OpenCV Examples Part 1

Nov 4, 2008 | Tags: OpenCV | del.icio.us del.icio.us | digg Digg

6. Access Image Data

Listing 5 below shows you a simple way to access image data using pointers. It loads an image, setup the pointer, and perform some manipulation to the image.

Listing 5: Access image data

  1. /* load the image */
  2. IplImage *img = cvLoadImage( argv[1], CV_LOAD_IMAGE_COLOR );
  3.  
  4. /* retrieve properties */
  5. int width     = img->width;
  6. int height    = img->height;
  7. int nchannels = img->nChannels;
  8. int step      = img->widthStep;
  9.  
  10. /* setup the pointer to access image data */
  11. uchar *data = ( uchar* )img->imageData;    
  12.  
  13. /* convert to grayscale manually */
  14. int i, j, r, g, b, byte;
  15. for( i = 0 ; i < height ; i++ ) {
  16.     for( j = 0 ; j < width ; j++ ) {
  17.         r = data[i*step + j*nchannels + 0];
  18.         g = data[i*step + j*nchannels + 1];
  19.         b = data[i*step + j*nchannels + 2];
  20.        
  21.         byte = ( r + g + b ) / 3;
  22.        
  23.         data[i*step + j*nchannels + 0] = byte;
  24.         data[i*step + j*nchannels + 1] = byte;
  25.         data[i*step + j*nchannels + 2] = byte;
  26.     }
  27. }

The code above converts the image to grayscale manually only for an example. It is more efficient and much simpler if we just use built-in OpenCV functions to perform image manipulations (using cvCvtColor in this case).

5. Convert RGB to Grayscale 7. Conclusion

In this page

  1. Introduction
  2. Load an Image
  3. Retrieve Image Properties
  4. Copy an Image
  5. Convert RGB to Grayscale
  6. Access Image Data
  7. Conclusion
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