Trying to compute my own Histogram without opencv calcHist() - c++

Question

What I'm trying to do is writing a function that calculates a Histogram of a greyscale image with a forwarded Number of Bins (anzBin) which the histograms range is divided in. Then I'm running through the Image Pixels compairing their value to the different Bins and in case a value fits, increasing the value of the Bin by 1
vector<int> calcuHisto(const IplImage *src_pic, int anzBin)
{
CvSize size = cvGetSize(src_pic);
int binSize = (size.width / 256)*anzBin;
vector<int> histogram(anzBin,0);
for (int y = 0; y<size.height; y++)
{
const uchar *src_pic_point =
(uchar *)(src_pic->imageData + y*src_pic->widthStep);
for (int x = 0; x<size.width; x++)
{
for (int z = 0; z < anzBin; z++)
{
if (src_pic_point[x] <= z*binSize)
{
histogram[src_pic_point[x]]++;
}
}
}
}
return histogram;
}
But unfortunately it's not working...
What is wrong here?
Please help

Answer 1

There are a few issues I can see
Your binSize calculation is wrong
Your binning algorithm is one sided, and should be two sided
You aren't incrementing the proper bin when you find a match
1. binsize calculation
bin size = your range / number of bins
2. two sided binning
if (src_pic_point[x] <= z*binSize)
you need a two sided range of values, not a one sided inequality. Imagine you have 4 bins and values from 0 to 255. Your bins should have the following ranges
bin low high
0 0 63.75
1 63.75 127.5
2 127.5 191.25
3 191.25 255
For example: a value of 57 should go in bin 0. Your code says the value goes in all the bins! Because its always <= z*binsize You need something something with a lower and upper bound.
3. Incrementing the appropriate bin
You are using z to loop over each bin, so when you find a match you should increment bin z, you don't use the actual pixel value except when determining which bin it belongs to
this would likely be buffer overrun imagine again you have 4 bins, and the current pixel has a value of 57. This code says increment bin 57. But you only have 4 bins (0-3)
histogram[src_pic_point[x]]++;
you want to increment only the bin the pixel value falls into
histogram[z]++;
CODE
With that in mind here is revised code (it is untested, but should work)
vector<int> calcuHisto(const IplImage *src_pic, int anzBin)
{
CvSize size = cvGetSize(src_pic);
double binSize = 256.0 / anzBin; //new definition
vector<int> histogram(anzBin,0); //i don't know if this works so I
//so I will leave it
//goes through all rows
for (int y = 0; y<size.height; y++)
{
//grabs an entire row of the imageData
const uchar *src_pic_point = (uchar *)(src_pic->imageData + y*src_pic->widthStep);
//goes through each column
for (int x = 0; x<size.width; x++)
{
//for each bin
for (int z = 0; z < anzBin; z++)
{
//check both upper and lower limits
if (src_pic_point[x] >= z*binSize && src_pic_point[x] < (z+1)*binSize)
{
//increment the index that contains the point
histogram[z]++;
}
}
}
}
return histogram;
}