How to access a RGB image data - c++

Question

I want the data at pixel to be compared with the colour and then i want to find contour then take centroid points of the contour ,so i am using like this to find countourdata am i wrong at this statement
int pos = i * w * Channels + j; //channels is 3 as rgb
// if any data exists
if (data->imageData[pos]>0)
Code is like this
for (int i = x; i < x+h; i++) //height of frame pixels
{
for (int j = y; j < y+w; j++)//width of frame pixels
{
int pos = i * w * Channels + j; //channels is 3 as rgb
// if any data exists
if (data->imageData[pos]>0) //Taking data (here is the problem how to take)
{
xPos += j;
yPos += i;
nPix++;
}
}
}

Answer 1

I use the following code structure
/**
* #brief Calculate greeness from an RGB image
*
* Performs the greeness pixelwise transform on the input image.
* Greeness is defined as
* Greeness = 255*G/sqrt(R^2+G^2+B^2)
* The function assumes that the resolution of the two images are identical.
*
* #param imSrc Input RGB image.
* #param imDst Output grayscale (greeness) image.
*/
void rgbToGreeness( IplImage *imSrc , IplImage* imDst) {
// Allocate variables
int tmp_pix;
uchar * _SrcPtr, * _DstPtr;
// Iterate over the image line by line
for(int y = 0 ; y < imSrc->height ; y++ )
{
// Locate pointers to the first data element in the current line
_SrcPtr = ( uchar* )( imSrc->imageData + y * imSrc->widthStep );
_DstPtr = ( uchar* )( imDst->imageData + y * imDst->widthStep );
// Iterate over the elements in the current line
for( int x = 0 ; x < imSrc->width ; x++ )
{
//2*G-B-R - Excessive green
tmp_pix = (int) (255*_SrcPtr[3*x+1]/pow(pow((float)_SrcPtr[3*x],2) + pow((float)_SrcPtr[3*x+1], 2) + pow((float)_SrcPtr[3*x+2], 2), (float) 0.5));
//If value is larger than 255, set it to 255 and lower than 0 set it to 0
_DstPtr[x] = (uchar) ( ( tmp_pix < 0 ) ? 0 : ( ( tmp_pix > 255 ) ? 255 : tmp_pix ) );
}
}
}

Answer 2

Here is some code to access RGB data of a pixel in an image
IplImage* img=cvLoadImage(fileName);
CvScalar s;
s=cvGet2D(img,i,j); // get the (i,j) pixel value
s.val[0]=111; // B-channel
s.val[1]=111; // G-channel
s.val[2]=111; // R-channel
cvSet2D(img,i,j,s); // set the (i,j) pixel value
Source (modified a little): http://www.cs.iit.edu/~agam/cs512/lect-notes/opencv-intro/opencv-intro.html#SECTION00053000000000000000

Answer 3

As requested here is my exact code where i want to calculate centroids from contour
My exact code is like this
1) Taking RGB image as input
2) x=0,y=0,w=width of frame,h=height of frame.are the data passing
void cRecursiveCentroids::ComputeCentroid(int x, int y, int w, int h, IplImage *data, bool splitOnUpDown, int level, int id, int addToId){
if (level == m_Levels-1 ) return;
int Channels = data->nChannels; // Number of channels
std::cout << "Channels: " << Channels << "\n";
int xPos = 0;
int yPos = 0;
int nPix = 0;
for (int i = x; i < x+h; i++) //Tracing the contour
{
for (int j = y; j < y+w; j++)
{
int pos = i * m_Wid * Channels + j; // Here may be the error i am thinking
// if any data exists
if (data->imageData[pos]>0)
{
xPos += j;
//std::cout << "xPos: " << xPos << "\n";
yPos += i;
// std::cout << "yPos: " << yPos << "\n";
nPix++;
}
}
}
Check = nPix;
if (nPix > 0){ // Calculating Position
xPos = (int)((float)xPos / (float)nPix);
yPos = (int)((float)yPos / (float)nPix);
int num = ( id + addToId ) > 16 ? 16 : (id+addToId);
m_Cent[num].posx = xPos;
m_Cent[num].posy = yPos;
m_Cent[num].level = level;
splitOnUpDown = !splitOnUpDown;
level = level+1;
if (splitOnUpDown) //Recursive calling for centroids
{
id *= 2;
ComputeCentroid(x,y,w,(yPos - y), data, splitOnUpDown, level, id, addToId);
ComputeCentroid(x,yPos,w,h-(yPos-y), data, splitOnUpDown, level, id+1, addToId);
} else {
id *= 2;
ComputeCentroid(x,y,(xPos-x),h, data, splitOnUpDown, level, id, addToId);
ComputeCentroid(xPos,y,w - (xPos-x),h, data, splitOnUpDown, level, id+1, addToId);
}
}
DrawCentroidPoints(); //Draw Centroid Points
}