libjpeg的交叉编译以及jpeg图片的缩放（缩略图）

# 2.1 切换到下载目录，执行
tar -xzvf jpegsrc.v9a.tar.gz

# 2.2 切换到源码目录
cd jpeg-9a/ 

# 3.1 设置交叉编译器的环境变量
export CC=/home/jarvischu/arm-linux-uclibcgnueabi-gcc

# 3.2 执行configure，其中--prefix 用来指定编译结果的存放位置; --host 用来指明交叉编译
./configure --prefix=/home/jarvischu/jpeg --enable-shared --enable-static --host=arm-unknown-linux

# 3.3 执行make命令
make

# 3.4 执行make install 命令
sudo make install

#include <stdio.h>
#include "jpeglib.h"
#include <setjmp.h>

struct my_error_mgr {
    struct jpeg_error_mgr pub; /* "public" fields */
    jmp_buf setjmp_buffer; /* for return to caller */
};

typedef struct my_error_mgr * my_error_ptr;

/*
* Here's the routine that will replace the standard error_exit method:
*/

METHODDEF(void) my_error_exit (j_common_ptr cinfo)
{
    /* cinfo->err really points to a my_error_mgr struct, so coerce pointer */
    my_error_ptr myerr = (my_error_ptr) cinfo->err;

    /* Always display the message. */
    /* We could postpone this until after returning, if we chose. */
    (*cinfo->err->output_message) (cinfo);

    /* Return control to the setjmp point */
    longjmp(myerr->setjmp_buffer, 1);
}

//读取Jpeg图片的数据并返回，如果出错，返回NULL
unsigned char* ReadJpeg(const char* path, int& width, int& height)
{
    FILE *file = fopen( path, "rb" );
    if ( file == NULL ) {
        return NULL;
    }

    struct jpeg_decompress_struct info; //for our jpeg info

    // struct jpeg_error_mgr err; //the error handler
    // info.err = jpeg_std_error(&err);

    struct my_error_mgr my_err;

    info.err = jpeg_std_error(&my_err.pub);
    my_err.pub.error_exit = my_error_exit;

    /* Establish the setjmp return context for my_error_exit to use. */
    if (setjmp(my_err.setjmp_buffer)) {
        /* If we get here, the JPEG code has signaled an error.
        * We need to clean up the JPEG object, close the input file, and return.
        */
        printf("Error occured\n");
        jpeg_destroy_decompress(&info);
        fclose(file);
        return NULL;
    }

    jpeg_create_decompress( &info ); //fills info structure
    jpeg_stdio_src( &info, file ); //void

    int ret_Read_Head = jpeg_read_header( &info, 1 ); //int

    if(ret_Read_Head != JPEG_HEADER_OK){
        printf("jpeg_read_header failed\n");
        fclose(file);
        jpeg_destroy_decompress(&info);
        return NULL;
    }

    bool bStart = jpeg_start_decompress( &info );
    if(!bStart){
        printf("jpeg_start_decompress failed\n");
        fclose(file);
        jpeg_destroy_decompress(&info);
        return NULL;
    }
    int w = width = info.output_width;
    int h = height = info.output_height;
    int numChannels = info.num_components; // 3 = RGB, 4 = RGBA
    unsigned long dataSize = w * h * numChannels;

    // read RGB(A) scanlines one at a time into jdata[]
    unsigned char *data = (unsigned char *)malloc( dataSize );
    if(!data) return NULL;

    unsigned char* rowptr;
    while ( info.output_scanline < h )
    {
        rowptr = data + info.output_scanline * w * numChannels;
        jpeg_read_scanlines( &info, &rowptr, 1 );
    }

    jpeg_finish_decompress( &info );

    fclose( file );

    return data;
}

/* 图片的线性缩放
*参数为：
*返回图片的宽度(w_Dest),
*返回图片的高度(h_Dest),
*返回图片的位深(bit_depth),
*源图片的RGB数据(src),
*源图片的宽度(w_Src),
*源图片的高度(h_Src)
*/
unsigned char* do_Stretch_Linear(int w_Dest,int h_Dest,int bit_depth,unsigned char *src,int w_Src,int h_Src)
{
    int sw = w_Src-1, sh = h_Src-1, dw = w_Dest-1, dh = h_Dest-1;
    int B, N, x, y;
    int nPixelSize = bit_depth/8;
    unsigned char *pLinePrev,*pLineNext;
    unsigned char *pDest = new unsigned char[w_Dest*h_Dest*bit_depth/8];
    unsigned char *tmp;
    unsigned char *pA,*pB,*pC,*pD;

    for(int i=0;i<=dh;++i)
    {
        tmp =pDest + i*w_Dest*nPixelSize;
        y = i*sh/dh;
        N = dh - i*sh%dh;
        pLinePrev = src + (y++)*w_Src*nPixelSize;
        //pLinePrev =(unsigned char *)aSrc->m_bitBuf+((y++)*aSrc->m_width*nPixelSize);
        pLineNext = (N==dh) ? pLinePrev : src+y*w_Src*nPixelSize;
        //pLineNext = ( N == dh ) ? pLinePrev : (unsigned char *)aSrc->m_bitBuf+(y*aSrc->m_width*nPixelSize);
        for(int j=0;j<=dw;++j)
        {
            x = j*sw/dw*nPixelSize;
            B = dw-j*sw%dw;
            pA = pLinePrev+x;
            pB = pA+nPixelSize;
            pC = pLineNext + x;
            pD = pC + nPixelSize;
            if(B == dw)
            {
                pB=pA;
                pD=pC;
            }

            for(int k=0;k<nPixelSize;++k)
            {
                *tmp++ = ( unsigned char )( int )(
                ( B * N * ( *pA++ - *pB - *pC + *pD ) + dw * N * *pB++
                + dh * B * *pC++ + ( dw * dh - dh * B - dw * N ) * *pD++
                + dw * dh / 2 ) / ( dw * dh ) );
            }
        }
    }
    return pDest;
}

bool write_JPEG_file (const char * filename, unsigned char* image_buffer, int quality, int image_width, int image_height)
{

    if(filename == NULL || image_buffer == NULL) return false;

    struct jpeg_compress_struct cinfo;
    struct jpeg_error_mgr jerr;
    FILE * outfile; /* target file */
    JSAMPROW row_pointer[1]; /* pointer to JSAMPLE row[s] */
    int row_stride; /* physical row width in image buffer */
    cinfo.err = jpeg_std_error(&jerr);
    /* Now we can initialize the JPEG compression object. */
    jpeg_create_compress(&cinfo);

    if ((outfile = fopen(filename, "wb")) == NULL) {
        fprintf(stderr, "can't open %s\n", filename);
        return false;
    }
    jpeg_stdio_dest(&cinfo, outfile);

    cinfo.image_width = image_width; /* image width and height, in pixels */
    cinfo.image_height = image_height;
    cinfo.input_components = 3; /* # of color components per pixel */
    cinfo.in_color_space = JCS_RGB; /* colorspace of input image */

    jpeg_set_defaults(&cinfo);
    jpeg_set_quality(&cinfo, quality, TRUE /* limit to baseline-JPEG values */);

    jpeg_start_compress(&cinfo, TRUE);

    row_stride = image_width * 3; /* JSAMPLEs per row in image_buffer */

    while (cinfo.next_scanline < cinfo.image_height) {
        row_pointer[0] = & image_buffer[cinfo.next_scanline * row_stride];
        (void) jpeg_write_scanlines(&cinfo, row_pointer, 1);
    }

    jpeg_finish_compress(&cinfo);
    fclose(outfile);

    jpeg_destroy_compress(&cinfo);

    return true;
}

//生成图片的缩略图（图片的一个缩小版本）
bool generate_image_thumbnail(const char* inputFile, const char* outputFile)
{
    if(inputFile == NULL || outputFile == NULL) return false;

    //读取jpeg图片像素数组
    int w=0,h=0;
    unsigned char* buff = ReadJpeg(inputFile,w,h);
    if(buff == NULL) {
        printf("ReadJpeg Failed\n");
        return false;
    }

    //缩放图片，缩放后的大小为(tb_w,tb_h)
    int tb_w = 160, tb_h = 160;
    unsigned char * img_buf = do_Stretch_Linear(tb_w,tb_h,24,buff,w,h);
    free(buff);

    //将缩放后的像素数组保存到jpeg文件
    bool bRetWrite = write_JPEG_file(outputFile,img_buf,10,tb_w,tb_h);
    delete[] img_buf;

    if(bRetWrite){
        return true;
    }else{
        printf("GenerateImageThumbnail: write failed\n");
        return true;
    }
}