OpenCV开发笔记（二十八）：带你学习图像识别之阈值化

前言

对于图像识别来说，首先要做预处理，预处理第一步是去噪，在之前的篇章中介绍了大量的去噪算法，接下来越过算法，进入识别的第二步：阈值化，阈值化是图像识别比较重要的操作之一。

Demo

阈值化

概述

图像阈值化通常是图像识别预处理阶段的开始，它消除了所有的颜色信息，大多数的OpoenCV的函数需要有用信息处被填入白色，背景被填入黑色。

在对各种各项进行处理操作的过程中，通道也需要对图像中的像素做出取舍与角色，直接提出一些地域或者高于一定值的像素。

阈值可以被视作做简单的图像分割方法，比如从一副图像中利用阈值分割出我们需要的物体部分，这样的图像风格防范基于图像中物体与背景之间的灰度差异，而且此分割属于像素级的分割。

为了从一副图像中提取出需要的部分，需要调整调整灰度值与阈值进行比较，并作出相应的判断，尤其注意：图像处理中，对于具体的图像处理其实是很依赖问题本身的，阈值化就依赖于具体的问题，物体在不同的图像中存在不同的灰度值，我们需要找到需要分割物体的像素点，可以对这些像素点设定一些特定的阈值。

为了突出分割，阈值化之后只会存在两种结果，超过阈值的和低于阈值的，通常的操作就是达到要求的设置为黑色或者白色，而对应的就设置为白色或者黑色，俗称“黑白配”。

如下图：虚线代表阈值：

double threshold( InputArray src,
               OutputArray dst,
               double thresh,
               double maxval,
               int type );

参数一：InputArray类型，一般是cv::Mat，且可以处理多通道，8或者32位浮点（注意：当使用THRESH_BINARY处理多通道的时候，每个通道都会进行阈值化，比如RGB三通道，那么可能R比G,B大，当阈值设置为大于G、B小于R时，则R为最大是，显示红色，其他类型的阈值形式类推）。
参数二；OutputArray类型，输出的目标图像，需要和原图片有一样的尺寸和类型。
参数三：double类型的thresh，阈值。
参数四：double类型的maxval，与“THRESH_BINARY”枚举和“THRESH_BINARY_INV”枚举一起使用才有效果，其他枚举忽略。
参数五：int类型的type，阈值类型。

颜色空间转换函数原型

void cvtColor( InputArray src,
            OutputArray dst,
            int code,
            int dstCn = 0 );

参数一：InputArray类型，一般是cv::Mat，8位无符号，16位无符号（CV_16UC…）或单精度浮点，主要与code对颜色空间的转换一一对应。
参数二；OutputArray类型，输出的目标图像，主要与code对颜色空间的转换一一对应（通道数），与输入图像的尺寸大小和深度是一样的，当参数四dstCn不等于0时，则依据设置生成。
参数三：int类型的code，代码颜色空间转换代码（请参见ColorConversionCodes枚举）。
参数四：int类型的dstCn，目标图像中的通道数；如果参数为0，则通道是从src和代码自动派生的。

Demo源码

void OpenCVManager::testThreshold()
{
    QString fileName1 = "I:/qtProject/openCVDemo/openCVDemo/modules/openCVManager/images/1.jpg";
    cv::Mat srcMat = cv::imread(fileName1.toStdString());

    cv::Mat thresh1Mat = cv::imread("I:/qtProject/openCVDemo/openCVDemo/modules/openCVManager/images/thresh1.png");
    cv::Mat thresh2Mat = cv::imread("I:/qtProject/openCVDemo/openCVDemo/modules/openCVManager/images/thresh2.png");
    cv::Mat thresh3Mat = cv::imread("I:/qtProject/openCVDemo/openCVDemo/modules/openCVManager/images/thresh3.png");
    cv::Mat thresh4Mat = cv::imread("I:/qtProject/openCVDemo/openCVDemo/modules/openCVManager/images/thresh4.png");
    cv::Mat thresh5Mat = cv::imread("I:/qtProject/openCVDemo/openCVDemo/modules/openCVManager/images/thresh5.png");
    cv::resize(thresh1Mat, thresh1Mat, cv::Size(160, 80));
    cv::resize(thresh2Mat, thresh2Mat, cv::Size(160, 80));
    cv::resize(thresh3Mat, thresh3Mat, cv::Size(160, 80));
    cv::resize(thresh4Mat, thresh4Mat, cv::Size(160, 80));
    cv::resize(thresh5Mat, thresh5Mat, cv::Size(160, 80));

    int width = 200;
    int height = 160;
    cv::resize(srcMat, srcMat, cv::Size(width, height));

    cv::String windowName = _windowTitle.toStdString();
    cvui::init(windowName);

    if(!srcMat.data)
    {
        qDebug() << __FILE__ << __LINE__
                 << "Failed to load image:" << fileName1;
        return;
    }

    qDebug() << __FILE__ << __LINE__
             << "Succeed to load image, type =" << srcMat.type()
             << "channels = " << srcMat.channels();

    cv::Mat dstMat;
    dstMat = cv::Mat::zeros(srcMat.size(), srcMat.type());
    cv::Mat windowMat = cv::Mat(cv::Size(dstMat.cols * 6, dstMat.rows * 4),
                                srcMat.type());

    int thresh = 100;
    int maxval = 255;

    while(true)
    {
        windowMat = cv::Scalar(0, 0, 0);
        // 原图先copy到左边
        cv::Mat leftMat = windowMat(cv::Range(0, srcMat.rows),
                                    cv::Range(0, srcMat.cols));
        cv::addWeighted(leftMat, 1.0f, srcMat, 1.0f, 0.0f, leftMat);

        // 调整阈值化的参数thresh
        cvui::printf(windowMat, width * 2 + 100, 20 + height * 0, "thresh");
        cvui::trackbar(windowMat, width * 2 + 100, 40 + height * 0, 250, &thresh, 0, 255);

        // 调整阈值化的参数maxval
        cvui::printf(windowMat, width * 2 + 100, 80 + height * 0, "maxval");
        cvui::trackbar(windowMat, width * 2 + 100, 100 + height * 0, 250, &maxval, 0, 255);

        cv::Mat tempMat;
        cvui::printf(windowMat, width * 0 + 40, 10 + height * 1, "THRESH_BINARY");
        cvui::printf(windowMat, width * 0 + 40, 25 + height * 1, "thresh = %d", thresh);
        cvui::printf(windowMat, width * 0 + 40, 40 + height * 1, "maxval = %d", maxval);
        tempMat = windowMat(cv::Range(srcMat.rows * 1 + 70, srcMat.rows * 2 - 10),
                            cv::Range(srcMat.cols * 0 + 20, srcMat.cols * 1 - 20));
        cv::addWeighted(tempMat, 0.0f, thresh1Mat, 1.0f, 0.0f, tempMat);
        cvui::printf(windowMat, width * 1 + 40, 10 + height * 1, "THRESH_BINARY_INV");
        cvui::printf(windowMat, width * 1 + 40, 25 + height * 1, "thresh = %d", thresh);
        cvui::printf(windowMat, width * 1 + 40, 40 + height * 1, "maxval = %d", maxval);
        tempMat = windowMat(cv::Range(srcMat.rows * 1 + 70, srcMat.rows * 2 - 10),
                            cv::Range(srcMat.cols * 1 + 20, srcMat.cols * 2 - 20));
        cv::addWeighted(tempMat, 0.0f, thresh2Mat, 1.0f, 0.0f, tempMat);
        cvui::printf(windowMat, width * 2 + 40, 10 + height * 1, "THRESH_TRUNC");
        cvui::printf(windowMat, width * 2 + 40, 25 + height * 1, "thresh = %d", thresh);
        cvui::printf(windowMat, width * 2 + 40, 40 + height * 1, "maxval = %d", maxval);
        tempMat = windowMat(cv::Range(srcMat.rows * 1 + 70, srcMat.rows * 2 - 10),
                            cv::Range(srcMat.cols * 2 + 20, srcMat.cols * 3 - 20));
        cv::addWeighted(tempMat, 0.0f, thresh3Mat, 1.0f, 0.0f, tempMat);
        cvui::printf(windowMat, width * 3 + 40, 10 + height * 1, "THRESH_TOZERO");
        cvui::printf(windowMat, width * 3 + 40, 25 + height * 1, "thresh = %d", thresh);
        cvui::printf(windowMat, width * 3 + 40, 40 + height * 1, "maxval = %d", maxval);
        tempMat = windowMat(cv::Range(srcMat.rows * 1 + 70, srcMat.rows * 2 - 10),
                            cv::Range(srcMat.cols * 3 + 20, srcMat.cols * 4 - 20));
        cv::addWeighted(tempMat, 0.0f, thresh4Mat, 1.0f, 0.0f, tempMat);
        cvui::printf(windowMat, width * 4 + 40, 10 + height * 1, "THRESH_TOZERO_INV");
        cvui::printf(windowMat, width * 4 + 40, 25 + height * 1, "thresh = %d", thresh);
        cvui::printf(windowMat, width * 4 + 40, 40 + height * 1, "maxval = %d", maxval);
        tempMat = windowMat(cv::Range(srcMat.rows * 1 + 70, srcMat.rows * 2 - 10),
                            cv::Range(srcMat.cols * 4 + 20, srcMat.cols * 5 - 20));
        cv::addWeighted(tempMat, 0.0f, thresh5Mat, 1.0f, 0.0f, tempMat);
        cvui::printf(windowMat, width * 5 + 40, 10 + height * 1, "THRESH_MASK");
        cvui::printf(windowMat, width * 5 + 40, 25 + height * 1, "thresh = %d", thresh);
        cvui::printf(windowMat, width * 5 + 40, 40 + height * 1, "maxval = %d", maxval);

        // 转换成灰度图像
        cv::Mat grayMat;    // 多通道
        cv::Mat grayMat2;   // 单通道
#if 1
        // CV_XXXX 与 cv::COLOR_BGR2GRAY 实际并没有区别 是高低版本表现形式的问题
        cv::cvtColor(srcMat, grayMat2, CV_BGR2GRAY);
        cv::cvtColor(grayMat2, grayMat, CV_GRAY2BGR);
#else
        cv::cvtColor(srcMat, grayMat2, cv::COLOR_BGR2GRAY);
        cv::cvtColor(grayMat2, grayMat, cv::COLOR_GRAY2BGR);
#endif
        // 效果图copy
        cv::Mat rightMat = windowMat(cv::Range(srcMat.rows * 0, srcMat.rows * 1),
                                     cv::Range(srcMat.cols * 1, srcMat.cols * 2));
        cv::addWeighted(rightMat, 0.0f, grayMat, 1.0f, 0.0f, rightMat);

        {
            cv::threshold(srcMat, dstMat, thresh, maxval, cv::THRESH_BINARY);
            // 效果图copy
            cv::Mat center = windowMat(cv::Range(srcMat.rows * 2, srcMat.rows * 3),
                                         cv::Range(srcMat.cols * 0, srcMat.cols * 1));
            cv::addWeighted(center, 0.0f, dstMat, 1.0f, 0.0f, center);

            cv::threshold(srcMat, dstMat, thresh, maxval, cv::THRESH_BINARY_INV);
            // 效果图copy
            cv::Mat center2 = windowMat(cv::Range(srcMat.rows * 2, srcMat.rows * 3),
                                         cv::Range(srcMat.cols * 1, srcMat.cols * 2));
            cv::addWeighted(center2, 0.0f, dstMat, 1.0f, 0.0f, center2);

            cv::threshold(srcMat, dstMat, thresh, maxval, cv::THRESH_TRUNC);
            // 效果图copy
            cv::Mat center3 = windowMat(cv::Range(srcMat.rows * 2, srcMat.rows * 3),
                                         cv::Range(srcMat.cols * 2, srcMat.cols * 3));
            cv::addWeighted(center3, 0.0f, dstMat, 1.0f, 0.0f, center3);

            cv::threshold(srcMat, dstMat, thresh, maxval, cv::THRESH_TOZERO);
            // 效果图copy
            cv::Mat center4 = windowMat(cv::Range(srcMat.rows * 2, srcMat.rows * 3),
                                         cv::Range(srcMat.cols * 3, srcMat.cols * 4));
            cv::addWeighted(center4, 0.0f, dstMat, 1.0f, 0.0f, center4);

            cv::threshold(srcMat, dstMat, thresh, maxval, cv::THRESH_TOZERO_INV);
            // 效果图copy
            cv::Mat center5 = windowMat(cv::Range(srcMat.rows * 2, srcMat.rows * 3),
                                         cv::Range(srcMat.cols * 4, srcMat.cols * 5));
            cv::addWeighted(center5, 0.0f, dstMat, 1.0f, 0.0f, center5);


            cv::threshold(srcMat, dstMat, thresh, maxval, cv::THRESH_MASK);
            // 效果图copy
            cv::Mat center6 = windowMat(cv::Range(srcMat.rows * 2, srcMat.rows * 3),
                                         cv::Range(srcMat.cols * 5, srcMat.cols * 6));
            cv::addWeighted(center6, 0.0f, dstMat, 1.0f, 0.0f, center6);
        }

        {
            cv::threshold(grayMat, dstMat, thresh, maxval, cv::THRESH_BINARY);
            // 效果图copy
            cv::Mat center = windowMat(cv::Range(srcMat.rows * 3, srcMat.rows * 4),
                                         cv::Range(srcMat.cols * 0, srcMat.cols * 1));
            cv::addWeighted(center, 0.0f, dstMat, 1.0f, 0.0f, center);

            cv::threshold(grayMat, dstMat, thresh, maxval, cv::THRESH_BINARY_INV);
            // 效果图copy
            cv::Mat center2 = windowMat(cv::Range(srcMat.rows * 3, srcMat.rows * 4),
                                         cv::Range(srcMat.cols * 1, srcMat.cols * 2));
            cv::addWeighted(center2, 0.0f, dstMat, 1.0f, 0.0f, center2);

            cv::threshold(grayMat, dstMat, thresh, maxval, cv::THRESH_TRUNC);
            // 效果图copy
            cv::Mat center3 = windowMat(cv::Range(srcMat.rows * 3, srcMat.rows * 4),
                                         cv::Range(srcMat.cols * 2, srcMat.cols * 3));
            cv::addWeighted(center3, 0.0f, dstMat, 1.0f, 0.0f, center3);

            cv::threshold(grayMat, dstMat, thresh, maxval, cv::THRESH_TOZERO);
            // 效果图copy
            cv::Mat center4 = windowMat(cv::Range(srcMat.rows * 3, srcMat.rows * 4),
                                         cv::Range(srcMat.cols * 3, srcMat.cols * 4));
            cv::addWeighted(center4, 0.0f, dstMat, 1.0f, 0.0f, center4);

            cv::threshold(grayMat, dstMat, thresh, maxval, cv::THRESH_TOZERO_INV);
            // 效果图copy
            cv::Mat center5 = windowMat(cv::Range(srcMat.rows * 3, srcMat.rows * 4),
                                         cv::Range(srcMat.cols * 4, srcMat.cols * 5));
            cv::addWeighted(center5, 0.0f, dstMat, 1.0f, 0.0f, center5);


            cv::threshold(grayMat, dstMat, thresh, maxval, cv::THRESH_MASK);
            // 效果图copy
            cv::Mat center6 = windowMat(cv::Range(srcMat.rows * 3, srcMat.rows * 4),
                                         cv::Range(srcMat.cols * 5, srcMat.cols * 6));
            cv::addWeighted(center6, 0.0f, dstMat, 1.0f, 0.0f, center6);
        }

        // 更新
        cvui::update();
        // 显示
        cv::imshow(windowName, windowMat);
        // esc键退出
        if(cv::waitKey(25) == 27)
        {
            break;
        }
    }
}