使用libjpeg库实现jpeg图片的缩放（缩略图） |
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libjpeg库的交叉编译
libjpeg库主要用于jpeg格式图片的编解码，其交叉编译过程如下 1. 下载源码从官方网站http://www.ijg.org/files/ 下载libjpeg库的源码，本次编译过程使用的是 jpegsrc.v9a.tar.gz 2. 解压源码2.1 切换到下载目录，执行 tar -xzvf jpegsrc.v9a.tar.gz2.2 切换到源码目录 cd jpeg-9a/ 3. 交叉编译3.1 设置交叉编译器的环境变量 export CC=/home/jarvischu/arm-linux-uclibcgnueabi-gcc3.2 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-linux3.3 执行make命令 make3.4 执行make install 命令 sudo make install至此，交叉编译结束，编译结果存放在/home/jarvischu/jpeg目录下，该目录包含了编译得到的lib和include 使用libjpeg库实现jpeg图片的缩放（缩略图） #include #include "jpeglib.h" #include 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 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 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 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 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; } } |
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