Automatic Contrast Adjustment

SharathKum_D_Intel — Wed, 10 Jul 2013 17:57:38 GMT

Hello All,

I am trying to perform Auto Contrast in my grey images using Intel IPP functions. I am not sure where I am going wrong but the image becomes too dark. Here is the code.

#define GrayLevels 256

int main()

{

std::string path("C:\\sharath\\Images\\");

std::string inputImageFile = path + "DiffFing.bmp";

IppImage<Ipp8u,1> imgsrc;

ippiLoadBMP(inputImageFile.c_str(),imgsrc);

IppImage<Ipp8u,1> imgDst;

IppiSize imgSize;

Ipp32s imgLevels[GrayLevels+1], imgHist[GrayLevels];

Ipp32s imgValues[GrayLevels];

int w = imgsrc.Width();

int h = imgsrc.Height();

int nlevels = GrayLevels;

float ww=0;

int min=0;

int max=GrayLevels-1;

int slope;

float cut_low = 0.05f;

float cut_high = 0.95f;

imgSize.width = w;

imgSize.height = h;

ippiHistogramEven(imgsrc, imgSize, imgHist, imgLevels, GrayLevels+1, 0, GrayLevels);

while(ww<cut_low)

{

ww+=imgHist[min++];

}

while(ww<cut_high)

{

ww+=imgHist[max--];

}

slope = (GrayLevels-1)/(max-min);

for(int i=0;i<GrayLevels;i++)

{

if(i<=min)

{

imgValues = 0;

}

else if(i>=max)

{

imgValues = GrayLevels-1;

}

else

{

imgValues = slope*(i-min);

}

imgDst.ResizeNE(imgsrc.Width(),imgsrc.Height());

ippiLUT(imgsrc, imgDst, imgSize, imgValues, imgLevels, GrayLevels+1);

std::string out4f = path + "DiffFing_Auto.bmp";

ippiSaveBMP(out4f.c_str(),imgDst);

return(EXIT_SUCCESS);

}

Can somebody help me figure out why my image gets darker instead of enhancing the contrast.

I have also attached the files for reference. DiffFing_AutoContrast.bmp is the image after autocontrast.

Thanks,

Sharath

In the while-statement,

Thomas_Jensen1 — Fri, 12 Jul 2013 14:26:57 GMT

In the while-statement,

while(ww<cut_low)
      {
            ww+=imgHist[min++];
      }

If I'm not mistaken, there is an error in datatype; ww is accumulating 32s values from imgHist, but cut_low is expressed in percentile, so a proper way would be to first compute the sum of all imgHist bins as 64u, and then have ww also as 64u, with value computed from sum*percentile.

Speaking about corrected

SergeyKostrov — Sat, 13 Jul 2013 17:45:14 GMT

Speaking about corrected image ( difffing-autocontrast.bmp ) it looks good. My question is what Contrast value do you set in case of Auto Contrast correction? For example, if contrast is set to 25% the resulting image will be too pale ( details are lost ), and if contrast is set to 75% the resulting image will be darker and this is what you have.

topic In the while-statement, in Intel® Integrated Performance Primitives

Automatic Contrast Adjustment

In the while-statement,

Speaking about corrected