Histogram not accurate for dark images - c++

Question

I'm using the next algorithm to calculate the histogram from a YUV420sp image. Seems to work but the result is not 100% accurate for a fully dark image. When the image is dark I would expect to have on the left side of the histogram a high pick showing that the image is too dark, but the algorithm in such scenario shows instead a flat line, no pick. On the other light scenarios the histogram seems to be accurate.
void calculateHistogram(const unsigned char* yuv420sp, const int yuvWidth, const int yuvHeight, const int histogramControlHeight, int* outHistogramData)
{
const int BINS = 256;
// Clear the output
memset(outHistogramData, 0, BINS * sizeof(int));
// Get YUV brightness values
const int totalPixels = yuvWidth * yuvHeight;
for (int index = 0; index < totalPixels; index++)
{
char brightness = yuv420sp[index];
outHistogramData[brightness]++;
}
// Get the maximum brightness
int maxBrightness = 0;
for (int index = 0; index < BINS; index++)
{
if (outHistogramData[index] > maxBrightness)
{
maxBrightness = outHistogramData[index];
}
}
// Normalize to fit the UI control height
const int maxNormalized = BINS * histogramControlHeight / maxBrightness;
for(int index = 0; index < BINS; index++)
{
outHistogramData[index] = (outHistogramData[index] * maxNormalized) >> 8;
}
}
[SOLVED by galop1n] Though Galop1n implementation is much nicer I'm updating this one with the corrections in case is of use to anyone.
Changes:
1) Reading brightness values into an unsigned char instead of a char.
2) Placed UI normalization division into the normalization loop.
void calculateHistogram(const unsigned char* yuv420sp, const int yuvWidth, const int yuvHeight, const int histogramCanvasHeight, int* outHistogramData)
{
const int BINS = 256;
// Clear the output
memset(outHistogramData, 0, BINS * sizeof(int));
// Get YUV brightness values
const int totalPixels = yuvWidth * yuvHeight;
for (int index = 0; index < totalPixels; index++)
{
unsigned char brightness = yuv420sp[index];
outHistogramData[brightness]++;
}
// Get the maximum brightness
int maxBrightness = 0;
for (int index = 0; index < BINS; index++)
{
if (outHistogramData[index] > maxBrightness)
{
maxBrightness = outHistogramData[index];
}
}
// Normalize to fit the UI control height
for(int index = 0; index < BINS; index++)
{
outHistogramData[index] = outHistogramData[index] * histogramCanvasHeight / maxBrightness;
}
}

Answer 1

There is at least two bugs in your implementation.
The indexing by the brightness because of using a temporary of type signed char.
The final normalization result can be influence by the value of control height and the maximum count of pixel in a bin. The division cannot really be put outside of the loop because of that.
I recommend also to use a std::array ( need c++11 ) to store the result instead of a raw pointer as there is a risk the caller do not allocate enough space for what will use the function.
#include <algorithm>
#include <array>
void calculateHistogram(const unsigned char* yuv420sp, const int yuvWidth, const int yuvHeight, const int histogramControlHeight, std::array<int, 256> &outHistogramData ) {
outHistogramData.fill(0);
std::for_each( yuv420sp, yuv420sp + yuvWidth * yuvHeight, [&](int e) {
outHistogramData[e]++;
} );
int maxCountInBins = * std::max_element( begin(outHistogramData), end(outHistogramData) );
for( int &bin : outHistogramData )
bin = bin * histogramControlHeight / maxCountInBins;
}

Answer 2

If the maximum brightness of the image maxBrightness is zero, your calculation of maxNormalized becomes a division by zero. I suspect this is where your problem is.
Without better understanding what normalization conditions you are trying to establish, I am not sure what alternative to suggest to you right now.