自己学习了下JPEG理论知识以后，找了个简单的解码器(NanoJPEG)试试看，原始地址在这里http://keyj.emphy.de/nanojpeg/，短短几百行，还是比较容易看懂的，本人理解详细参见代码注释，如理解有误欢迎指出，有问题/兴趣也可以留言和我讨论。
$ gcc -O3 -D_NJ_EXAMPLE_PROGRAM -o nanojpeg nanojpeg.c $ ./nanojpeg testorig.jpg
跟着main函数，边理论边实践，不错的方法！
// NanoJPEG -- KeyJ's Tiny Baseline JPEG Decoder
// version 1.3 (2012-03-05)
// by Martin J. Fiedler <martin.fiedler@gmx.net>
//
// This software is published under the terms of KeyJ's Research License,
// version 0.2. Usage of this software is subject to the following conditions:
// 0. There's no warranty whatsoever. The author(s) of this software can not
// be held liable for any damages that occur when using this software.
// 1. This software may be used freely for both non-commercial and commercial
// purposes.
// 2. This software may be redistributed freely as long as no fees are charged
// for the distribution and this license information is included.
// 3. This software may be modified freely except for this license information,
// which must not be changed in any way.
// 4. If anything other than configuration, indentation or comments have been
// altered in the code, the original author(s) must receive a copy of the
// modified code.
///////////////////////////////////////////////////////////////////////////////
// DOCUMENTATION SECTION //
// read this if you want to know what this is all about //
///////////////////////////////////////////////////////////////////////////////
// INTRODUCTION
// ============
//
// This is a minimal decoder for baseline JPEG images. It accepts memory dumps
// of JPEG files as input and generates either 8-bit grayscale or packed 24-bit
// RGB images as output. It does not parse JFIF or Exif headers; all JPEG files
// are assumed to be either grayscale or YCbCr. CMYK or other color spaces are
// not supported. All YCbCr subsampling schemes with power-of-two ratios are
// supported, as are restart intervals. Progressive or lossless JPEG is not
// supported.
// Summed up, NanoJPEG should be able to decode all images from digital cameras
// and most common forms of other non-progressive JPEG images.
// The decoder is not optimized for speed, it's optimized for simplicity and
// small code. Image quality should be at a reasonable level. A bicubic chroma
// upsampling filter ensures that subsampled YCbCr images are rendered in
// decent quality. The decoder is not meant to deal with broken JPEG files in
// a graceful manner; if anything is wrong with the bitstream, decoding will
// simply fail.
// The code should work with every modern C compiler without problems and
// should not emit any warnings. It uses only (at least) 32-bit integer
// arithmetic and is supposed to be endianness independent and 64-bit clean.
// However, it is not thread-safe.
// COMPILE-TIME CONFIGURATION
// ==========================
//
// The following aspects of NanoJPEG can be controlled with preprocessor
// defines:
//
// _NJ_EXAMPLE_PROGRAM = Compile a main() function with an example
// program.
// _NJ_INCLUDE_HEADER_ONLY = Don't compile anything, just act as a header
// file for NanoJPEG. Example:
// #define _NJ_INCLUDE_HEADER_ONLY
// #include "nanojpeg.c"
// int main(void) {
// njInit();
// // your code here
// njDone();
// }
// NJ_USE_LIBC=1 = Use the malloc(), free(), memset() and memcpy()
// functions from the standard C library (default).
// NJ_USE_LIBC=0 = Don't use the standard C library. In this mode,
// external functions njAlloc(), njFreeMem(),
// njFillMem() and njCopyMem() need to be defined
// and implemented somewhere.
// NJ_USE_WIN32=0 = Normal mode (default).
// NJ_USE_WIN32=1 = If compiling with MSVC for Win32 and
// NJ_USE_LIBC=0, NanoJPEG will use its own
// implementations of the required C library
// functions (default if compiling with MSVC and
// NJ_USE_LIBC=0).
// NJ_CHROMA_FILTER=1 = Use the bicubic chroma upsampling filter
// (default). // 图像resize的一种算法
// NJ_CHROMA_FILTER=0 = Use simple pixel repetition for chroma upsampling
// (bad quality, but faster and less code).
// API
// ===
//
// For API documentation, read the "header section" below.
// EXAMPLE
// =======
//
// A few pages below, you can find an example program that uses NanoJPEG to
// convert JPEG files into PGM or PPM. To compile it, use something like
// gcc -O3 -D_NJ_EXAMPLE_PROGRAM -o nanojpeg nanojpeg.c
// You may also add -std=c99 -Wall -Wextra -pedantic -Werror, if you want 
///////////////////////////////////////////////////////////////////////////////
// HEADER SECTION //
// copy and pase this into nanojpeg.h if you want //
///////////////////////////////////////////////////////////////////////////////
#ifndef _NANOJPEG_H
#define _NANOJPEG_H
// nj_result_t: Result codes for njDecode().
typedef enum _nj_result {
NJ_OK = 0, // no error, decoding successful
NJ_NO_JPEG, // not a JPEG file
NJ_UNSUPPORTED, // unsupported format
NJ_OUT_OF_MEM, // out of memory
NJ_INTERNAL_ERR, // internal error
NJ_SYNTAX_ERROR, // syntax error
__NJ_FINISHED, // used internally, will never be reported
} nj_result_t;
// njInit: Initialize NanoJPEG.
// For safety reasons, this should be called at least one time before using
// using any of the other NanoJPEG functions.
void njInit(void);
// njDecode: Decode a JPEG image.
// Decodes a memory dump of a JPEG file into internal buffers.
// Parameters:
// jpeg = The pointer to the memory dump.
// size = The size of the JPEG file.
// Return value: The error code in case of failure, or NJ_OK (zero) on success.
nj_result_t njDecode(const void* jpeg, const int size);
// njGetWidth: Return the width (in pixels) of the most recently decoded
// image. If njDecode() failed, the result of njGetWidth() is undefined.
int njGetWidth(void);
// njGetHeight: Return the height (in pixels) of the most recently decoded
// image. If njDecode() failed, the result of njGetHeight() is undefined.
int njGetHeight(void);
// njIsColor: Return 1 if the most recently decoded image is a color image
// (RGB) or 0 if it is a grayscale image. If njDecode() failed, the result
// of njGetWidth() is undefined.
int njIsColor(void);
// njGetImage: Returns the decoded image data.
// Returns a pointer to the most recently image. The memory layout it byte-
// oriented, top-down, without any padding between lines. Pixels of color
// images will be stored as three consecutive bytes for the red, green and
// blue channels. This data format is thus compatible with the PGM or PPM
// file formats and the OpenGL texture formats GL_LUMINANCE8 or GL_RGB8.
// If njDecode() failed, the result of njGetImage() is undefined.
unsigned char* njGetImage(void);
// njGetImageSize: Returns the size (in bytes) of the image data returned
// by njGetImage(). If njDecode() failed, the result of njGetImageSize() is
// undefined.
int njGetImageSize(void);
// njDone: Uninitialize NanoJPEG.
// Resets NanoJPEG's internal state and frees all memory that has been
// allocated at run-time by NanoJPEG. It is still possible to decode another
// image after a njDone() call.
void njDone(void);
#endif//_NANOJPEG_H
///////////////////////////////////////////////////////////////////////////////
// CONFIGURATION SECTION //
// adjust the default settings for the NJ_ defines here //
///////////////////////////////////////////////////////////////////////////////
#ifndef NJ_USE_LIBC
#define NJ_USE_LIBC 1
#endif
#ifndef NJ_USE_WIN32
#ifdef _MSC_VER
#define NJ_USE_WIN32 (!NJ_USE_LIBC)
#else
#define NJ_USE_WIN32 0
#endif
#endif
#ifndef NJ_CHROMA_FILTER
#define NJ_CHROMA_FILTER 1
#endif
///////////////////////////////////////////////////////////////////////////////
// EXAMPLE PROGRAM //
// just define _NJ_EXAMPLE_PROGRAM to compile this (requires NJ_USE_LIBC) //
///////////////////////////////////////////////////////////////////////////////
#ifdef _NJ_EXAMPLE_PROGRAM
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char* argv[]) {
int size;
char *buf;
FILE *f;
if (argc < 2) {
printf("Usage: %s <input.jpg> [<output.ppm>]\n", argv[0]);
return 2;
}
f = fopen(argv[1], "rb");
if (!f) {
printf("Error opening the input file.\n");
return 1;
}
fseek(f, 0, SEEK_END);
size = (int) ftell(f); // 字节
buf = malloc(size);
fseek(f, 0, SEEK_SET);
size = (int) fread(buf, 1, size, f); // 读取整个文件内容到buf
fclose(f);
njInit(); // 初始化nj_context_t
if (njDecode(buf, size)) {
printf("Error decoding the input file.\n");
return 1;
}
f = fopen((argc > 2) ? argv[2] : (njIsColor() ? "nanojpeg_out.ppm" : "nanojpeg_out.pgm"), "wb");
if (!f) {
printf("Error opening the output file.\n");
return 1;
}
fprintf(f, "P%d\n%d %d\n255\n", njIsColor() ? 6 : 5, njGetWidth(), njGetHeight());
fwrite(njGetImage(), 1, njGetImageSize(), f);
fclose(f);
njDone();
return 0;
}
#endif
// 解释什么是stride http://msdn.microsoft.com/en-us/library/windows/desktop/aa473780(v=vs.85).aspx
///////////////////////////////////////////////////////////////////////////////
// IMPLEMENTATION SECTION //
// you may stop reading here //
///////////////////////////////////////////////////////////////////////////////
#ifndef _NJ_INCLUDE_HEADER_ONLY
#ifdef _MSC_VER
#define NJ_INLINE static __inline
#define NJ_FORCE_INLINE static __forceinline
#else
#define NJ_INLINE static inline
#define NJ_FORCE_INLINE static inline
#endif
#if NJ_USE_LIBC
#include <stdlib.h>
#include <string.h>
#define njAllocMem malloc
#define njFreeMem free
#define njFillMem memset
#define njCopyMem memcpy
#elif NJ_USE_WIN32
#include <windows.h>
#define njAllocMem(size) ((void*) LocalAlloc(LMEM_FIXED, (SIZE_T)(size)))
#define njFreeMem(block) ((void) LocalFree((HLOCAL) block))
NJ_INLINE void njFillMem(void* block, unsigned char value, int count) { __asm {
mov edi, block
mov al, value
mov ecx, count
rep stosb
} }
NJ_INLINE void njCopyMem(void* dest, const void* src, int count) { __asm {
mov edi, dest
mov esi, src
mov ecx, count
rep movsb
} }
#else
extern void* njAllocMem(int size);
extern void njFreeMem(void* block);
extern void njFillMem(void* block, unsigned char byte, int size);
extern void njCopyMem(void* dest, const void* src, int size);
#endif
typedef struct _nj_code {
unsigned char bits, code;
} nj_vlc_code_t;
typedef struct _nj_cmp {
int cid;
int ssx, ssy; // 水平/垂直因子
int width, height;
int stride;
int qtsel; // Quantization Table量化表
int actabsel, dctabsel; // AC/DC Huffman Table
int dcpred;
unsigned char *pixels;
} nj_component_t; // 颜色分量
typedef struct _nj_ctx {
nj_result_t error;
const unsigned char *pos; // 待解码数据指针(按字节来)
int size; // 整个数据的长度
int length; // 某一个marker内容的长度
int width, height; // 图片宽和高度
int mbwidth, mbheight; // MCU水平/垂直个数
int mbsizex, mbsizey; // MCU宽/高
int ncomp; // 颜色分量数
nj_component_t comp[3]; // YCbCr
int qtused, qtavail; // 这两个目前看不出来很大用处
unsigned char qtab[4][64]; // 但是目前似乎只有2个
nj_vlc_code_t vlctab[4][65536]; // 构造所有16位数的Huffman基数
// 目前基本上是4个(直/交/0/1)
int buf, bufbits; // 这是用来做什么的 buf是存放内容的 bufbits是计数器，存放了多少个bits
int block[64];
int rstinterval;
unsigned char *rgb; // 解析出来的RGB所要占用的内存 // 每1个点包含3个字节，按找RGB的顺序
} nj_context_t;
static nj_context_t nj;
static const char njZZ[64] = { 0, 1, 8, 16, 9, 2, 3, 10, 17, 24, 32, 25, 18,
11, 4, 5, 12, 19, 26, 33, 40, 48, 41, 34, 27, 20, 13, 6, 7, 14, 21, 28, 35,
42, 49, 56, 57, 50, 43, 36, 29, 22, 15, 23, 30, 37, 44, 51, 58, 59, 52, 45,
38, 31, 39, 46, 53, 60, 61, 54, 47, 55, 62, 63 };
/*
0 1 2 3 4 5 6 7
8 9 10 11 12 13 14 15
16 17 18 19 20 21 22 23
24 25 26 27 28 29 30 31
32 33 34 35 36 37 38 39
40 41 42 43 44 45 46 47
48 49 50 51 52 53 54 55
56 57 58 59 60 61 62 63
*/
NJ_FORCE_INLINE unsigned char njClip(const int x) { // 限定范围是0 ~ 255之间
return (x < 0) ? 0 : ((x > 0xFF) ? 0xFF : (unsigned char) x);
}
#define W1 2841
#define W2 2676
#define W3 2408
#define W5 1609
#define W6 1108
#define W7 565
NJ_INLINE void njRowIDCT(int* blk) { // 按行来操作的 0 ~ 7 // 8 ~ 15
int x0, x1, x2, x3, x4, x5, x6, x7, x8;
if (!((x1 = blk[4] << 11)
| (x2 = blk[6])
| (x3 = blk[2])
| (x4 = blk[1])
| (x5 = blk[7])
| (x6 = blk[5])
| (x7 = blk[3])))
{
blk[0] = blk[1] = blk[2] = blk[3] = blk[4] = blk[5] = blk[6] = blk[7] = blk[0] << 3;
return;
}
x0 = (blk[0] << 11) + 128;
x8 = W7 * (x4 + x5);
x4 = x8 + (W1 - W7) * x4;
x5 = x8 - (W1 + W7) * x5;
x8 = W3 * (x6 + x7);
x6 = x8 - (W3 - W5) * x6;
x7 = x8 - (W3 + W5) * x7;
x8 = x0 + x1;
x0 -= x1;
x1 = W6 * (x3 + x2);
x2 = x1 - (W2 + W6) * x2;
x3 = x1 + (W2 - W6) * x3;
x1 = x4 + x6;
x4 -= x6;
x6 = x5 + x7;
x5 -= x7;
x7 = x8 + x3;
x8 -= x3;
x3 = x0 + x2;
x0 -= x2;
x2 = (181 * (x4 + x5) + 128) >> 8;
x4 = (181 * (x4 - x5) + 128) >> 8;
blk[0] = (x7 + x1) >> 8;
blk[1] = (x3 + x2) >> 8;
blk[2] = (x0 + x4) >> 8;
blk[3] = (x8 + x6) >> 8;
blk[4] = (x8 - x6) >> 8;
blk[5] = (x0 - x4) >> 8;
blk[6] = (x3 - x2) >> 8;
blk[7] = (x7 - x1) >> 8;
}
NJ_INLINE void njColIDCT(const int* blk, unsigned char *out, int stride) {
int x0, x1, x2, x3, x4, x5, x6, x7, x8;
if (!((x1 = blk[8*4] << 8)
| (x2 = blk[8*6])
| (x3 = blk[8*2])
| (x4 = blk[8*1])
| (x5 = blk[8*7])
| (x6 = blk[8*5])
| (x7 = blk[8*3])))
{
x1 = njClip(((blk[0] + 32) >> 6) + 128);
for (x0 = 8; x0; --x0) {
*out = (unsigned char) x1;
out += stride;
}
return;
}
x0 = (blk[0] << 8) + 8192;
x8 = W7 * (x4 + x5) + 4;
x4 = (x8 + (W1 - W7) * x4) >> 3;
x5 = (x8 - (W1 + W7) * x5) >> 3;
x8 = W3 * (x6 + x7) + 4;
x6 = (x8 - (W3 - W5) * x6) >> 3;
x7 = (x8 - (W3 + W5) * x7) >> 3;
x8 = x0 + x1;
x0 -= x1;
x1 = W6 * (x3 + x2) + 4;
x2 = (x1 - (W2 + W6) * x2) >> 3;
x3 = (x1 + (W2 - W6) * x3) >> 3;
x1 = x4 + x6;
x4 -= x6;
x6 = x5 + x7;
x5 -= x7;
x7 = x8 + x3;
x8 -= x3;
x3 = x0 + x2;
x0 -= x2;
x2 = (181 * (x4 + x5) + 128) >> 8; // Y，Cb和Cr的值都范围都是-128 ~ 127，并且在FDCT的时候有先减去128，所以现在要IDCT之后再加上128
x4 = (181 * (x4 - x5) + 128) >> 8;
*out = njClip(((x7 + x1) >> 14) + 128); out += stride;
*out = njClip(((x3 + x2) >> 14) + 128); out += stride;
*out = njClip(((x0 + x4) >> 14) + 128); out += stride;
*out = njClip(((x8 + x6) >> 14) + 128); out += stride;
*out = njClip(((x8 - x6) >> 14) + 128); out += stride;
*out = njClip(((x0 - x4) >> 14) + 128); out += stride;
*out = njClip(((x3 - x2) >> 14) + 128); out += stride;
*out = njClip(((x7 - x1) >> 14) + 128);
}
#define njThrow(e) do { nj.error = e; return; } while (0)
#define njCheckError() do { if (nj.error) return; } while (0)
static int njShowBits(int bits) { // 能放得下大于32位的值么？
unsigned char newbyte;
if (!bits) return 0;
while (nj.bufbits < bits) { // 也就是说要buf的位数小于已经buf的位数的时候，就直接读出来？
if (nj.size <= 0) {
nj.buf = (nj.buf << 8) | 0xFF;
nj.bufbits += 8;
continue;
}
newbyte = *nj.pos++; // 数据指针是按字节
nj.size--;
nj.bufbits += 8;
nj.buf = (nj.buf << 8) | newbyte; // 高位最终会被覆盖掉，比如我要buf一个64位的值怎么办？
if (newbyte == 0xFF) {
if (nj.size) {
unsigned char marker = *nj.pos++;
nj.size--;
switch (marker) {
case 0x00:
case 0xFF:
break;
case 0xD9: nj.size = 0; break;
default:
if ((marker & 0xF8) != 0xD0)
nj.error = NJ_SYNTAX_ERROR;
else {
nj.buf = (nj.buf << 8) | marker;
nj.bufbits += 8;
}
}
} else
nj.error = NJ_SYNTAX_ERROR;
}
}
return (nj.buf >> (nj.bufbits - bits)) & ((1 << bits) - 1);
}
NJ_INLINE void njSkipBits(int bits) {
if (nj.bufbits < bits)
(void) njShowBits(bits);
nj.bufbits -= bits;
}
NJ_INLINE int njGetBits(int bits) {
int res = njShowBits(bits);
njSkipBits(bits);
return res;
}
NJ_INLINE void njByteAlign(void) {
nj.bufbits &= 0xF8; // (1111 1000)8的倍数，不满8的部分丢弃
}
static void njSkip(int count) {
nj.pos += count; // 数据指针增加
nj.size -= count; // 总体数据大小减去count
nj.length -= count; // 当前marker长度减去count
if (nj.size < 0) nj.error = NJ_SYNTAX_ERROR;
}
NJ_INLINE unsigned short njDecode16(const unsigned char *pos) {
return (pos[0] << 8) | pos[1]; // 00000000 00001101
}
static void njDecodeLength(void) { // decode长度字段，这个方法调用一般都是已经进入到特定的marker之后
if (nj.size < 2) njThrow(NJ_SYNTAX_ERROR);
nj.length = njDecode16(nj.pos); // 该marker的长度(除去marker名字所占用的2个字节)
if (nj.length > nj.size) njThrow(NJ_SYNTAX_ERROR);
njSkip(2);
}
NJ_INLINE void njSkipMarker(void) {
njDecodeLength();
njSkip(nj.length);
}
NJ_INLINE void njDecodeSOF(void) { // 解析Start of Frame的时候就会把所需要的内存都分配好
int i, ssxmax = 0, ssymax = 0;
nj_component_t* c;
njDecodeLength(); // 解析长度并移动数据指针
if (nj.length < 9) njThrow(NJ_SYNTAX_ERROR);
if (nj.pos[0] != 8) njThrow(NJ_UNSUPPORTED); // 样本精度，一般都是8
nj.height = njDecode16(nj.pos + 1); // 图片高度/宽度
nj.width = njDecode16(nj.pos + 3);
nj.ncomp = nj.pos[5]; // 颜色分量数据，一般都是3
njSkip(6); // 之前共6个字节数据，所以移动数据指针6个字节
switch (nj.ncomp) { // 目前只支持1和3这两种
case 1:
case 3:
break;
default:
njThrow(NJ_UNSUPPORTED);
}
if (nj.length < (nj.ncomp * 3)) njThrow(NJ_SYNTAX_ERROR); // 数据量肯定是要大于颜色分量数 multiply 3，因为接着存颜色分量信息的每个结构占3个字节
// 颜色分量ID占用1个字节，水平/垂直因子占用1个字节(高4位水平，低4位垂直)，量化表占用1个字节
for (i = 0, c = nj.comp; i < nj.ncomp; ++i, ++c) {
c->cid = nj.pos[0]; // 颜色分量ID
if (!(c->ssx = nj.pos[1] >> 4)) njThrow(NJ_SYNTAX_ERROR); // 高4位(水平因子)
if (c->ssx & (c->ssx - 1)) njThrow(NJ_UNSUPPORTED); // non-power of two
if (!(c->ssy = nj.pos[1] & 15)) njThrow(NJ_SYNTAX_ERROR); // (00001111)低4位(垂直因子)
if (c->ssy & (c->ssy - 1)) njThrow(NJ_UNSUPPORTED); // non-power of two
if ((c->qtsel = nj.pos[2]) & 0xFC) njThrow(NJ_SYNTAX_ERROR); // (11111101) 这里0xFC是用在这里干什么的？
njSkip(3); // 移动数据指针到下一个颜色分量
nj.qtused |= 1 << c->qtsel; // 这里是做什么用的？看不出来
if (c->ssx > ssxmax) ssxmax = c->ssx; // 记录最大水平因子
if (c->ssy > ssymax) ssymax = c->ssy; // 记录最大垂直因子
}
if (nj.ncomp == 1) { // 只有一种颜色分量的时候就简单啦
c = nj.comp;
c->ssx = c->ssy = ssxmax = ssymax = 1;
}
nj.mbsizex = ssxmax << 3; // MCU宽 是 水平采样因子最大值 multiply 8
nj.mbsizey = ssymax << 3; // MCU高 是 垂直采样因子最大值 multiply 8
nj.mbwidth = (nj.width + nj.mbsizex - 1) / nj.mbsizex; // 分子采用+ nj.mbsizex - 1就取到大于但是最接近(等于)宽度的值，
// 并且这个值是MCU宽度整数倍 // 这里是水平方向MCU的个数
nj.mbheight = (nj.height + nj.mbsizey - 1) / nj.mbsizey; // 这里是垂直方向MCU的个数
for (i = 0, c = nj.comp; i < nj.ncomp; ++i, ++c) {
c->width = (nj.width * c->ssx + ssxmax - 1) / ssxmax; // 采样宽度？ 最大水平/垂直因子的值就是图片原来的值，否则就会根据因子做相应的减少
c->stride = (c->width + 7) & 0x7FFFFFF8; // (0111 1111 1111 1111 1111 1111 1111 1000) 做什么？以1234567结尾的都省略掉？
// 变成8的整数
// 补齐8位，注意前面有加7，所以总是不会比原来的少，比如原来是227，那么这里就会变成232
// 这是按照数据单元计算的，所以不对
printf("%d, stride %d\n", i, c->stride);
c->height = (nj.height * c->ssy + ssymax - 1) / ssymax;
c->stride = nj.mbwidth * nj.mbsizex * c->ssx / ssxmax; // 再计算一遍stride有什么用？前面计算的是错误的，没有考虑MCU宽度
// 这里都已经是round过的了，所以直接计算
printf("%d, stride again %d\n", i, c->stride);
if (((c->width < 3) && (c->ssx != ssxmax)) || ((c->height < 3) && (c->ssy != ssymax))) njThrow(NJ_UNSUPPORTED);
if (!(c->pixels = njAllocMem(c->stride * (nj.mbheight * nj.mbsizey * c->ssy / ssymax)))) njThrow(NJ_OUT_OF_MEM); // 为分量分配内存
// 大小是所有MCU的
// 可能比图片实际
// 尺寸大
}
if (nj.ncomp == 3) { // 只有有3个颜色分量的时候才需要
nj.rgb = njAllocMem(nj.width * nj.height * nj.ncomp);
if (!nj.rgb) njThrow(NJ_OUT_OF_MEM);
}
njSkip(nj.length);
}
NJ_INLINE void njDecodeDHT(void) {
int codelen, currcnt, remain, spread, i, j;
nj_vlc_code_t *vlc;
static unsigned char counts[16]; // 码字
njDecodeLength();
while (nj.length >= 17) { // 码字的数量(16) + 类型和ID(1)
i = nj.pos[0]; // 类型和ID
if (i & 0xEC) njThrow(NJ_SYNTAX_ERROR); // (11101100)
if (i & 0x02) njThrow(NJ_UNSUPPORTED); // (00000010)
i = (i | (i >> 3)) & 3; // combined DC/AC + tableid value
// 直流0，直流1，交流0，交流1
for (codelen = 1; codelen <= 16; ++codelen) // 码字长度
counts[codelen - 1] = nj.pos[codelen]; // 读取码字
njSkip(17);
vlc = &nj.vlctab[i][0];
remain = spread = 65536;
for (codelen = 1; codelen <= 16; ++codelen) {
spread >>= 1; // 干什么？
currcnt = counts[codelen - 1];
if (!currcnt) continue; // 如果该位数没有码字
if (nj.length < currcnt) njThrow(NJ_SYNTAX_ERROR);
remain -= currcnt << (16 - codelen);
if (remain < 0) njThrow(NJ_SYNTAX_ERROR);
for (i = 0; i < currcnt; ++i) { // 码字个数，同样位数的码字可以有多个
register unsigned char code = nj.pos[i];
for (j = spread; j; --j) { // 保存这么多个有什么作用？
vlc->bits = (unsigned char) codelen; // 码字位数
vlc->code = code; // 码字值
++vlc;
}
}
njSkip(currcnt);
}
while (remain--) {
vlc->bits = 0;
++vlc;
}
}
if (nj.length) njThrow(NJ_SYNTAX_ERROR);
}
NJ_INLINE void njDecodeDQT(void) {
int i;
unsigned char *t;
njDecodeLength();
while (nj.length >= 65) {
i = nj.pos[0]; // QT信息，高4位为QT精度，低4位为QT号
if (i & 0xFC) njThrow(NJ_SYNTAX_ERROR); // (1111 1110)这个用来检测QT号码是否正确的吗？目前精度好像都为0，所以这么写？
nj.qtavail |= 1 << i; // XXX 直接通过这里转换为数量？
t = &nj.qtab[i][0];
for (i = 0; i < 64; ++i)
t[i] = nj.pos[i + 1]; // 读取到QT数组当中，但应该还是按照文件流当中的排列
njSkip(65);
}
if (nj.length) njThrow(NJ_SYNTAX_ERROR);
}
NJ_INLINE void njDecodeDRI(void) {
njDecodeLength();
if (nj.length < 2) njThrow(NJ_SYNTAX_ERROR);
nj.rstinterval = njDecode16(nj.pos);
njSkip(nj.length);
}
static int njGetVLC(nj_vlc_code_t* vlc, unsigned char* code) { // Variable Length Coding
int value = njShowBits(16);
int bits = vlc[value].bits;
if (!bits) { nj.error = NJ_SYNTAX_ERROR; return 0; }
njSkipBits(bits);
value = vlc[value].code;
if (code) *code = (unsigned char) value;
bits = value & 15;
if (!bits) return 0;
value = njGetBits(bits);
if (value < (1 << (bits - 1)))
value += ((-1) << bits) + 1;
return value;
}
NJ_INLINE void njDecodeBlock(nj_component_t* c, unsigned char* out) {
unsigned char code = 0;
int value, coef = 0;
njFillMem(nj.block, 0, sizeof(nj.block));
c->dcpred += njGetVLC(&nj.vlctab[c->dctabsel][0], NULL); // DC 0/1 不会和AC重复
nj.block[0] = (c->dcpred) * nj.qtab[c->qtsel][0]; // DC // 这里是反量化？
do {
value = njGetVLC(&nj.vlctab[c->actabsel][0], &code); // DC 2/3
if (!code) break; // EOB
if (!(code & 0x0F) && (code != 0xF0)) njThrow(NJ_SYNTAX_ERROR);
coef += (code >> 4) + 1; // coefficient 系数
if (coef > 63) njThrow(NJ_SYNTAX_ERROR);
nj.block[(int) njZZ[coef]] = value * nj.qtab[c->qtsel][coef]; // AC 这里是反量化？
} while (coef < 63);
for (coef = 0; coef < 64; coef += 8)
njRowIDCT(&nj.block[coef]); // 上面先Huffman解码/反量化，这里行(反DCT)
for (coef = 0; coef < 8; ++coef)
njColIDCT(&nj.block[coef], &out[coef], c->stride);
}
NJ_INLINE void njDecodeScan(void) {
int i, mbx, mby, sbx, sby;
int rstcount = nj.rstinterval, nextrst = 0;
nj_component_t* c;
njDecodeLength();
if (nj.length < (4 + 2 * nj.ncomp)) njThrow(NJ_SYNTAX_ERROR);
if (nj.pos[0] != nj.ncomp) njThrow(NJ_UNSUPPORTED);
njSkip(1); // 颜色分量数量
for (i = 0, c = nj.comp; i < nj.ncomp; ++i, ++c) {
if (nj.pos[0] != c->cid) njThrow(NJ_SYNTAX_ERROR); // 颜色分量ID
if (nj.pos[1] & 0xEE) njThrow(NJ_SYNTAX_ERROR);
c->dctabsel = nj.pos[1] >> 4; // 高4位为直流表DC Table
c->actabsel = (nj.pos[1] & 1) | 2; // 低4位为交流表AC Table(这里有做特殊处理，所以AC的表名不会和DC相同)
printf("DC/AC Huffman table ids: %d/%d\n", c->dctabsel, c->actabsel);
njSkip(2);
}
if (nj.pos[0] || (nj.pos[1] != 63) || nj.pos[2]) njThrow(NJ_UNSUPPORTED);
njSkip(nj.length); // 忽略3个字节 通常为 00 3F 00
// 2 + 1 + 6 + 3为12字节，这个marker的长度刚好为12字节
// 接下来都是编码过的图像数据
for (mbx = mby = 0;;) {
for (i = 0, c = nj.comp; i < nj.ncomp; ++i, ++c) // 每个分量都要decode
for (sby = 0; sby < c->ssy; ++sby) // 水平/垂直因子
for (sbx = 0; sbx < c->ssx; ++sbx) {
njDecodeBlock(c, &c->pixels[((mby * c->ssy + sby) * c->stride + mbx * c->ssx + sbx) << 3]); // 读取原始编码过
// 的图片数据到block中
// 并反量化，反离散余弦变换
njCheckError();
}
if (++mbx >= nj.mbwidth) { // 读完所有的MCU，到达最右就返回从下一行开始
mbx = 0;
if (++mby >= nj.mbheight) break; // 到达最底行的时候推出，decode结束
}
if (nj.rstinterval && !(--rstcount)) { // restart marker
njByteAlign();
i = njGetBits(16);
if (((i & 0xFFF8) != 0xFFD0) || ((i & 7) != nextrst)) njThrow(NJ_SYNTAX_ERROR);
nextrst = (nextrst + 1) & 7;
rstcount = nj.rstinterval;
for (i = 0; i < 3; ++i)
nj.comp[i].dcpred = 0;
}
}
nj.error = __NJ_FINISHED;
}
#if NJ_CHROMA_FILTER
#define CF4A (-9)
#define CF4B (111)
#define CF4C (29)
#define CF4D (-3)
#define CF3A (28)
#define CF3B (109)
#define CF3C (-9)
#define CF3X (104)
#define CF3Y (27)
#define CF3Z (-3)
#define CF2A (139)
#define CF2B (-11)
#define CF(x) njClip(((x) + 64) >> 7)
// 通常我们放大图片的时候就需要upsampling，缩小的时候就downsampling，通称为resampling
// 这里Cb/Cr分量的会少些，所以需要upsampling
NJ_INLINE void njUpsampleH(nj_component_t* c) {
printf("njUpsampleH %d\n", c->cid);
const int xmax = c->width - 3;
unsigned char *out, *lin, *lout;
int x, y;
out = njAllocMem((c->width * c->height) << 1);
if (!out) njThrow(NJ_OUT_OF_MEM);
lin = c->pixels;
lout = out;
for (y = c->height; y; --y) {
lout[0] = CF(CF2A * lin[0] + CF2B * lin[1]);
lout[1] = CF(CF3X * lin[0] + CF3Y * lin[1] + CF3Z * lin[2]);
lout[2] = CF(CF3A * lin[0] + CF3B * lin[1] + CF3C * lin[2]);
for (x = 0; x < xmax; ++x) {
lout[(x << 1) + 3] = CF(CF4A * lin[x] + CF4B * lin[x + 1] + CF4C * lin[x + 2] + CF4D * lin[x + 3]);
lout[(x << 1) + 4] = CF(CF4D * lin[x] + CF4C * lin[x + 1] + CF4B * lin[x + 2] + CF4A * lin[x + 3]);
}
lin += c->stride;
lout += c->width << 1;
lout[-3] = CF(CF3A * lin[-1] + CF3B * lin[-2] + CF3C * lin[-3]);
lout[-2] = CF(CF3X * lin[-1] + CF3Y * lin[-2] + CF3Z * lin[-3]);
lout[-1] = CF(CF2A * lin[-1] + CF2B * lin[-2]);
}
c->width <<= 1;
c->stride = c->width;
njFreeMem(c->pixels);
c->pixels = out;
}
NJ_INLINE void njUpsampleV(nj_component_t* c) {
printf("njUpsampleV %d\n", c->cid);
const int w = c->width, s1 = c->stride, s2 = s1 + s1;
unsigned char *out, *cin, *cout;
int x, y;
out = njAllocMem((c->width * c->height) << 1);
if (!out) njThrow(NJ_OUT_OF_MEM);
for (x = 0; x < w; ++x) {
cin = &c->pixels[x];
cout = &out[x];
*cout = CF(CF2A * cin[0] + CF2B * cin[s1]); cout += w;
*cout = CF(CF3X * cin[0] + CF3Y * cin[s1] + CF3Z * cin[s2]); cout += w;
*cout = CF(CF3A * cin[0] + CF3B * cin[s1] + CF3C * cin[s2]); cout += w;
cin += s1;
for (y = c->height - 3; y; --y) {
*cout = CF(CF4A * cin[-s1] + CF4B * cin[0] + CF4C * cin[s1] + CF4D * cin[s2]); cout += w;
*cout = CF(CF4D * cin[-s1] + CF4C * cin[0] + CF4B * cin[s1] + CF4A * cin[s2]); cout += w;
cin += s1;
}
cin += s1;
*cout = CF(CF3A * cin[0] + CF3B * cin[-s1] + CF3C * cin[-s2]); cout += w;
*cout = CF(CF3X * cin[0] + CF3Y * cin[-s1] + CF3Z * cin[-s2]); cout += w;
*cout = CF(CF2A * cin[0] + CF2B * cin[-s1]);
}
c->height <<= 1;
c->stride = c->width;
njFreeMem(c->pixels);
c->pixels = out;
}
#else
NJ_INLINE void njUpsample(nj_component_t* c) {
printf("njUpsample %d\n", c->cid);
int x, y, xshift = 0, yshift = 0;
unsigned char *out, *lin, *lout;
while (c->width < nj.width) { c->width <<= 1; ++xshift; }
while (c->height < nj.height) { c->height <<= 1; ++yshift; }
out = njAllocMem(c->width * c->height); // 放大后的尺寸
if (!out) njThrow(NJ_OUT_OF_MEM);
lin = c->pixels;
lout = out;
for (y = 0; y < c->height; ++y) {
lin = &c->pixels[(y >> yshift) * c->stride];
for (x = 0; x < c->width; ++x)
lout[x] = lin[x >> xshift];
lout += c->width;
}
c->stride = c->width;
njFreeMem(c->pixels);
c->pixels = out;
}
#endif
NJ_INLINE void njConvert() {
int i;
nj_component_t* c;
for (i = 0, c = nj.comp; i < nj.ncomp; ++i, ++c) { // 如果需要的话就upsampling
#if NJ_CHROMA_FILTER
while ((c->width < nj.width) || (c->height < nj.height)) {
if (c->width < nj.width) njUpsampleH(c);
njCheckError();
if (c->height < nj.height) njUpsampleV(c);
njCheckError();
}
#else
if ((c->width < nj.width) || (c->height < nj.height))
njUpsample(c);
#endif
if ((c->width < nj.width) || (c->height < nj.height)) njThrow(NJ_INTERNAL_ERR);
}
if (nj.ncomp == 3) { // SEE njGetImage()
// convert to RGB
int x, yy;
unsigned char *prgb = nj.rgb;
const unsigned char *py = nj.comp[0].pixels;
const unsigned char *pcb = nj.comp[1].pixels;
const unsigned char *pcr = nj.comp[2].pixels;
// 多余的数据(编/解码是对齐用的)会被丢弃吗？
for (yy = nj.height; yy; --yy) { // 列
for (x = 0; x < nj.width; ++x) { // 行
register int y = py[x] << 8; // 这是为什么？ 色彩空间转换公式计算需要
register int cb = pcb[x] - 128; // YCbCr的Cb和Cr一般都是有符号数，但是在JPEG当中都是无符号数
register int cr = pcr[x] - 128;
*prgb++ = njClip((y + 359 * cr + 128) >> 8); // 色彩空间转换，YCbCr到RGB
*prgb++ = njClip((y - 88 * cb - 183 * cr + 128) >> 8);
*prgb++ = njClip((y + 454 * cb + 128) >> 8);
}
py += nj.comp[0].stride; // 移动YCbCr数据指针，每一行都是有stride的，所以当需要的数据都得到时，后面的就不管，直接丢弃，移动到下一行
pcb += nj.comp[1].stride;
pcr += nj.comp[2].stride;
}
} else if (nj.comp[0].width != nj.comp[0].stride) { // 如果宽度和stride都一样，什么都不用做
// grayscale -> only remove stride
unsigned char *pin = &nj.comp[0].pixels[nj.comp[0].stride];
unsigned char *pout = &nj.comp[0].pixels[nj.comp[0].width];
int y;
for (y = nj.comp[0].height - 1; y; --y) {
njCopyMem(pout, pin, nj.comp[0].width);
pin += nj.comp[0].stride;
pout += nj.comp[0].width;
}
nj.comp[0].stride = nj.comp[0].width;
}
}
void njInit(void) {
njFillMem(&nj, 0, sizeof(nj_context_t)); // 初始化nj_context_t
}
void njDone(void) {
int i;
for (i = 0; i < 3; ++i)
if (nj.comp[i].pixels) njFreeMem((void*) nj.comp[i].pixels);
if (nj.rgb) njFreeMem((void*) nj.rgb);
njInit();
}
nj_result_t njDecode(const void* jpeg, const int size) {
njDone();
nj.pos = (const unsigned char*) jpeg;
nj.size = size & 0x7FFFFFFF; // ？
if (nj.size < 2) return NJ_NO_JPEG;
if ((nj.pos[0] ^ 0xFF) | (nj.pos[1] ^ 0xD8)) return NJ_NO_JPEG; // 不以0xFFD8打头(为什么要用异或来判断？)
njSkip(2);
while (!nj.error) { // 有“错误”的时候离开
if ((nj.size < 2) || (nj.pos[0] != 0xFF)) return NJ_SYNTAX_ERROR; // 太小，或者不以0xFF打头
njSkip(2); // 移动到标签的后面(长度字段的前面)
switch (nj.pos[-1]) {
case 0xC0: njDecodeSOF(); break;
case 0xC4: njDecodeDHT(); break;
case 0xDB: njDecodeDQT(); break;
case 0xDD: njDecodeDRI(); break;
case 0xDA: njDecodeScan(); break;
case 0xFE: njSkipMarker(); break;
default:
if ((nj.pos[-1] & 0xF0) == 0xE0) // JPG0和APP0字段，目前都忽略
njSkipMarker();
else
return NJ_UNSUPPORTED;
}
}
if (nj.error != __NJ_FINISHED) return nj.error;
nj.error = NJ_OK;
njConvert();
return nj.error;
}
int njGetWidth(void) { return nj.width; }
int njGetHeight(void) { return nj.height; }
int njIsColor(void) { return (nj.ncomp != 1); }
unsigned char* njGetImage(void) { return (nj.ncomp == 1) ? nj.comp[0].pixels : nj.rgb; } // 一/三个分量
int njGetImageSize(void) { return nj.width * nj.height * nj.ncomp; }
#endif // _NJ_INCLUDE_HEADER_ONLY
请问需要怎样设定输入数据开始调试？
你想什么样方式的调试？调试什么？
我是在visual studio环境下运行，但是显示一个问题，说没有程序入口，而且这个代码是实现jpeg解码功能，我们在调试的时候应该将待解码数据流放在什么位置呢，还有其他的需要一些什么设置呢？
它默认编译是没有main函数的，它的main函数是通过宏_NJ_EXAMPLE_PROGRAM包起来的，所以你编译的时候要启用这个宏，在GCC下通过-D_NJ_EXAMPLE_PROGRAM就可以启用这个宏，所以在VS下你也需要启用这个宏，这样main函数才会参与编译。
运行的时候，这个程序是接受输入参数的，比如我这里./nanojpeg testorig.jpg就有传递一个待解码的图片路径，如果解码成功就会在程序目录下生成一张PPM的图片，也就是你解码出来的结果。
请问这个代码运行完毕后可以得到什么结果，如何查看
在上面评论出回答了，建议你多读读http://keyj.emphy.de/nanojpeg/主页，代码里面的注释，还有这篇博客上面编译运行的命令。
今天仔细研究了代码，终于弄明白整个过程了，多谢了