使用libjpeg库读取JPEG图像数据

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// 这些和错误处理有关，不用管
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;

METHODDEF(void) my_error_exit(j_common_ptr cinfo)
{
	my_error_ptr myerr = (my_error_ptr)cinfo->err;
	(*cinfo->err->output_message) (cinfo);
	longjmp(myerr->setjmp_buffer, 1);
}

// 读取图像的函数
// 我添加了一个参数，是我库里的zMatrix类对象，用于保存读取的图片数据
GLOBAL(int) read_JPEG_file(char * filename, z::Matrix8u & img)
{
	struct jpeg_decompress_struct cinfo;
	struct my_error_mgr jerr;
	FILE * infile;		
	JSAMPARRAY buffer;		
	int row_stride;	

	if ((infile = fopen(filename, "rb")) == NULL) {
		fprintf(stderr, "can't open %sn", filename);
		return 0;
	}
	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 0;
	}
	jpeg_create_decompress(&cinfo);

	jpeg_stdio_src(&cinfo, infile);

	// 这个函数获取了读取图片的信息，包括图片的高和宽
	(void)jpeg_read_header(&cinfo, TRUE);
	// 在这里添加你自己的代码，获取或用户到图像信息
	img.create(cinfo.image_height, cinfo.image_width, 3);

	(void)jpeg_start_decompress(&cinfo);
	
	row_stride = cinfo.output_width * cinfo.output_components;
	buffer = (*cinfo.mem->alloc_sarray)((j_common_ptr)&cinfo, JPOOL_IMAGE, row_stride, 1);
	
	while (cinfo.output_scanline < cinfo.output_height) {
	    // 一行一行的读取
		(void)jpeg_read_scanlines(&cinfo, buffer, 1);
		
		// 在这里添加代码获取到图片的像素数据
		// buffer保存了读取的当前行的数据，保存顺序是RGB
		// output_scanline是已经读取过的行数
		for (int i = 0; i < img.cols; ++i) {
			img[cinfo.output_scanline - 1][i * 3 + 2] = buffer[0][i * 3 + 0];
			img[cinfo.output_scanline - 1][i * 3 + 1] = buffer[0][i * 3 + 1];
			img[cinfo.output_scanline - 1][i * 3 + 0] = buffer[0][i * 3 + 2];
		}
	}

	(void)jpeg_finish_decompress(&cinfo);
	jpeg_destroy_decompress(&cinfo);
	fclose(infile);
	return 1;
}