How to compute 2D log-chromaticity? - c++

Question

My goal is to remove shadows from image. I use C++ and OpenCV. Sure I lack enough math background and not being native English speaker makes everything harder to understand.
After reading different approaches to remove shadows I found method which should work for me but it relies on something they call "2D chromaticity" and "2D log-chromaticity space" but even this term seems to be inconsistent in different sources. Many papers on topic, few are listed here:
http://www.cs.cmu.edu/~efros/courses/LBMV09/Papers/finlayson-eccv-04.pdf
http://www2.cmp.uea.ac.uk/Research/compvis/Papers/DrewFinHor_ICCV03.pdf
http://www.cvc.uab.es/adas/publications/alvarez_2008.pdf
http://ivrgwww.epfl.ch/alumni/fredemba/papers/FFICPR06.pdf
I teared Google into strips by searching right words and explanations. Best I found is Illumination invariant image which did not help me much.
I tried to repeat formula log(G/R), log(B/R) described in first paper, page 3 to get figures similar to 2b.
As input I used http://en.wikipedia.org/wiki/File:Gretag-Macbeth_ColorChecker.jpg
Output I get is
My source code:
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>
#include <stdio.h>
using namespace std;
using namespace cv;
int main( int argc, char** argv ) {
Mat src;
src = imread( argv[1], 1 );
if( !src.data )
{ return -1; }
Mat image( 600, 600, CV_8UC3, Scalar(127,127,127) );
int cn = src.channels();
uint8_t* pixelPtr = (uint8_t*)src.data;
for(int i=0 ; i< src.rows;i++) {
for(int j=0 ; j< src.cols;j++) {
Scalar_<uint8_t> bgrPixel;
bgrPixel.val[0] = pixelPtr[i*src.cols*cn + j*cn + 0]; // B
bgrPixel.val[1] = pixelPtr[i*src.cols*cn + j*cn + 1]; // G
bgrPixel.val[2] = pixelPtr[i*src.cols*cn + j*cn + 2]; // R
if(bgrPixel.val[2] !=0 ) { // avoid division by zero
float a= image.cols/2+50*(log((float)bgrPixel.val[0] / (float)bgrPixel.val[2])) ;
float b= image.rows/2+50*(log((float)bgrPixel.val[1] / (float)bgrPixel.val[2])) ;
if(!isinf(a) && !isinf(b))
image.at<Vec3b>(a,b)=Vec3b(255,2,3);
}
}
}
imshow("log-chroma", image );
imwrite("log-chroma.png", image );
waitKey(0);
}
What I am missing or misunderstand?

Answer 1

By reading the paper Recovery of Chromaticity Image Free from Shadows via Illumination Invariance that you've posted, and your code, I guess the problem is that your coordinate system (X/Y axis) are linear while in the paper the coordinate system are log(R/G) by log(B/G).

Answer 2

This is the closest I can figure. Reading through this:
http://www2.cmp.uea.ac.uk/Research/compvis/Papers/DrewFinHor_ICCV03.pdf
I came across the sentence:
"Fig. 2(a) shows log-chromaticities for the 24 surfaces of a Macbeth ColorChecker Chart, (the six neutral patches all belong to the same
cluster). If we now vary the lighting and plot median values
for each patch, we see the curves in Fig. 2(b)."
If you look closely at the log-chromaticity plot, you see 19 blobs, corresponding to each of the 18 colors in the Macbeth chart, plus the sum of all the 6 grayscale targets in the bottom row:
Explanation of Log Chromaticities
Explanation of Log Chromaticities
With 1 picture, we can only get 1 point of each blob: We take the median value inside each target and plot it. To get plot from the paper, we would have to create multiple images with different lighting. We might be able to do this by varying the temperature of the image in an image editor.
For now, I just looked at the color patches in the original image and plotted the points:
Input:
Color Patches Used
Output:
Log Chromaticity
The graph dots are not all in the same place as the paper, but I figure it's fairly close. Would someone please check my work to see if this makes sense?

Answer 3

In that OpenCV code I got a "undefined Identifier error" for the function ifinf() and I solved it by replacing it with _finite(). That might be the issue with the Visual studio version.
if(!isinf(a) && !isinf(b)) ----> if(_finite(a) && _finite(b))
Include this header:
#include<float.h>