Image processing using OpenCV #1: Invert an image

Code to open an image and invert it using OpenCV:

img = cvLoadImage(argv[1]);

// get the image data
height    = img->height;
width     = img->width;
step      = img->widthStep;
channels  = img->nChannels;
data      = (uchar *)img->imageData;

for( i=0;i<height;i++ )
{
    for( j=0;j<width;j++ )
    {
        for( k=0;k<channels;k++ )
            data[i*step+j*channels+k] = 255 - data[i*step+j*channels+k];   
    }
}