如何獲得墊圖像的像素區域

Question

我是使用Opencv和Java的新手。我有一個Mat圖像，我試圖讀取特定區域中的像素，以便稍後我可以遍歷該區域並確定HSV，我嘗試使用CvRect獲取我想要的區域的座標和大小圖片。我將如何獲得圖像的該區域？

Mat firstImage = Imgcodecs.imread("firstImage.png"); 
    CvRect topL = new CvRect(firstImage.get(160, 120, 30, 30)); 

Answer 1

有兩種方法可以做到這一點：一個讀取像素之一，或者得到一個矩形的所有像素爲Java對整個圖像，然後用更大的陣列工作。哪一個最好取決於矩形的大小。下面的代碼首先將圖像的指定rect部分獲取到Java數組中。

import java.awt.Color; 
import org.opencv.core.Core; 
import org.opencv.core.Mat; 
import org.opencv.core.Rect; 
import org.opencv.highgui.Highgui; 

public class OpenCVThing 
{ 
    public static void main(String[] args) 
    { 
     String opencvpath = System.getProperty("user.dir") + "\\lib\\"; 
     System.load(opencvpath + Core.NATIVE_LIBRARY_NAME + ".dll"); 
     // Get the whole rect into smallImg 
     Mat firstImage = Highgui.imread("capture.png"); 
     System.out.println("total pxels:" + firstImage.total()); 
     // We are getting a column 30 high and 30 wide 
     int width = 30; 
     int height = 30; 
     Rect roi = new Rect(120, 160, width, height); 
     Mat smallImg = new Mat(firstImage, roi); 
     int channels = smallImg.channels(); 
     System.out.println("small pixels:" + smallImg.total()); 
     System.out.println("channels:" + smallImg.channels()); 
     int totalBytes = (int)(smallImg.total() * smallImg.channels()); 
     byte buff[] = new byte[totalBytes]; 
     smallImg.get(0, 0, buff); 

     // assuming it's of CV_8UC3 == BGR, 3 byte/pixel 
     // Effectively assuming channels = 3 
     for (int i=0; i< height; i++) 
     { 
      // stride is the number of bytes in a row of smallImg 
      int stride = channels * width; 
      for (int j=0; j<stride; j+=channels) 
      { 
       int b = buff[(i * stride) + j]; 
       int g = buff[(i * stride) + j + 1]; 
       int r = buff[(i * stride) + j + 2]; 
       float[] hsv = new float[3]; 
       Color.RGBtoHSB(r,g,b,hsv); 
       // Do something with the hsv. 
       System.out.println("hsv: " + hsv[0]); 
      } 
     } 
    } 
} 

注1：在這種情況下，在淺黃色每個字節表示一個像素的第三，因爲我假定格式是CV_8UC3。

total pxels:179305 
small pixels:900 
channels:3 
hsv: 0.5833333 
hsv: 0.5833333 
hsv: 0.5833333 
hsv: 0.5833333 
hsv: 0.5833333 

etc ... 

見this page和docs的更詳細一點