OpenCV Examples Part 2

Dec 20, 2008 | Tags: OpenCV | del.icio.us del.icio.us | digg Digg

This article is Part 2 of my OpenCV Example series. In this article, I will explain some basic things to do with OpenCV like: adding text to image, drawing shapes, handling keyboard and mouse events.

1. Adding Text to Image

Listing 1 below shows you how to add text to image. The result is shown in Fig.1

Listing 1: add_text.c

  1. #include <stdio.h>
  2. #include "cv.h"
  3. #include "highgui.h"
  4.  
  5. int main(int argc, char** argv)
  6. {
  7.     IplImage *img = cvLoadImage("2.jpg", CV_LOAD_IMAGE_COLOR);
  8.  
  9.     /* initialize font and add text */
  10.     CvFont font;
  11.     cvInitFont(&font, CV_FONT_HERSHEY_SIMPLEX, 1.0, 1.0, 0, 1, CV_AA);
  12.     cvPutText(img, "Hello World!", cvPoint(10, 130), &font, cvScalar(255, 255, 255, 0));
  13.  
  14.     /* display the image */
  15.     cvNamedWindow("image", CV_WINDOW_AUTOSIZE);
  16.     cvShowImage("image", img);
  17.     cvWaitKey(0);
  18.     cvDestroyWindow("image");
  19.     cvReleaseImage( &img );
  20.  
  21.     return 0;
  22. }

The code above simply loads an image, setup font and add text, then displays the image. Consult OpenCV Reference for details of cvInitFont and cvPutText parameters. The result is shown in Fig.1 below.

The result image.
Fig 1. The result image.

2. Drawing Shapes

Listing 2 below shows you how to draw some shapes. The result is shown in Figure 2.

Listing 2: shapes.c

  1. #include <stdio.h>
  2. #include "cv.h"
  3. #include "highgui.h"
  4.  
  5. int main(int argc, char** argv)
  6. {
  7.     /* create an image */
  8.     IplImage *img = cvCreateImage(cvSize(200, 100), IPL_DEPTH_8U, 3);
  9.    
  10.     /* draw a green line */
  11.     cvLine(img,                         /* the dest image */
  12.            cvPoint(10, 10),             /* start point */
  13.            cvPoint(150, 80),            /* end point */
  14.            cvScalar(0, 255, 0, 0),      /* the color; green */
  15.            1, 8, 0);                    /* thickness, line type, shift */
  16.    
  17.     /* draw a blue box */
  18.     cvRectangle(img,                    /* the dest image */
  19.                 cvPoint(20, 15),        /* top left point */
  20.                 cvPoint(100, 70),       /* bottom right point */
  21.                 cvScalar(255, 0, 0, 0), /* the color; blue */
  22.                 1, 8, 0);               /* thickness, line type, shift */
  23.    
  24.     /* draw a red circle */
  25.     cvCircle(img,                       /* the dest image */
  26.              cvPoint(110, 60), 35,      /* center point and radius */
  27.              cvScalar(0, 0, 255, 0),    /* the color; red */
  28.              1, 8, 0);                  /* thickness, line type, shift */
  29.    
  30.     /* display the image */
  31.     cvNamedWindow("img", CV_WINDOW_AUTOSIZE);
  32.     cvShowImage("img", img);
  33.     cvWaitKey(0);
  34.     cvDestroyWindow("img");
  35.     cvReleaseImage(&img);
  36.  
  37.     return 0;
  38. }

The code above draws a line, a rectangle and a circle. It is also possible to draw more complex shapes like ellipses and pollygons. Consult the OpenCV Reference for the functions. The result image is shown below

The result image.
Fig 2. The result image.

3. Handling Keyboard Input

Listing 3 shows you a simple example of keyboard input handling. While there is no button or such in OpenCV GUI,you can employ input from keyboard to control your application.

Listing 3: keybd.c

  1. #include <stdio.h>
  2. #include "cv.h"
  3. #include "highgui.h"
  4.  
  5. int main(int argc, char** argv)
  6. {
  7.     IplImage *img0, *img1;
  8.     int       key;
  9.    
  10.     /* load an image */
  11.     img0 = cvLoadImage("2.jpg", CV_LOAD_IMAGE_COLOR);
  12.    
  13.     /* create a copy */
  14.     img1 = cvCloneImage(img0);
  15.    
  16.     /* display original image */
  17.     cvNamedWindow("image", CV_WINDOW_AUTOSIZE);
  18.     cvShowImage("image", img0);
  19.    
  20.     while(1) {
  21.         /* wait for keyboard input */
  22.         key = cvWaitKey(0);
  23.        
  24.         /* 'q' pressed, quit the program */
  25.         if (key == 'q') break;
  26.        
  27.         switch(key) {
  28.             /* '1' pressed, display the original image */
  29.             case '1':
  30.                 cvShowImage("image", img0);
  31.                 break;
  32.            
  33.             /* '2' pressed, flip the image horizontally */
  34.             case '2':
  35.                 cvFlip(img1, NULL, 1);
  36.                 cvShowImage("image", img1);
  37.                 break;
  38.            
  39.             /* '3' pressed, flip the image vertically */
  40.             case '3':
  41.                 cvFlip(img1, NULL, 0);
  42.                 cvShowImage("image", img1);
  43.                 break;
  44.         }
  45.     }
  46.    
  47.     /* free memory */
  48.     cvDestroyWindow("image");
  49.     cvReleaseImage(&img0);
  50.     cvReleaseImage(&img1);
  51.    
  52.     return 0;
  53. }

The code above loads an image and waits for user input. If the user press '1', it displays the original image. If the user press '2', it flips the image horizontally. If the user press '3', it flips the image vertically. And it quits when the user press 'q'.

4. Handling Mouse Events

Employing mouse for you OpenCV application is very useful, since you can select object, make selection, slicing image and such things.

To do this, you need to perform 2 things:

  • Write a mouse handler function
  • Register the function to a specified window

Listing 4 below shows you a simple example.

Listing 4: mouse.c

  1. #include <stdio.h>
  2. #include "cv.h"
  3. #include "highgui.h"
  4.  
  5. IplImage *img0, *img1;
  6.  
  7. void mouseHandler(int event, int x, int y, int flags, void *param)
  8. {
  9.     switch(event) {
  10.         /* left button down */
  11.         case CV_EVENT_LBUTTONDOWN:
  12.             fprintf(stdout, "Left button down (%d, %d).\n", x, y);
  13.             break;
  14.        
  15.         /* right button down */
  16.         case CV_EVENT_RBUTTONDOWN:
  17.             fprintf(stdout, "Right button down (%d, %d).\n", x, y);
  18.             break;
  19.        
  20.         /* mouse move */
  21.         case CV_EVENT_MOUSEMOVE:
  22.             /* draw a rectangle */
  23.             img1 = cvCloneImage(img0);
  24.             cvRectangle(img1,
  25.                         cvPoint(x - 15, y - 15),
  26.                         cvPoint(x + 15, y + 15),
  27.                         cvScalar(0, 0, 255, 0), 2, 8, 0);
  28.             cvShowImage("image", img1);
  29.             break;
  30.     }
  31. }
  32.  
  33. int main(int argc, char** argv)
  34. {
  35.     /* load an image */
  36.     img0 = cvLoadImage("2.jpg", CV_LOAD_IMAGE_COLOR);
  37.    
  38.     /* create new window and register mouse handler */
  39.     cvNamedWindow("image", CV_WINDOW_AUTOSIZE);
  40.     cvSetMouseCallback( "image", mouseHandler, NULL );
  41.    
  42.     /* display the image */
  43.     cvShowImage("image", img0);
  44.     cvWaitKey(0);
  45.    
  46.     cvDestroyWindow("image");
  47.     cvReleaseImage(&img0);
  48.     cvReleaseImage(&img1);
  49.    
  50.     return 0;
  51. }
  52.  

The code above loads an image and waits for mouse events. It displays a message when left and right button is pressed. It also draws a red rectangle that follows the mouse pointer when the mouse move over the image.

Related Articles

Recommended Books

book cover Learning OpenCV: Computer Vision with the OpenCV Library
By: Gary Bradski, Adrian Kaehler
This book is the "de facto" OpenCV User's Manual. It provides a practical, pragmatic, accessible book on computer vision, with algorithmic explanation and concrete example code snippets. Written by the creators of OpenCV, no doubt you should obtain a copy.

book cover Machine Vision: Theory, Algorithms, Practicalities (Signal Processing and its Applications)
By: E. R. Davies
This book provides a solid and concrete foundation to computer vision from engineering point of view. Use Learning OpenCV from Gary Bradsky to learn how to use OpenCV, and use this book to understand how OpenCV works behind the screen.

book cover C Programming Language (2nd Edition)
By: Brian W. Kernighan
For you who new to C programming, this is a must-have book for you. Written by the C creator himself, this book is concise and powerful, just like C itself.

The Downloads

13 Comments

Enrique Araiza on Feb 10, 2009:

Thank you very much for these examples, they helped me a lot.

Duncan Boehle on May 26, 2009:

Thank you for the examples, they were very helpful for learning OpenCV! I do have a question about the mouse-handling example, though: wouldn't the line "img1 = cvCloneImage(img0);" continuously use up tons of memory if the user kept moving the mouse around? I'm trying to work off of your example and I can't find a way to make a "cursor" in OpenCV without wasting memory.
Thanks!

Nash on May 27, 2009:

Not really. For this:

img1 = cvCloneImage(img0);

cvCloneImage will read img0 and overwrite it to img1 repeatedly. This is true since img1 is a global variable.

Also it will use tons of memory if you use array of images. e.g:

img[i] = cvCloneImage(img0);

Huda on Jul 13, 2009:

Thank u nash for ur helpful example, I have aquestion about mouse handler: How can i make amouse handler function member of c++ class ?

Nash on Jul 14, 2009:

Sorry but I don't know how. However, I think you should use native GUI capabilities from VC++/wxWidgets/Qt for complex applications since OpenCV GUIs are very basic and simple.

dewesh on Oct 8, 2009:

thanks

Stefan on Oct 12, 2009:

Hi, I've tried a couple of these examples, but now I'm wondering how I can make a mouse cursor in the shape of a circle or a line for example. For instance I am doing a project where I have to mark a ball on an image, so now I want to make a mouse cursor the size of the ball so i can just drag the cursor over the ball and click.. Replies would really be appreciated!

Nash on Oct 14, 2009:

Hello,

I've made the code for selecting ROI at runtime using mouse. Please check it out.

larun on Jan 7, 2010:

hello

any one is having a code to track a car in a video in open cv
thanks,

Yang Mulia on Jan 8, 2010:

makasih bos ^^

Maui on May 23, 2010:

hi nash,
i'm reading your tutorial and codes... they are very interesting and really helpfull! thank you very much

m.

hajar on Nov 23, 2010:

insted of using "imfill" and "bwareaopen" in MATLAB what function can I use in OpenCV??

Rida Shamasneh on Nov 27, 2010:

Thanx man for your codes, it is so so helpful

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