img_processing.cpp

#define RED_INTENCITY_COEF 0.2126F
#define GREEN_INTENCITY_COEF 0.7152F
#define BLUE_INTENCITY_COEF 0.0722F

extern "C"{
    void otsuBinarize ( unsigned char* data, unsigned int len );
}

void otsuBinarize ( unsigned char* data, unsigned int len ) {
    // histogram and greyscale
    int histogram[256] = {0};

    for (unsigned int i = 0; i < len; i += 4) {
        unsigned char brightness =  ( data[i] * RED_INTENCITY_COEF ) + 
                            ( data[i + 1] * GREEN_INTENCITY_COEF ) +
                            ( data[i + 2] * BLUE_INTENCITY_COEF );
        histogram[brightness]++;
        data[i] = brightness;
    }

    // otsu binarization threshold calculation

    unsigned int total = len / 4; // H x W; num of pixels
    unsigned int sum = 0;
    for( short i = 0; i < 256; ++i )
        sum += i * histogram[i];

    unsigned int sumB = 0;
    unsigned int wB = 0;
    unsigned int wF = 0;

    float varMax = 0;
    unsigned char threshold_1 = 0;
    unsigned char threshold_2 = 0;

    for ( short i = 0; i < 256; ++i ) {
        wB += histogram[i];            // Weight-Background
        if (wB == 0) continue;

        wF = total - wB;                 // Weight-Foreground
        if (wF == 0) break;

        sumB += i * histogram[i];

        float mB = sumB / wB;            // Mean-Background
        float mF = (sum - sumB) / wF;    // Mean-Foreground

        // Calculate Between Class Variance
        float varBetween = 1.0F * wB * wF * (mB - mF) * (mB - mF);

        // Check if new maximum found
        if ( varBetween >= varMax ) {
            threshold_1 = i;
            if ( varBetween > varMax ) threshold_2 = i;
            varMax = varBetween;
        }
    }

    // Calculated threshold
    unsigned char threshold = ( threshold_1 + threshold_2 ) / 2;

    // binarization based on threshold value 
    for (unsigned int i = 0; i < len; i += 4) {
        unsigned char det = data[i] > threshold; 
        data[i] = data[i + 1] = data[i + 2] = ( det * 255 ) + ( det * 0 );;
    }
}