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color.cpp

//==============================================
//  copyright            : (C) 2003-2005 by Will Stokes
//==============================================
//  This program is free software; you can redistribute it
//  and/or modify it under the terms of the GNU General
//  Public License as published by the Free Software
//  Foundation; either version 2 of the License, or
//  (at your option) any later version.
//==============================================

//Systemwide includes
#include <qimage.h>
#include <qstring.h>
#include <qapplication.h>

//Projectwide includes
#include "color.h"
#include "../../gui/statusWidget.h"

//----------------------------------------------
// Inputs:
// -------
// QString filename - location of original image on disk
// StatusWidget* status - widget for making progress visible to user
//
// Outputs:
// --------
// QImage* returned - constructed image
//
// Description:
// ------------
// This method simply stretchs the color values to use up the entire dynamic range, in our
// case red/green/blue values ranging from [0,255].
//
// The algorithm can be broken up into two steps:
// 1.) finding the endpoints of the curretn color values (e.g. [6,120])
// 2.) stretching the current image colors to fit the full [0,255] range
//
// One could simply iterate over the entire image and find the minimum and 
// maximum red, gree, and blue color values. However; often there is residual 
// noise at the endpoints since camera sensors always provide some noise, 
// while the majority of pixels fall somewhere in the middle. If we constructed 
// a histogram for a particular color channel it might look like this:
//  5 |                      .
//  . |                     . .
//  . |                    .   .
//  1 | .    .          .        .              .
//  0 |. .... ........             ............. .....
//    |-X------------*--------------*-----------X-----
//     0            100             175            255            
//
//
// Here color values are plotted across the x axis, while the y axis indicates 
// how offten a particular color value was seen. Note we're plotting one of the 
// three color channels here (red, green, blue).
//
// In this example we notice most pixels that were observed fall in the 100-175 
// range (that is the red/green/blue color value was most often observed with a 
// value between 100-175). A few pixels with values of 5 or 10 were seen, and a 
// few with around 230 were seen. We want to stretch the common case, 100-175, 
// out to the full 0-255 range. That will make the pixels that were 175 now 255, 
// those that wer 100 now 0, thus darks will become darker, brights will become
// brigther. We'll lineraly scale all pixels inbetween. Any pixels beyond the 
// 100-175 range will get clamped at 0 and 255 respectively.
//
// Now, if we simply find the smallest and largest values we'll get soemthinglike 
// 5 and 235, and stretching will have very little effect on the image. Instead, 
// we first actually populate a histogram array for each color channel (an int 
// array of 256 elements). We then determine the 1% and 99% boundaries but walking 
// from the end points of hte histogram array and adding up the elements. Once 1% 
// of the pixels have been accounted for we set the the boundary endpoint to the index, 
// thus giving us 100,175 in this case.
//
// Actually scaling the colors is relatively trivial. A second pass over the 
// image takes place during which a pixels color is fetched, modified, then written 
// back out according to the following equations. It should be noted that as a 
// sanity check we clamp scaled color values to the [0,255] range.
//
// r' = [255*(r-rLow)] / [rHigh - rLow]
// g' = [255*(g-gLow)] / [gHigh - gLow]
// b' = [255*(b-bLow)] / [bHigh - bLow]
//
// where [r/g/b][Low/High] refer to the 1% and 99% endpoints for the red, 
// green, and blue color changes respectively.
//
//----------------------------------------------

//==============================================
QImage* improveColorBalance( QString filename, StatusWidget* status )
{
  //load original image
  QImage* editedImage = new QImage( filename );
  
  //convert to 32-bit depth if necessary
  if( editedImage->depth() < 32 )
  {
    QImage* tmp = editedImage;
    editedImage = new QImage( tmp->convertDepth( 32, Qt::AutoColor ) );
    delete tmp; tmp=NULL;
  }

  //setup progress bar
  QString statusMessage = qApp->translate( "improveColorBalance", "Enhancing Color Balance:" );
  status->showProgressBar( statusMessage, 100 );
  qApp->processEvents();  
  
  //update progress bar for every 1% of completion
  const int updateIncrement = (int) ( 0.01 * editedImage->width() * editedImage->height() );
  int newProgress = 0; 

  //construct intensity histographs for each color channel
  int redVals[256];
  int greenVals[256];
  int blueVals[256];
  int i=0;
  for(i=0; i<256; i++) 
  {
    redVals[i] = 0;
    greenVals[i] = 0;
    blueVals[i] = 0;
  }  
  
  //populate histogram by iterating over all image pixels
  int numPixels = editedImage->width()*editedImage->height();  
  QRgb* rgb;
  uchar* scanLine;
  int x, y;
  for( y=0; y<editedImage->height(); y++)
  {   
    //iterate over each selected pixel in scanline
    scanLine = editedImage->scanLine(y);
    for( x=0; x<editedImage->width(); x++)
    {
      rgb = ((QRgb*)scanLine+x);
      redVals[qRed(*rgb)]++;
      greenVals[qGreen(*rgb)]++;
      blueVals[qBlue(*rgb)]++;
    } //for x
  } //for y
  
  //find 1% and 99% precenticles
  //we'll stretch these values so we avoid outliers from affecting the stretch
  int sumR=0;
  int sumG=0;
  int sumB=0;
  int indexLowR, indexHighR;
  int indexLowG, indexHighG;
  int indexLowB, indexHighB;
  indexLowR = -1; indexHighR = -1;
  indexLowG = -1; indexHighG = -1;
  indexLowB = -1; indexHighB = -1;
  for(i=0; i<256; i++)
  {
    sumR+=redVals[i];
    sumG+=greenVals[i];
    sumB+=blueVals[i];
    
    //if 1% not found yet and criteria met set index
    if(indexLowR < 0 && sumR >= 0.01*numPixels)
    { indexLowR = i; }
    if(indexLowG < 0 && sumG >= 0.01*numPixels)
    { indexLowG = i; }
    if(indexLowB < 0 && sumB >= 0.01*numPixels)
    { indexLowB = i; }
   
    //if 99% not found yet and criteria met set index
    if(indexHighR < 0 && sumR >= 0.99*numPixels)
    { indexHighR = i; }
    if(indexHighG < 0 && sumG >= 0.99*numPixels)
    { indexHighG = i; }
    if(indexHighB < 0 && sumB >= 0.99*numPixels)
    { indexHighB = i; }
  }
  
  //run through all image pixels a second time, this time scaling coordinates as necessary
  for( y=0; y<editedImage->height(); y++)
  {   
    //iterate over each selected pixel in scanline
    scanLine = editedImage->scanLine(y);
    for( x=0; x<editedImage->width(); x++)
    {
      //get color coordinates and convert to 0-1 scale
      rgb = ((QRgb*)scanLine+x);
      double r = ((double)qRed(*rgb)   );
      double g = ((double)qGreen(*rgb) );
      double b = ((double)qBlue(*rgb)  );

      if(indexHighR != indexLowR) { r = (255*(r-indexLowR))/(indexHighR-indexLowR); }
      if(indexHighG != indexLowG) { g = (255*(g-indexLowG))/(indexHighG-indexLowG); }
      if(indexHighB != indexLowB) { b = (255*(b-indexLowB))/(indexHighB-indexLowB); }
        
      int rp = (int) QMIN( QMAX(r, 0), 255 );
      int gp = (int) QMIN( QMAX(g, 0), 255 );
      int bp = (int) QMIN( QMAX(b, 0), 255 );
      
      //set adjusted color value
      *rgb = qRgb(rp,gp,bp);
      
      //update status bar if significant progress has been made since last update
      newProgress++;
      if(newProgress >= updateIncrement)
      {
        newProgress = 0;
        status->incrementProgress();
        qApp->processEvents();  
      }
      
    } //for x
  } //for y
  
  //remove status bar
  status->setStatus( "" );
  qApp->processEvents();

  //return pointer to edited image
  return editedImage;    
}
//==============================================

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