calcHist with multiple channels in openCV - c++

Question

I'm trying to compute a 3D 2x2x2 RGB histogram with openCV with cv::calcHist. However I don't understand how calcHist works with multiple channels. I want it to select one of two red layer of bins i, one of two green layer of bins j and one of two blue layer of bins k and store the result into the corresponding triple-indexed bin bins[i][j][k]. But that's not what it's doing.
The following loop works fine
int bin_index(const float x, unsigned int nbins)
{
return std::min(static_cast<unsigned int>(x*nbins), nbins - 1);
}
//...
for(cv::Point pixel_idx : all possible pixel positions)
{
int red_idx = bin_index(image(pixel_idx)[0], number_of_red_bins);
int green_idx = bin_index(image(pixel_idx)[0], number_of_green_bins);
int blue_idx = bin_index(image(pixel_idx)[0], number_of_blue_bins);
histogram[blue_idx * number_of_green_bins * number_of_red_bins
+ green_idx * number_of_red_bins + red_idx] += 1;
}
But the following usage of cv::calcHist yield a different result
cv::Mat block = image(rect of the patch);
const int channels[] = {0, 1, 2};
const int histSize[] = {number_of_red_bins, number_of_green_bins, number_of_blue_bins};
constexpr int histSizeDims = sizeof(histSize) / sizeof(const int);
const float range[] {0.0f, 1.001f};
const float* histRanges[] = {range, range, range};
cv::Mat result;
cv::calcHist(
&block,
1,
channels,
cv::Mat(),
result,
histSizeDims,
histSize,
histRanges,
true, false);
Can you tell me how how calcHist behaves with multi-dimentional histograms ? Thank you for your help !