OpenCV cvSet2d.....what does this do

I was browsing over some code in the OpenCV page when it came to accessing Pixel Data

IplImage* img=cvCreateImage(cvSize(640,480),IPL_DEPTH_32F,3);
CvScalar s;
s=cvGet2D(img,i,j); // get the (i,j) pixel value
printf("B=%f, G=%f, R=%f\n",s.val[0],s.val[1],s.val[2]);
s.val[0]=111;
s.val[1]=111;
s.val[2]=111;
cvSet2D(img,i,j,s); // set the (i,j) pixel value

I had done something similar, but I used the Template Class provided to access pixel data......anyways Im not sure I understand the part s.val[0]=111....etc? if s.val[0] contains the B value, what exactly is s.val[0]=111 doing? is it setting it to black?........I dont understand exactly what it's supposed to be?

Im used to开发者_Python百科 CVscalars and such but I dont understand this format? Specifically what 111 means?

thanks

The cvSet2D(img, i, j, s) functions to not access the (i,j)th pixel. It accesses the (j,i)th pixel. That is because images are stored as a matrix - you need to specify the row first (the Y coordinate) and then the column (the X coordinate).

Instead of using the cvGet/Set functions, did you try using pointers to access data within an image?

If you want direct access to the pixels, after loading an image you could do something like:

// This example converts a colored image to its grayscale version.
// Let's say that rgb_img is your previously loaded image.
IplImage* gray_frame = 0;
gray_frame = cvCreateImage(cvSize(rgb_img->width, rgb_img->height), rgb_img->depth, rgb_img->nChannels);
if (!gray_frame)
{
  fprintf(stderr, "!!! cvCreateImage failed!\n" );
  return NULL;
}

for (int i = 0; i < rgb_img->width * rgb_img->height * rgb_img->nChannels; i += rgb_img->nChannels)
{
    gray_frame->imageData[i] = (rgb_img->imageData[i] + rgb_img->imageData[i+1] + rgb_img->imageData[i+2])/3;   //B
    gray_frame->imageData[i+1] = (rgb_img->imageData[i] + rgb_img->imageData[i+1] + rgb_img->imageData[i+2])/3; //G
    gray_frame->imageData[i+2] = (rgb_img->imageData[i] + rgb_img->imageData[i+1] + rgb_img->imageData[i+2])/3; //R
}