opencv 手写选择题阅卷 (一)表格设计与识别

(一)答题表格设计与识别

实际设计好的表格如下图

为了图像精确,表格和四角的标记都是由程序生成的,文字和数据是后期排版软件添加上去的.

图中四角的四个黑方块主要用来定位表格,然后就可以切割出每个单元格,最后去做字符识别.

具体步骤为:

1,灰度化并二值化;

2,查找轮廓,把找出四个定位标记;

3,透视变换,校正变形;

4,切割表格,分别识别每个表格;

实际操作中发现最关键的是表格一定要平整,变形对识别影响较大;

部分代码:

int table_recognition(IplImage* img,unsigned char * result)
{
    //大图二值化
    IplImage* bin_img = cvCloneImage(img);
    image_threshold(bin_img);
    //去噪
    IplImage* tmp_img = cvCloneImage(bin_img);
    cvErode(tmp_img, tmp_img, NULL, 1); //腐蚀
    cvDilate(tmp_img, tmp_img, NULL, 1); //膨胀
    //查找轮廓
    CvSeq* contours;
    CvMemStorage * storage = cvCreateMemStorage(0);

    cvSetImageROI(tmp_img, cvRect(0, 0, bin_img->width, bin_img->height));
    cvFindContours(tmp_img, storage, &contours, sizeof(CvContour), CV_RETR_LIST, CV_CHAIN_APPROX_SIMPLE, cvPoint(0, 0));

    int ids[5];
    int rotates[5];
    int vals[5];
    CvPoint points[5][4];
    int n = 0;
    // 检测每个轮廓
    for (; contours; contours = contours->h_next)
    {
        //用指定精度逼近多边形曲线
        CvSeq* result;
        result = cvApproxPoly(contours, sizeof(CvContour), storage, CV_POLY_APPROX_DP, cvContourPerimeter(contours)*0.01, 0);

        //不是四边形的不要
        if (result->total != 4)
            continue;
        //不是凸多边形不要
        if (!cvCheckContourConvexity(result))
            continue;
        //面积大小或小于指定值的排除
        double s = fabs(cvContourArea(result, CV_WHOLE_SEQ, 0));
        if (s<60)
            continue;

        //解码每个轮廓标志,正确的保存下来**********************************************
        CvPoint2D32f  srcQuad[4];
        for (int i = 0; i < 4; i++){
            CvPoint* pt = (CvPoint*)cvGetSeqElem(result, i);//取标记四边形的四个顶点
            points[n][i] = *pt;
            srcQuad[i].x = (float)pt->x;
            srcQuad[i].y = (float)pt->y;
        }
        //透视变换取出marker
        IplImage * mark_img = cvCreateImage(cvSize(40,40), 8, 1);
        perspective(bin_img, mark_img, srcQuad);

        //
        int rt = marker_decode(mark_img, &ids[n], &rotates[n], &vals[n]);
        if (rt !=0)
            continue;

        //
        n++;
        if (n>4)
            break;
    }
    if (n != 4)//发现四个标记
        return -1;
    //if (rotates[0] != rotates[1] || rotates[1] != rotates[2] || rotates[2] != rotates[3])//四个标记旋转一致
    //    return -1;
    //marker 0123
    if (ids[0] != 0 && ids[1] != 0 && ids[2] != 0 && ids[3] != 0)
        return -1;
    if (ids[0] != 1 && ids[1] != 1 && ids[2] != 1 && ids[3] != 1)
        return -1;
    if (ids[0] != 2 && ids[1] != 2 && ids[2] != 2 && ids[3] != 2)
        return -1;
    if (ids[0] != 3 && ids[1] != 3 && ids[2] != 3 && ids[3] != 3)
        return -1;

    //确定表格四个点
    CvPoint2D32f  pts[4];
    for (int i = 0; i < 4; i++)
    {
        int id = ids[i];
        int rotate = rotates[i];
        CvPoint pt;
        if (id == 0){
            pt = points[i][(1 + rotate)%4];
        }
        else if (id == 1){
            pt = points[i][(0 + rotate) % 4];
        }
        else if (id == 2){
            pt = points[i][(3 + rotate) % 4];
        }
        else if (id == 3){
            pt = points[i][(2 + rotate) % 4];
        }
        pts[id].x = pt.x;
        pts[id].y = pt.y;
    }

    //CvPoint2D32f tmp_ptf = pts[1];
    //pts[1] = pts[3];
    //pts[3] = tmp_ptf;

    IplImage * table_img = cvCreateImage(cvSize(64*15, 64*4+32), 8, 1);
    perspective(img, table_img, pts);

    //表格分割
    int nt = 0;
    IplImage* gird_img = cvCreateImage(cvSize(64, 64), 8, 1);
    for (int j = 1; j < 4; j+=2)
    {
        for (int i = 0; i < 15; i++)
        {
            cvSetImageROI(table_img, cvRect(0+64*i, 15+64*j, 64, 64));
            cvCopy(table_img, gird_img);
#ifdef _WIN32
            save_gird(gird_img, nt);
#endif
            int rt = svm_recognition(gird_img);
            result[nt] = rt;
            nt++;
        }
    }

    //cvNamedWindow("Image", CV_WINDOW_NORMAL);
    //cvShowImage("Image", gird_img);
    //cvWaitKey(0);

    cvReleaseImage(&bin_img);
    cvClearMemStorage(storage);

    return 0;
}