使用LibJpeg图像解码

1 简介

libjpeg一个图片解码库，在项目中需要读入图片，但不想依赖opencv的接口，这时可以libjpeg完成解码。libjpeg有两个版本，一个时原装的libjpeg，另一个则是libjpeg-turbo，这是一个使用 SIMD指令加速的解码库，大约是libjpeg的3倍的速度，代码参见 https://github.com/libjpeg-turbo/libjpeg-turbo

2 编译

下载源代码，cmake，nasm 参考（https://github.com/libjpeg-turbo/libjpeg-turbo/blob/master/BUILDING.md）

3代码

struct JPEGINFO {
unsigned int width;
unsigned int height;
ColorType colortype;
unsigned char* dstImg;
};
/*
*  输入图片名，输出unsigned char*图片数据
*
*/
int readjpeg(std::string file_name,
JPEGINFO &jpeginfo)
{
struct jpeg_decompress_struct cinfo;
struct my_error_mgr jerr;
FILE * infile;
JSAMPARRAY buffer;
int row_stride;
errno_t err;
if ((err = fopen_s(&infile, file_name.c_str(), "rb")) != 0)
{
fprintf(stderr, "can't open %s\n", file_name);
return -1;
}
cinfo.err = jpeg_std_error(&jerr.pub);
jerr.pub.error_exit = my_error_exit;
if (setjmp(jerr.setjmp_buffer)) {
jpeg_destroy_decompress(&cinfo);
fclose(infile);
return -1;
}
jpeg_create_decompress(&cinfo);
jpeg_stdio_src(&cinfo, infile);

(void)jpeg_read_header(&cinfo, TRUE);

(void)jpeg_start_decompress(&cinfo);
row_stride = cinfo.output_width * cinfo.output_components;

int cols = cinfo.output_width;
int rows = cinfo.output_height;

buffer = (*cinfo.mem->alloc_sarray)((j_common_ptr)&cinfo, JPOOL_IMAGE, row_stride, 1);

jpeginfo.width = cinfo.output_width;
jpeginfo.height = cinfo.output_height;
if (cinfo.output_components == 3)
{
jpeginfo.colortype = RGB;
jpeginfo.dstImg = new unsigned char[jpeginfo.width*jpeginfo.height * 3+1];

}//gray
else{
jpeginfo.colortype = Gray;
jpeginfo.dstImg = new unsigned char[jpeginfo.width * jpeginfo.height+1];
}

while (cinfo.output_scanline < cinfo.output_height) {
(void)jpeg_read_scanlines(&cinfo, buffer, 1);
for (size_t i = 0; i < cinfo.output_width; i++)
{
//rgb
if (jpeginfo.colortype == RGB)
{
jpeginfo.dstImg[(cinfo.output_scanline - 1)*jpeginfo.width*3 + i*3] = buffer[0][i * 3];
jpeginfo.dstImg[(cinfo.output_scanline - 1)*jpeginfo.width*3 + i*3 + 1] = buffer[0][i * 3 + 1];
jpeginfo.dstImg[(cinfo.output_scanline - 1)*jpeginfo.width *3+ i*3 + 2] = buffer[0][i * 3 + 2];
}
else if (jpeginfo.colortype == Gray)
{
jpeginfo.dstImg[(cinfo.output_scanline - 1)*jpeginfo.width + i] = buffer[0][i];
}
}
}

(void)jpeg_finish_decompress(&cinfo);

jpeg_destroy_decompress(&cinfo);
fclose(infile);
return 0;

}