/* * 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 /* 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) >> 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) >> 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[-(int)uiSizeX]; /* go to bdeginning of next row */ } } 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); } } } }