Thresholding using OpenCV

門檻值過濾算是最簡單的影像切割方法，OpenCV提供兩個APIcv::threshold和cv::adaptiveThreshold讓開發者使用，threshold這個API必須自己設定門檻值，而adaptiveThreshold可以自動判斷適合的門檻值，依個人需求而決定使用哪一個。

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

• src 輸入影像
• dst 輸出影像
• thresh 門檻值
• maxval 滿足條件給予該像素點的值
• type 計算後輸出影像的呈現型態，型態如下：

  • THRESH_BINARY

  • THRESH_BINARY_INV

  • THRESH_TRUNC

  • THRESH_TOZERO

  • THRESH_TOZERO_INV

cv::Mat grayImg;
cv::cvtColor(srcImg, grayImg, cv::COLOR_BGR2GRAY);

cv::Mat binImg, binInvImg, truncImg, tozeroImg, tozeroInvImg, otsuImg, trianImg;
double th = 127, maxVal = 255;
cv::threshold( grayImg, binImg, th, maxVal, cv::THRESH_BINARY );
cv::threshold( grayImg, binInvImg, th, maxVal, cv::THRESH_BINARY_INV );
cv::threshold( grayImg, truncImg, th, maxVal, cv::THRESH_TRUNC );
cv::threshold( grayImg, tozeroImg, th, maxVal, cv::THRESH_TOZERO );
cv::threshold( grayImg, tozeroInvImg, th, maxVal, cv::THRESH_TOZERO_INV );
cv::threshold( grayImg, trianImg, th, maxVal, cv::THRESH_TRIANGLE );
cv::threshold( grayImg, otsuImg, 0, maxVal, cv::THRESH_BINARY+cv::THRESH_OTSU );

詳細Sample Code請參考GitHub

void adaptiveThreshold(InputArray src, OutputArray dst, double maxValue, int adaptiveMethod, int thresholdType, int blockSize, double C)

• src 輸入影像(灰階)

• dst 輸出影像

• maxValue 滿足條件給予該像素點的值

• adaptiveMethod 自動篩選門檻值的方法

  • ADAPTIVE_THRESH_MEAN_C 平均加權
  • ADAPTIVE_THRESH_GAUSSIAN_C 高斯函數

• thresholdType 計算後輸出影像的呈現型態，型態如下：

  • THRESH_BINARY

  • THRESH_BINARY_INV

• blockSize 鄰近像素需要加入參考的大小

• C

cv::Mat grayImg;
cv::cvtColor(srcImg, grayImg, cv::COLOR_BGR2GRAY);

cv::Mat adapMaenImg, adapGaussianImg;
double maxVal = 255;

cv::adaptiveThreshold( grayImg, adapMaenImg, maxVal, cv::ADAPTIVE_THRESH_MEAN_C, cv::THRESH_BINARY, 11, 2 );
cv::adaptiveThreshold( grayImg, adapGaussianImg, maxVal, cv::ADAPTIVE_THRESH_GAUSSIAN_C, cv::THRESH_BINARY, 11, 2 );

詳細Sample Code請參考GitHub

更多OpenCV文章請參考：OpenCV Tutorial (學習筆記)