Contrast Limited Adaptive Histogram Equalization

CLAHE.c

/*
 * ANSI C code from the article
 * "Contrast Limited Adaptive Histogram Equalization"
 * by Karel Zuiderveld, karel@cv.ruu.nl
 * in "Graphics Gems IV", Academic Press, 1994
 *
 *
 *  These functions implement Contrast Limited Adaptive Histogram Equalization.
 *  The main routine (CLAHE) expects an input image that is stored contiguously in
 *  memory;  the CLAHE output image overwrites the original input image and has the
 *  same minimum and maximum values (which must be provided by the user).
 *  This implementation assumes that the X- and Y image resolutions are an integer
 *  multiple of the X- and Y sizes of the contextual regions. A check on various other
 *  error conditions is performed.
 *
 *  #define the symbol BYTE_IMAGE to make this implementation suitable for
 *  8-bit images. The maximum number of contextual regions can be redefined
 *  by changing uiMAX_REG_X and/or uiMAX_REG_Y; the use of more than 256
 *  contextual regions is not recommended.
 *
 *  The code is ANSI-C and is also C++ compliant.
 *
 *  Author: Karel Zuiderveld, Computer Vision Research Group,
 *         Utrecht, The Netherlands (karel@cv.ruu.nl)
 */

#ifdef BYTE_IMAGE
typedef unsigned char kz_pixel_t;     /* for 8 bit-per-pixel images */
#define uiNR_OF_GREY (256)
#else
typedef unsigned short kz_pixel_t;     /* for 12 bit-per-pixel images (default) */
# define uiNR_OF_GREY (4096)
#endif

/******** Prototype of CLAHE function. Put this in a separate include file. *****/
int CLAHE(kz_pixel_t* pImage, unsigned int uiXRes, unsigned int uiYRes, kz_pixel_t Min,
      kz_pixel_t Max, unsigned int uiNrX, unsigned int uiNrY,
      unsigned int uiNrBins, float fCliplimit);

/*********************** Local prototypes ************************/
static void ClipHistogram (unsigned long*, unsigned int, unsigned long);
static void MakeHistogram (kz_pixel_t*, unsigned int, unsigned int, unsigned int,
        unsigned long*, unsigned int, kz_pixel_t*);
static void MapHistogram (unsigned long*, kz_pixel_t, kz_pixel_t,
           unsigned int, unsigned long);
static void MakeLut (kz_pixel_t*, kz_pixel_t, kz_pixel_t, unsigned int);
static void Interpolate (kz_pixel_t*, int, unsigned long*, unsigned long*,
    unsigned long*, unsigned long*, unsigned int, unsigned int, kz_pixel_t*);

/**************     Start of actual code **************/
#include <stdlib.h>             /* To get prototypes of malloc() and free() */

const unsigned int uiMAX_REG_X = 16;      /* max. # contextual regions in x-direction */
const unsigned int uiMAX_REG_Y = 16;      /* max. # contextual regions in y-direction */



/************************** main function CLAHE ******************/
int CLAHE (kz_pixel_t* pImage, unsigned int uiXRes, unsigned int uiYRes,
     kz_pixel_t Min, kz_pixel_t Max, unsigned int uiNrX, unsigned int uiNrY,
          unsigned int uiNrBins, float fCliplimit)
/*   pImage - Pointer to the input/output image
 *   uiXRes - Image resolution in the X direction
 *   uiYRes - Image resolution in the Y direction
 *   Min - Minimum greyvalue of input image (also becomes minimum of output image)
 *   Max - Maximum greyvalue of input image (also becomes maximum of output image)
 *   uiNrX - Number of contextial regions in the X direction (min 2, max uiMAX_REG_X)
 *   uiNrY - Number of contextial regions in the Y direction (min 2, max uiMAX_REG_Y)
 *   uiNrBins - Number of greybins for histogram ("dynamic range")
 *   float fCliplimit - Normalized cliplimit (higher values give more contrast)
 * The number of "effective" greylevels in the output image is set by uiNrBins; selecting
 * a small value (eg. 128) speeds up processing and still produce an output image of
 * good quality. The output image will have the same minimum and maximum value as the input
 * image. A clip limit smaller than 1 results in standard (non-contrast limited) AHE.
 */
{
    unsigned int uiX, uiY;          /* counters */
    unsigned int uiXSize, uiYSize, uiSubX, uiSubY; /* size of context. reg. and subimages */
    unsigned int uiXL, uiXR, uiYU, uiYB;  /* auxiliary variables interpolation routine */
    unsigned long ulClipLimit, ulNrPixels;/* clip limit and region pixel count */
    kz_pixel_t* pImPointer;           /* pointer to image */
    kz_pixel_t aLUT[uiNR_OF_GREY];        /* lookup table used for scaling of input image */
    unsigned long* pulHist, *pulMapArray; /* pointer to histogram and mappings*/
    unsigned long* pulLU, *pulLB, *pulRU, *pulRB; /* auxiliary pointers interpolation */

    if (uiNrX > uiMAX_REG_X) return -1;       /* # of regions x-direction too large */
    if (uiNrY > uiMAX_REG_Y) return -2;       /* # of regions y-direction too large */
    if (uiXRes % uiNrX) return -3;      /* x-resolution no multiple of uiNrX */
    if (uiYRes & uiNrY) return -4;      /* y-resolution no multiple of uiNrY */
    if (Max >= uiNR_OF_GREY) return -5;       /* maximum too large */
    if (Min >= Max) return -6;          /* minimum equal or larger than maximum */
    if (uiNrX < 2 || uiNrY < 2) return -7;/* at least 4 contextual regions required */
    if (fCliplimit == 1.0) return 0;      /* is OK, immediately returns original image. */
    if (uiNrBins == 0) uiNrBins = 128;      /* default value when not specified */

    pulMapArray=(unsigned long *)malloc(sizeof(unsigned long)*uiNrX*uiNrY*uiNrBins);
    if (pulMapArray == 0) return -8;      /* Not enough memory! (try reducing uiNrBins) */

    uiXSize = uiXRes/uiNrX; uiYSize = uiYRes/uiNrY;  /* Actual size of contextual regions */
    ulNrPixels = (unsigned long)uiXSize * (unsigned long)uiYSize;

    if(fCliplimit > 0.0) {          /* Calculate actual cliplimit     */
       ulClipLimit = (unsigned long) (fCliplimit * (uiXSize * uiYSize) / uiNrBins);
       ulClipLimit = (ulClipLimit < 1UL) ? 1UL : ulClipLimit;
    }
    else ulClipLimit = 1UL<<14;          /* Large value, do not clip (AHE) */
    MakeLut(aLUT, Min, Max, uiNrBins);      /* Make lookup table for mapping of greyvalues */
    /* Calculate greylevel mappings for each contextual region */
    for (uiY = 0, pImPointer = pImage; uiY < uiNrY; uiY++) {
    for (uiX = 0; uiX < uiNrX; uiX++, pImPointer += uiXSize) {
        pulHist = &pulMapArray[uiNrBins * (uiY * uiNrX + uiX)];
        MakeHistogram(pImPointer,uiXRes,uiXSize,uiYSize,pulHist,uiNrBins,aLUT);
        ClipHistogram(pulHist, uiNrBins, ulClipLimit);
        MapHistogram(pulHist, Min, Max, uiNrBins, ulNrPixels);
    }
    pImPointer += (uiYSize - 1) * uiXRes;          /* skip lines, set pointer */
    }

    /* Interpolate greylevel mappings to get CLAHE image */
    for (pImPointer = pImage, uiY = 0; uiY <= uiNrY; uiY++) {
    if (uiY == 0) {                      /* special case: top row */
        uiSubY = uiYSize >> 1;  uiYU = 0; uiYB = 0;
    }
    else {
        if (uiY == uiNrY) {                  /* special case: bottom row */
        uiSubY = uiYSize >> 1;    uiYU = uiNrY-1;     uiYB = uiYU;
        }
        else {                      /* default values */
        uiSubY = uiYSize; uiYU = uiY - 1; uiYB = uiYU + 1;
        }
    }
    for (uiX = 0; uiX <= uiNrX; uiX++) {
        if (uiX == 0) {                  /* special case: left column */
        uiSubX = uiXSize >> 1; uiXL = 0; uiXR = 0;
        }
        else {
        if (uiX == uiNrX) {              /* special case: right column */
            uiSubX = uiXSize >> 1;  uiXL = uiNrX - 1; uiXR = uiXL;
        }
        else {                      /* default values */
            uiSubX = uiXSize; uiXL = uiX - 1; uiXR = uiXL + 1;
        }
        }

        pulLU = &pulMapArray[uiNrBins * (uiYU * uiNrX + uiXL)];
        pulRU = &pulMapArray[uiNrBins * (uiYU * uiNrX + uiXR)];
        pulLB = &pulMapArray[uiNrBins * (uiYB * uiNrX + uiXL)];
        pulRB = &pulMapArray[uiNrBins * (uiYB * uiNrX + uiXR)];
        Interpolate(pImPointer,uiXRes,pulLU,pulRU,pulLB,pulRB,uiSubX,uiSubY,aLUT);
        pImPointer += uiSubX;              /* set pointer on next matrix */
    }
    pImPointer += (uiSubY - 1) * uiXRes;
    }
    free(pulMapArray);                      /* free space for histograms */
    return 0;                          /* return status OK */
}
void ClipHistogram (unsigned long* pulHistogram, unsigned int
             uiNrGreylevels, unsigned long ulClipLimit)
/* This function performs clipping of the histogram and redistribution of bins.
 * The histogram is clipped and the number of excess pixels is counted. Afterwards
 * the excess pixels are equally redistributed across the whole histogram (providing
 * the bin count is smaller than the cliplimit).
 */
{
    unsigned long* pulBinPointer, *pulEndPointer, *pulHisto;
    unsigned long ulNrExcess, ulUpper, ulBinIncr, ulStepSize, i;
    long lBinExcess;

    ulNrExcess = 0;  pulBinPointer = pulHistogram;
    for (i = 0; i < uiNrGreylevels; i++) { /* calculate total number of excess pixels */
    lBinExcess = (long) pulBinPointer[i] - (long) ulClipLimit;
    if (lBinExcess > 0) ulNrExcess += lBinExcess;      /* excess in current bin */
    };

    /* Second part: clip histogram and redistribute excess pixels in each bin */
    ulBinIncr = ulNrExcess / uiNrGreylevels;          /* average binincrement */
    ulUpper =  ulClipLimit - ulBinIncr;     /* Bins larger than ulUpper set to cliplimit */

    for (i = 0; i < uiNrGreylevels; i++) {
      if (pulHistogram[i] > ulClipLimit) pulHistogram[i] = ulClipLimit; /* clip bin */
      else {
      if (pulHistogram[i] > ulUpper) {        /* high bin count */
          ulNrExcess -= pulHistogram[i] - ulUpper; pulHistogram[i]=ulClipLimit;
      }
      else {                    /* low bin count */
          ulNrExcess -= ulBinIncr; pulHistogram[i] += ulBinIncr;
      }
       }
    }

    while (ulNrExcess) {   /* Redistribute remaining excess  */
    pulEndPointer = &pulHistogram[uiNrGreylevels]; pulHisto = pulHistogram;

    while (ulNrExcess && pulHisto < pulEndPointer) {
        ulStepSize = uiNrGreylevels / ulNrExcess;
        if (ulStepSize < 1) ulStepSize = 1;          /* stepsize at least 1 */
        for (pulBinPointer=pulHisto; pulBinPointer < pulEndPointer && ulNrExcess;
         pulBinPointer += ulStepSize) {
        if (*pulBinPointer < ulClipLimit) {
            (*pulBinPointer)++;     ulNrExcess--;      /* reduce excess */
        }
        }
        pulHisto++;          /* restart redistributing on other bin location */
    }
    }
}
void MakeHistogram (kz_pixel_t* pImage, unsigned int uiXRes,
        unsigned int uiSizeX, unsigned int uiSizeY,
        unsigned long* pulHistogram,
        unsigned int uiNrGreylevels, kz_pixel_t* pLookupTable)
/* This function classifies the greylevels present in the array image into
 * a greylevel histogram. The pLookupTable specifies the relationship
 * between the greyvalue of the pixel (typically between 0 and 4095) and
 * the corresponding bin in the histogram (usually containing only 128 bins).
 */
{
    kz_pixel_t* pImagePointer;
    unsigned int i;

    for (i = 0; i < uiNrGreylevels; i++) pulHistogram[i] = 0L; /* clear histogram */

    for (i = 0; i < uiSizeY; i++) {
    pImagePointer = &pImage[uiSizeX];
    while (pImage < pImagePointer) pulHistogram[pLookupTable[*pImage++]]++;
    pImagePointer += uiXRes;
    pImage = &pImagePointer[-uiSizeX];
    }
}

void MapHistogram (unsigned long* pulHistogram, kz_pixel_t Min, kz_pixel_t Max,
           unsigned int uiNrGreylevels, unsigned long ulNrOfPixels)
/* This function calculates the equalized lookup table (mapping) by
 * cumulating the input histogram. Note: lookup table is rescaled in range [Min..Max].
 */
{
    unsigned int i;  unsigned long ulSum = 0;
    const float fScale = ((float)(Max - Min)) / ulNrOfPixels;
    const unsigned long ulMin = (unsigned long) Min;

    for (i = 0; i < uiNrGreylevels; i++) {
    ulSum += pulHistogram[i]; pulHistogram[i]=(unsigned long)(ulMin+ulSum*fScale);
    if (pulHistogram[i] > Max) pulHistogram[i] = Max;
    }
}

void MakeLut (kz_pixel_t * pLUT, kz_pixel_t Min, kz_pixel_t Max, unsigned int uiNrBins)
/* To speed up histogram clipping, the input image [Min,Max] is scaled down to
 * [0,uiNrBins-1]. This function calculates the LUT.
 */
{
    int i;
    const kz_pixel_t BinSize = (kz_pixel_t) (1 + (Max - Min) / uiNrBins);

    for (i = Min; i <= Max; i++)  pLUT[i] = (i - Min) / BinSize;
}

void Interpolate (kz_pixel_t * pImage, int uiXRes, unsigned long * pulMapLU,
     unsigned long * pulMapRU, unsigned long * pulMapLB,  unsigned long * pulMapRB,
     unsigned int uiXSize, unsigned int uiYSize, kz_pixel_t * pLUT)
/* pImage      - pointer to input/output image
 * uiXRes      - resolution of image in x-direction
 * pulMap*     - mappings of greylevels from histograms
 * uiXSize     - uiXSize of image submatrix
 * uiYSize     - uiYSize of image submatrix
 * pLUT           - lookup table containing mapping greyvalues to bins
 * This function calculates the new greylevel assignments of pixels within a submatrix
 * of the image with size uiXSize and uiYSize. This is done by a bilinear interpolation
 * between four different mappings in order to eliminate boundary artifacts.
 * It uses a division; since division is often an expensive operation, I added code to
 * perform a logical shift instead when feasible.
 */
{
    const unsigned int uiIncr = uiXRes-uiXSize; /* Pointer increment after processing row */
    kz_pixel_t GreyValue; unsigned int uiNum = uiXSize*uiYSize; /* Normalization factor */

    unsigned int uiXCoef, uiYCoef, uiXInvCoef, uiYInvCoef, uiShift = 0;

    if (uiNum & (uiNum - 1))   /* If uiNum is not a power of two, use division */
    for (uiYCoef = 0, uiYInvCoef = uiYSize; uiYCoef < uiYSize;
     uiYCoef++, uiYInvCoef--,pImage+=uiIncr) {
    for (uiXCoef = 0, uiXInvCoef = uiXSize; uiXCoef < uiXSize;
         uiXCoef++, uiXInvCoef--) {
        GreyValue = pLUT[*pImage];           /* get histogram bin value */
        *pImage++ = (kz_pixel_t ) ((uiYInvCoef * (uiXInvCoef*pulMapLU[GreyValue]
                      + uiXCoef * pulMapRU[GreyValue])
                + uiYCoef * (uiXInvCoef * pulMapLB[GreyValue]
                      + uiXCoef * pulMapRB[GreyValue])) / uiNum);
    }
    }
    else {               /* avoid the division and use a right shift instead */
    while (uiNum >>= 1) uiShift++;           /* Calculate 2log of uiNum */
    for (uiYCoef = 0, uiYInvCoef = uiYSize; uiYCoef < uiYSize;
         uiYCoef++, uiYInvCoef--,pImage+=uiIncr) {
         for (uiXCoef = 0, uiXInvCoef = uiXSize; uiXCoef < uiXSize;
           uiXCoef++, uiXInvCoef--) {
           GreyValue = pLUT[*pImage];      /* get histogram bin value */
           *pImage++ = (kz_pixel_t)((uiYInvCoef* (uiXInvCoef * pulMapLU[GreyValue]
                      + uiXCoef * pulMapRU[GreyValue])
                + uiYCoef * (uiXInvCoef * pulMapLB[GreyValue]
                      + uiXCoef * pulMapRB[GreyValue])) >> uiShift);
        }
    }
    }
}