is providing a one-iMCU-height output buffer!
*/
if (prep->rows_to_go == 0 &&
*out_row_group_ctr < out_row_groups_avail) {
for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
ci++, compptr++) {
numrows = (compptr->v_samp_factor * compptr->DCT_v_scaled_size) /
cinfo->min_DCT_v_scaled_size;
expand_bottom_edge(output_buf[ci],
compptr->width_in_blocks * compptr->DCT_h_scaled_size,
(int) (*out_row_group_ctr * numrows),
(int) (out_row_groups_avail * numrows));
}
*out_row_group_ctr = out_row_groups_avail;
break; /* can exit outer loop without test */
}
}
}pre_process_data中主要涉及的就是color_convert和expand_bottom_edge兩個邏輯，在jpeg_start_compress --> jinit_compress_master -->jinit_color_converter中，通過判斷輸入影象的color_space去選擇相關color轉換的邏輯，此時color_convert則對應了相關處理的邏輯；而expand_bottom_edge則通過擴充套件input的bottom來將output的height
擴充套件至input的height
接著是compress_data函式，jpeg_start_compress --> jinit_compress_master -->jinit_c_coef_controller中，cinfo->coef->pub.start_pass = start_pass_coef; 接著在start_pass_coef 中 coef->pub.compress_data = compress_data;可以得到compress_data的邏輯
/*
* Process some data in the single-pass case.
* We process the equivalent of one fully interleaved MCU row ("iMCU" row)
* per call, ie, v_samp_factor block rows for each component in the image.
* Returns TRUE if the iMCU row is completed, FALSE if suspended.
*
* NB: input_buf contains a plane for each component in image,
* which we index according to the component's SOF position.
*/
METHODDEF(boolean)
compress_data (j_compress_ptr cinfo, JSAMPIMAGE input_buf)
{
my_coef_ptr coef = (my_coef_ptr) cinfo->coef;
JDIMENSION MCU_col_num; /* index of current MCU within row */
JDIMENSION last_MCU_col = cinfo->MCUs_per_row - 1;
JDIMENSION last_iMCU_row = cinfo->total_iMCU_rows - 1;
int blkn, bi, ci, yindex, yoffset, blockcnt;
JDIMENSION ypos, xpos;
jpeg_component_info *compptr;
forward_DCT_ptr forward_DCT;
/* Loop to write as much as one whole iMCU row */
for (yoffset = coef->MCU_vert_offset; yoffset < coef->MCU_rows_per_iMCU_row;
yoffset++) {
for (MCU_col_num = coef->mcu_ctr; MCU_col_num <= last_MCU_col;
MCU_col_num++) {
/* Determine where data comes from in input_buf and do the DCT thing.
* Each call on forward_DCT processes a horizontal row of DCT blocks
* as wide as an MCU; we rely on having allocated the MCU_buffer[] blocks
* sequentially. Dummy blocks at the right or bottom edge are filled in
* specially. The data in them does not matter for image reconstruction,
* so we fill them with values that will encode to the smallest amount of
* data, viz: all zeroes in the AC entries, DC entries equal to previous
* block's DC value. (Thanks to Thomas Kinsman for this idea.)
*/
blkn = 0;
for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
compptr = cinfo->cur_comp_info[ci];
forward_DCT = cinfo->fdct->forward_DCT[compptr->component_index];
blockcnt = (MCU_col_num < last_MCU_col) ? compptr->MCU_width
: compptr->last_col_width;
xpos = MCU_col_num * compptr->MCU_sample_width;
ypos = yoffset * compptr->DCT_v_scaled_size;
/* ypos == (yoffset+yindex) * DCTSIZE */
for (yindex = 0; yindex < compptr->MCU_height; yindex++) {
if (coef->iMCU_row_num < last_iMCU_row ||
yoffset+yindex < compptr->last_row_height) {
(*forward_DCT) (cinfo, compptr,
input_buf[compptr->component_index],
coef->MCU_buffer[blkn],
ypos, xpos, (JDIMENSION) blockcnt);
if (blockcnt < compptr->MCU_width) {
/* Create some dummy blocks at the right edge of the image. */
FMEMZERO((void FAR *) coef->MCU_buffer[blkn + blockcnt],
(compptr->MCU_width - blockcnt) * SIZEOF(JBLOCK));
for (bi = blockcnt; bi < compptr->MCU_width; bi++) {
coef->MCU_buffer[blkn+bi][0][0] = coef->MCU_buffer[blkn+bi-1][0][0];
}
}
} else {
/* Create a row of dummy blocks at the bottom of the image. */
FMEMZERO((void FAR *) coef->MCU_buffer[blkn],
compptr->MCU_width * SIZEOF(JBLOCK));
for (bi = 0; bi < compptr->MCU_width; bi++) {
coef->MCU_buffer[blkn+bi][0][0] = coef->MCU_buffer[blkn-1][0][0];
}
}
blkn += compptr->MCU_width;
ypos += compptr->DCT_v_scaled_size;
}
}
/* Try to write the MCU. In event of a suspension failure, we will
* re-DCT the MCU on restart (a bit inefficient, could be fixed...)
*/
if (! (*cinfo->entropy->encode_mcu) (cinfo, coef->MCU_buffer)) {
/* Suspension forced; update state counters and exit */
coef->MCU_vert_offset = yoffset;
coef->mcu_ctr = MCU_col_num;
return FALSE;
}
}
/* Completed an MCU row, but perhaps not an iMCU row */
coef->mcu_ctr = 0;
}
/* Completed the iMCU row, advance counters for next one */
coef->iMCU_row_num++;
start_iMCU_row(cinfo);
return TRUE;
}這裡就完成了forward_DCT的步驟，另外比較重要的就是encode_mcu了，與之前的形式一樣，encode_mcu在start_pass_huff中確定了相應的實現方式，也就完成了Huffman編碼的過程。另外，encode_mcu中，huffman編碼後的結果也被記入cinfo->dest的資料結構中，也就對應到了之前OpenCV呼叫libJpeg時的一個名為JpegDestication的資料結構中，從而完成了整個的編碼流程