This code calculates and plots the histograms of the red, green, and blue channels of an image. It takes an input image, creates histograms for each color channel, scales the histograms to fit within the bounds of an output image, then plots the histograms side by side in the output image. Key steps include extracting each color channel, calculating histograms of each channel, finding the maximum histogram value, scaling histograms based on this maximum, plotting the histograms onto the output image, and saving the final output image.

1. histogram.txt
code by Iry - document created on 13 Nov - what-o.com@hotmail.com
void CMy04Dlg::CalcHistogram(IplImage* img, int c)
{
IplImage* channel = cvCreateImage( cvGetSize(img), 8, 1 );
IplImage *hist_img = cvCreateImage(cvSize(100,100), 8, 3);
cvSet( hist_img, cvScalarAll(255), 0 );
CvHistogram *hist_red;
CvHistogram *hist_green;
CvHistogram *hist_blue;
int hist_size = 256;
float range[]={0,256};
float* ranges[] = { range };
float max_value = 0.0;
float max = 0.0;
float w_scale = 0.0;
/* Create a 1-D Arrays to hold the histograms */
hist_red = cvCreateHist(1, &hist_size, CV_HIST_ARRAY, ranges, 1);
hist_green = cvCreateHist(1, &hist_size, CV_HIST_ARRAY, ranges, 1);
hist_blue = cvCreateHist(1, &hist_size, CV_HIST_ARRAY, ranges, 1);
/* Set image to obtain RED as Channel of Interest (COI) */
cvSetImageCOI(img,3);
cvCopy(img,channel);
cvResetImageROI(img);
/* Calculate histogram of the Image and store it in the array */
cvCalcHist( &channel, hist_red, 0, NULL );
/* Calculate and Plot the histograms Green and Blue channels as well */
/* Green channel */
cvSetImageCOI(img,2);
cvCopy(img,channel);
cvResetImageROI(img);
cvCalcHist( &channel, hist_green, 0, NULL );
/* Blue channel */
cvSetImageCOI(img,1);
cvCopy(img,channel);
cvResetImageROI(img);
cvCalcHist( &channel, hist_blue, 0, NULL );
/* Find the minimum and maximum values of the histograms */
cvGetMinMaxHistValue( hist_red, 0, &max_value, 0, 0 );
cvGetMinMaxHistValue( hist_green, 0, &max, 0, 0 );
max_value = (max > max_value) ? max : max_value;
cvGetMinMaxHistValue( hist_blue, 0, &max, 0, 0 );
max_value = (max > max_value) ? max : max_value;
// The variable max_value has the maximum of the three histograms
/* Using the maximum value, Scale/Squeeze the histogram (to fit the image)
*/
cvScale( hist_red->bins, hist_red->bins,
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2. histogram.txt
((float)hist_img->height)/max_value, 0 );
cvScale( hist_green->bins, hist_green->bins,
((float)hist_img->height)/max_value, 0 );
cvScale( hist_blue->bins, hist_blue->bins,
((float)hist_img->height)/max_value, 0 );
/* Scale/Squeeze the histogram range to image width */
w_scale = ((float)hist_img->width)/hist_size;
/* Plot the Histograms */
for( int i = 0; i < hist_size; i++ )
{
cvRectangle( hist_img, cvPoint((int)i*w_scale ,
hist_img->height),cvPoint((int)(i+1)*w_scale, hist_img->height -
cvRound(cvGetReal1D(hist_red->bins,i))),CV_RGB(255,0,0), -1, 8, 0 );
cvRectangle( hist_img, cvPoint((int)i*w_scale ,
hist_img->height),cvPoint((int)(i+1)*w_scale, hist_img->height -
cvRound(cvGetReal1D(hist_green->bins,i))),CV_RGB(0,255,0), -1, 8, 0 );
cvRectangle( hist_img, cvPoint((int)i*w_scale ,
hist_img->height),cvPoint((int)(i+1)*w_scale, hist_img->height -
cvRound(cvGetReal1D(hist_blue->bins,i))),CV_RGB(0,0,255), -1, 8, 0 );
}
ShowImage( hist_img, IDC_IHist );
cvSaveImage("myoutput_10nov.jpg",hist_img);
}
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