Color Histogram Equalization

RGB 컬러 이미지의 gray level의 cdf를 이용해서 histogram equalization을 수행한다. 컬러 이미지의 gray level은 

$$gray = \frac{r + g+ b}{3}$$

으로 정의한다.

gray level에 기반한 equalization 결과;

각 color 채널 equalization 결과:

RGB color를 HSV color로 변환한 후 V에 대해서 equalization을 했을 때 결과:

void test_color_equalize_new(CRaster& raster, CRaster& out) {
    int hist[256] = {0};
    int chist[256], lut[256], partition[256 + 1];
    CSize sz = raster.GetSize();
    out = raster; // clone; 
    // pdf of gray level: g = (r + g + b) / 3
    for (int y = 0; y < sz.cy; y++) {
        BYTE *p = (BYTE *)raster.GetLinePtr(y);
        for (int x = 0; x < sz.cx; x++){
            int a = *p++; a += *p++; a += *p++;
            hist[a / 3]++;
        }
    };
    // cdf;
    chist[0] = hist[0];
    for (int i = 1; i < 256; i++)
        chist[i] = hist[i] + chist[i - 1];
    
    /* assign equal number of pixels in each gray levels;*/
    int num_pixels = sz.cx * sz.cy;
    double pixels_per_level = double(num_pixels) / 256;

    /* first and last in partition */
    partition[0]   = 0;
    partition[256] = 256;
    /* intermediate; */
    for (int j = 0, i = 1; i < 256; i++) {
        double desired = i * pixels_per_level;			
        while (chist[j + 1] <= desired) j++;
        /* nearest interpolation */
        if ((desired - chist[j]) < (chist[j + 1] - desired)) partition[i] = j;
        else partition[i] = j + 1;
    } 
    /* fill equalization LUT */
    for (int j = 0; j < 256; j++) {
        int i = 0;
        while (partition[i + 1] <= j) i++;
        lut[j] = i;
    } 
    // needs hue preserving processes;
    for (int y = 0; y < sz.cy; y++) {
        BYTE *p = (BYTE *)raster.GetLinePtr(y);
        BYTE *q = (BYTE *)out.GetLinePtr(y);
        for (int x = 0; x < sz.cx; x++) {
            *q++ = lut[*p++]; *q++ = lut[*p++]; *q++ = lut[*p++];
        }
    }
}

void test_color_equalize_HSV(CRaster& raster, CRaster& out) {
    int hist[256] = {0};
    int chist[256] = {0}, lut[256] = {0}, partition[256 + 1];
    CSize sz = raster.GetSize();
    out = raster; // cloning;
    std::vector<double> fH(sz.cx * sz.cy);
    std::vector<double> fV(sz.cx * sz.cy);
    std::vector<double> fS(sz.cx * sz.cy);

    int n = sz.cx * sz.cy;
    double r, g, b;
    for (int y = 0, k = 0; y < sz.cy; y++) {
        BYTE *p = (BYTE *)raster.GetLinePtr(y);
        for (int x = 0; x < sz.cx; x++, k++){
            b = *p++, g = *p++, r = *p++;
            RGBtoHSV(r / 255., g / 255., b / 255., fH[k], fS[k], fV[k]);
        }
    };	
    // make histogram of V-color;
    for (int k = 0; k < n; k++)
        hist[int(fV[k] * 255)]++;

    // cdf;
    chist[0] = hist[0];
    for (int i = 1; i < 256; i++) {
        chist[i] = hist[i] + chist[i - 1];
    }
    /* assign equal number of pixels in each V-color;*/
    int num_pixels = sz.cx * sz.cy;
    double pixels_per_level = double(num_pixels) / 256;

    /* first and last in partition */
    partition[0]   = 0;
    partition[256] = 256;
    /* intermediate; */
    for (int j = 0, i = 1; i < 256; i++) {
        double desired = i * pixels_per_level;			
        while (chist[j + 1] <= desired) j++;
        /* nearest interpolation */
        if ((desired - chist[j]) < (chist[j + 1] - desired)) partition[i] = j;
        else partition[i] = j + 1;
    } 
    /* fill equalization LUT */
    for (int j = 0; j < 256; j++) {
        int i = 0;
        while (partition[i + 1] <= j) i++;
        lut[j] = i;
    } 
    for (int k = 0; k < n; k++)
        fV[k]= lut[int(fV[k] * 255)] / 255.;

    for (int y = 0, k = 0; y < sz.cy; y++) {
        BYTE *q = (BYTE *)out.GetLinePtr(y);
        for (int x = 0; x < sz.cx; x++, k++) {
            HSVtoRGB(fH[k], fS[k], fV[k], r, g, b);
            *q++ = int(b * 255); 
            *q++ = int(g * 255);
            *q++ = int(r * 255);
        }
    }
}

/* fR Red component, range: [0, 1]
** fG Green component, range: [0, 1]
** fB Blue component, range: [0, 1]
** fH Hue component, range: [0, 360]
** fS Hue component, range: [0, 1]
** fV Hue component, range: [0, 1] */
void RGBtoHSV(double fR, double fG, double fB, double& fH, double& fS, double& fV);
void HSVtoRGB(double fH, double fS, double fV, double& fR, double& fG, double& fB);