libjpeg-turbo使用实例（编解码jpeg、jpg转bmp、bmp转jpg代码）

libjpeg-turbo库用于jpeg图像编解码，上一节说了编译过程：编译libjpeg-turbo 。现在说说jpeg的编码、解码使用方法。

Windows上GDI接口支持的都是位图格式（DDB\DIB）图像，这里只说bmp编码成jpeg格式图片并保存到本地和jpeg解码成bmp格式并保存到本地。

bmp转jpeg

int Bmp2Jpeg_Compress(void* lpBmpBuffer, int nWidth, int nHeight, OUT void** ppJpegBuffer, OUT unsigned long* pOutSize)
{
jpeg_compress_struct toWriteInfo;
jpeg_error_mgr errorMgr;
toWriteInfo.err = jpeg_std_error(&errorMgr);
//注册失败的回调函数
toWriteInfo.err->error_exit = error_exit;
jpeg_create_compress(&toWriteInfo);
//保存压缩后的图片
//FILE* fp = NULL;
//_wfopen_s(&fp, L"c:\\output.jpg", L"wb+");
//jpeg_stdio_dest(&toWriteInfo, fp);
//确定要用于输出压缩的jpeg的数据空间
jpeg_mem_dest(&toWriteInfo, (unsigned char**)ppJpegBuffer, pOutSize);
toWriteInfo.image_width = nWidth;
toWriteInfo.image_height = nHeight;
toWriteInfo.jpeg_width = nWidth / 2;
toWriteInfo.jpeg_height = nHeight / 2;
toWriteInfo.input_components = 4;// 在此为1,表示灰度图， 如果是彩色位图，则为4
toWriteInfo.in_color_space = JCS_EXT_BGRA; //JCS_GRAYSCALE表示灰度图，JCS_RGB表示彩色图像
jpeg_set_defaults(&toWriteInfo);
jpeg_set_quality(&toWriteInfo, 100, TRUE);	//设置压缩质量100表示100%
jpeg_start_compress(&toWriteInfo, TRUE);
int nRowStride = nWidth*4;	// 如果不是索引图,此处需要乘以4
JSAMPROW row_pointer[1];	// 一行位图
while (toWriteInfo.next_scanline < toWriteInfo.image_height)
{
row_pointer[0] = (JSAMPROW)((unsigned char*)lpBmpBuffer + toWriteInfo.next_scanline*nRowStride);
jpeg_write_scanlines(&toWriteInfo, row_pointer, 1);
}
jpeg_finish_compress(&toWriteInfo);
jpeg_destroy_compress(&toWriteInfo);
return 0;
}

传入读取的bmp文件二进制数据，输出编码后的jpeg流和大小。

input_components指定为4个字节，目前的Windows支持的都是32位位图；nRowStride = nWidth*4 表示每一行位图数据的字节数，每个像素4字节；还有一个关键的地方in_color_space = JCS_EXT_BGRA，涉及到Windows中数据在内存中的排列方式（little-endian,
低字节存放在内存的低位）ARGB在内存中为BGRA（PS 我尝试过使用JCS_EXT_ARGB，结果颜色全部取反了）。

编码完成后，直接写入文件即可保存为jpeg文件：

//libjpeg为我们压缩好了jpeg数据，只需要往文件里面写入即可
FILE* fpOut = NULL;
fopen_s(&fpOut, "c:\\out.jpg", "wb+");
if (fp)
{
fwrite(pOutBuffer, 1, lOutSize, fpOut);
fclose(fpOut);
}

jpeg转DIB

int Jpeg2DIB_DeCompress(void* lpJpegBuffer, unsigned long nInSize, OUT void** ppDibBuffer, OUT unsigned long* pOutSize, OUT int* pWidth, OUT int* pHeight)
{
jpeg_decompress_struct cInfo;
jpeg_create_decompress(&cInfo);
jpeg_error_mgr errorMgr;
cInfo.err = jpeg_std_error(&errorMgr);
cInfo.err->error_exit = error_exit;
jpeg_mem_src(&cInfo, (const unsigned char*)lpJpegBuffer, nInSize);
jpeg_read_header(&cInfo, TRUE);
jpeg_start_decompress(&cInfo);
JSAMPROW row_pointer[1];
int nBitCounts = cInfo.num_components * 8;
int nWidthBits = cInfo.image_width*cInfo.num_components;// ((cInfo.image_width*nBitCounts + 31) >> 5) << 2;
unsigned long lOutSize = nWidthBits*cInfo.image_height;
unsigned char* pOutBuffer = (unsigned char*)malloc(lOutSize);
row_pointer[0] = pOutBuffer;
while (cInfo.output_scanline<cInfo.output_height)
{
row_pointer[0] = pOutBuffer + (cInfo.image_height - cInfo.output_scanline-1)*nWidthBits;
jpeg_read_scanlines(&cInfo, row_pointer, 1);
}
jpeg_finish_decompress(&cInfo);
jpeg_destroy_decompress(&cInfo);
*ppDibBuffer = pOutBuffer;
*pOutSize = lOutSize;
*pWidth = abs((int)cInfo.image_width);
*pHeight = abs((int)cInfo.image_height);
return 0;
}

int Jpeg2DIB_DeCompress2(const char* pFile, OUT void** ppDibBuffer, OUT unsigned long* pOutSize, OUT int* pWidth, OUT int* pHeight)
{
FILE* fp = NULL;
fopen_s(&fp, pFile, "rb");
if (NULL == fp)
return -1;
fseek(fp, 0, SEEK_END);
long lSize = ftell(fp);
fseek(fp, 0, SEEK_SET);
void* lpBuffer = malloc(lSize);
if (NULL == lpBuffer)
return -1;
size_t nRead = fread(lpBuffer, 1, lSize, fp);
fclose(fp);
int nRet = Jpeg2DIB_DeCompress(lpBuffer, nRead, ppDibBuffer, pOutSize, pWidth, pHeight);
free(lpBuffer);
return nRet;
}

unsigned long lOutSize = nWidthBits*cInfo.image_height 通过jpeg图片的尺寸计算DIB数据区大小，通常是高度*宽度*像素字节数。

和编码一样，解码时jpeg库也是一行一行进行的循环调用 jpeg_read_scanlines。

这样解码后，并不是真正的DIB数据，因为cInfo.num_components=3，也就是说解码的数据是RGB的，每个像素占用3个字节。我们需要再次转换成ARGB的Windows上支持的DIB格式。

//RGB to ARGB
unsigned long nDestSize = nWidth * 4 * nHeight;
DWORD* pArgbData = (DWORD*)malloc(nDestSize);
DWORD* pArgbDataTemp = pArgbData;
unsigned char* pRgbData = (unsigned char*)lpOutData;
int nOffset = nOutSize-3, i = 0;
while (nOffset>=0)
{
/*注意，在window系统中内存以little-endian存储，即低字节存放在内存的低位 0xARGB -- 0xBGRA
/除去忽略的A 即alpha通道位  读取内存中的数据为 BGR 需要转换成 RGB
/bmp位图会忽略掉alpha通道位，设置成任意数值都可以以
*/
DWORD dwColor = 0x00000000 + RGB(pRgbData[nOffset+2], pRgbData[nOffset + 1], pRgbData[nOffset]);
*pArgbDataTemp = dwColor;
pArgbDataTemp++;
nOffset -= 3;
}

循环转换后，得到的就是标准的DIB数据了，可以调用GDI API创建与之关联的位图句柄，然后贴图绘制到界面上。

BITMAP bmp = { 0 };
bmp.bmWidth = nWidth;
bmp.bmHeight = nHeight;
bmp.bmWidthBytes = nWidth * 4;
bmp.bmPlanes = 1;
bmp.bmBitsPixel = 32;
bmp.bmBits = pArgbData;

HBITMAP hBitmap = CreateBitmapIndirect(&bmp);

还可以把DIB数据保存到本地，bmp位图图像。

BID保存为位图文件

int SaveDIBToBmpFile(const char* pFile, void* pDibBuffer, unsigned long nBufferSize, int nWidth, int nHeight)
{
BITMAPFILEHEADER fHeader;
int nStructSize1 = sizeof(BITMAPFILEHEADER);
int nStructSize2 = sizeof(BITMAPINFO)-sizeof(RGBQUAD);
memset(&fHeader, 0, nStructSize1);
memcpy(&fHeader, "BM", 2);
fHeader.bfSize = nStructSize1 + nStructSize2 + nBufferSize;
fHeader.bfOffBits = nStructSize1 + nStructSize2;
BITMAPINFO bmpInfo = { nStructSize2 };
bmpInfo.bmiHeader.biWidth = nWidth;
bmpInfo.bmiHeader.biHeight = nHeight;
bmpInfo.bmiHeader.biPlanes = 1;
bmpInfo.bmiHeader.biBitCount = 32;
bmpInfo.bmiHeader.biCompression = BI_RGB;
bmpInfo.bmiHeader.biSizeImage = 0;
FILE* fp = NULL;
fopen_s(&fp, pFile, "wb+");
if (NULL == fp)
return -1;
/*	写入 bmp位图文件，
*	文件格式：位图文件头+位图信息头+位图数据
*	写入数据头前，需要填充相应的字段
*	详细结构说明，见 http://blog.csdn.net/mfcing/article/details/7451670 */
fwrite(&fHeader, 1, nStructSize1, fp);
fwrite(&bmpInfo, 1, nStructSize2, fp);
fwrite(pDibBuffer, 1, nBufferSize, fp);
fclose(fp);
return 0;
}

程序运行图

jpeg图片转换成bmp图片后，保存成功



本实例说的不够清楚，完整源码下载：http://download.csdn.net/detail/mfcing/9638178