OpenCV Utility: Reading Image Pixels Value

This is a utility to read the pixel value of an image at the specified location. When the user's mouse clicks on the image, it reads the corresponding pixel and displays the RGB value. Useful for debugging.

Listing 1: readpixel.c

1. #include "cv.h"
2. #include "highgui.h"
4. void
5. mouseHandler(int event, int x, int y, int flags, void* param)
6. {
7.     IplImage* img0, * img1;
8.     CvFont    font;
9.     uchar*    ptr;
10.     char      label[20];
12.     img0 = (IplImage*) param;
13.     img1 = cvCloneImage(img0);
15.     cvInitFont(&font, CV_FONT_HERSHEY_PLAIN, .8, .8, 0, 1, 8);
17.     if (event == CV_EVENT_LBUTTONDOWN)
18.     {
19.         /* read pixel */
20.         ptr = cvPtr2D(img1, y, x, NULL);
22.         /*
23.          * display the BGR value
24.          */
25.         sprintf(label, "(%d, %d, %d)", ptr[0], ptr[1], ptr[2]);
27.         cvRectangle(
28.             img1,
29.             cvPoint(x, y - 12),
30.             cvPoint(x + 100, y + 4),
31.             CV_RGB(255, 0, 0),
32.             CV_FILLED,
33.             8, 0
34.         );
36.         cvPutText(
37.             img1,
38.             label,
39.             cvPoint(x, y),
40.             &font,
41.             CV_RGB(255, 255, 0)
42.         );
44.         cvShowImage("img", img1);
45.     }
46. }
48. int
49. main(int argc, char** argv)
50. {
51.     IplImage* img;
53.     /* usage: <prog_name> <image> */
54.     if (argc != 2) {
55.         printf("Usage: %s <image>\n", argv[0]);
56.         return 1;
57.     }
59.     /* load image */
60.     img = cvLoadImage(argv[1], 1);
62.     /* always check */
63.     assert(img);
65.     /* create a window and install mouse handler */
66.     cvNamedWindow("img", 1);
67.     cvSetMouseCallback("img", mouseHandler, (void*)img);
69.     cvShowImage("img", img);
71.     cvWaitKey(0);
73.     /* be tidy */
74.     cvDestroyAllWindows();
75.     cvReleaseImage(&img);
77.     return 0;
78. }