## Sunday, March 25, 2012

### Cubic spline interpolation

It's been a long since I actually coded any interpolation method.
Matlab is notorious for making you lazy since it's so easy to get things done and you tend to stop looking under the hood. A friend asked me for help on a cubic interpolation problem and since that was too easy I expanded it so I can use it on my projects.

The math behind cubic spline is really simple. You piecewise fit cubic polynomials using 4 data values (two points and two tangents) in order to create a smooth spline that passes from all given points. The wikipedia sources are really good so I won't dive into the math. Instead I'll provide some matlab code for doing the dirty deed. Matlab (as always) has a command for this (spline) but we wont be using it because I like getting my hands dirty.
function [ yy, xx] = cubicSpline(N,points,gradients)
% CUBICSPLINE - returns N interpolation points using cubic spline
%   interpolation method, this method cannot be used for space curves
%
% Input
% N         : number of interpolation points
% points    : given points [x y]
%
% Output
% xx        : uniform spaced function interpolation at N points
% yy        : uniform spaced N points
%
%-------------------------------------------------------------------------
%% Validate input arguments
if isempty(N) || N < 1
error('N must be >= 1');
end

if isempty(points) || size(points,1) < 1
error('point must have >= 1 rows');
end

if isempty(points) || size(points,2) ~= 2
error('point must have 2 collumns');
end

end
%% coefficient calculation part

% get number of points
[rows ~] = size(points);

% compute inverse matrix to be used
matrix = inv([1 0 0 0 ; 0 1 0 0 ; 1 1 1 1; 0 1 2 3]);

% initialize coefficients structure
coefficients = zeros(rows-1,4);

% given n points we must calculate n-1 polynomials
for i = 2 : rows
pEnd = [];
pStart = [];

% calculate gradient using finite central differences
if (i-1) == 1
else
pStart = (points(i,2) - points(i-2,2))/2;
end

if i == rows
else
pEnd = (points(i+1,2) - points(i-1,2))/2;
end

% create vector [Pi P'i Pi+1 P'i+1]'
vector = [points(i-1,2);pStart;points(i,2);pEnd];

% calculate polynomial coefficients
coefficients(i-1,:) = (matrix * vector)';
end

%% interpolation part

% get max X and min X and interval
minX = points(1,1);
maxX = points(end,1);
intervalX = (maxX - minX) / (N - 1);
xx = minX : intervalX : maxX;

% interpolate at given locations
yy = zeros(1,N);
splineIndex = 1;
for i = 2 : N-1
x = xx(i);
% find the index of the used spline
for j = splineIndex : rows
if x >= points(j,1) && x < points(j+1,1)
splineIndex = j;
break;
end
end
splineCoeffs = coefficients(splineIndex,:);
% compute m
m = (xx(i) - points(splineIndex,1))/...
(points(splineIndex+1,1) - points(splineIndex,1));
% compute value with given spline and m
yy(i) = splineCoeffs(1) + splineCoeffs(2) * m + ...
splineCoeffs(3) * m^2 + splineCoeffs(4) * m^3;
end
yy(1) = points(1,2);
yy(end) = points(end,2);
end



This code can be used to interpolate y=f(x) functions. For example :

%% Demonstration of cubic splines
N = 100;
x = [0:1:10];
y = sin(x);
xOriginal = [0:0.1:10];
yOriginal = sin(xOriginal);
figure;
plot(xOriginal,yOriginal,xx,yy,'--',x,y,'o','LineWidth',2);
legend('Original function','Interpolation spline','Given points');
%%



gives us the following graph :

any errors at the beginning and the end are due to the fact that I entered zero gradient at those points but provided the correct gradients the result should be much more precise.

To to make it somewhat useful in my projects I should use this function as a basis for calculating space curves. This excellent source explains that space curves are functions of u such as y = f(u) and x = f(u).

function [ yy,xx ] = cubicSpline2d(N, points, gradients )
% CUBICSPLINE - returns N interpolation points using cubic spline
%   interpolation method, this method cann be used for space curves
%
% Input
% N         : number of interpolation points
% points    : given points [x y]
%
% Output
% xx        : uniform spaced function interpolation at N points
% yy        : uniform spaced N points
%
%-------------------------------------------------------------------------
%% Validate input arguments
if isempty(N) || N < 1
error('N must be >= 1');
end

if isempty(points) || size(points,1) < 1
error('point must have >= 1 rows');
end

if isempty(points) || size(points,2) ~= 2
error('point must have 2 collumns');
end

end
%%
% get number of points
[rows ~] = size(points);

% get total length of points
u = [1 : rows]';
x = [points(:,1)];
y = [points(:,2)];

end



and the test script

%% Demonstration of cubic splines 2d
u = 0:0.5:2*pi;
N = numel(u)*10;
y = sin(u);
x = sin(u) + cos(u);
gradient = [0 0; 0 0];
plot(xx,yy,'--r',x,y,'ob','LineWidth',2);
legend('Interpolation spline','Given points');
title('Cubic space curve interpolation')
xlabel('x-axis');
ylabel('y-axis');
%%



gives us the following plot :

## Friday, March 2, 2012

### Latest results

As part of my hand tracking project I post these last videos. I believe I reached the top of the performance of the particle filter algorithm.

Unfortunately I want even better results so I should move to more complex algorithms. The problem is that real time performance is going to be much more difficult to achieve.

### Contour Refinement

In my hand tracking project I've used color segmentation to get the hand contour.
The problem is that usually the contour is not perfect, there are parts missing and/or there is some noise, for that reason I coded this general purpose contour refining function that fits the contour on a feature map moving each point along its normal.

        /// <summary>
/// Find better positions for each better at contour by searching on the edge normals on
/// the feature map
/// </summary>
/// <param name="_objectContour">The contour to be refined</param>
/// <param name="_featureMap">The feature map to be refined unto</param>
/// <param name="_normalOffset">The maximum number of pixels to offset</param>
/// <param name="_featureThreshold">The minimum feature value acceptable</param>
/// <returns>Refined Contour</returns>
public static Seq<Point> ContourRefine(
Seq<Point> _objectContour,
Image<Gray, float> _featureMap,
int _normalOffset = 5,
float _featureThreshold = float.MaxValue,
float _inertiaCoeff = 1.0f,
float _multiplierCoeff = -1.0f)
{
List<Point> pointsFitted = new List<Point>();
Point[] pointsArray = _objectContour.ToArray();
for (int i = 0; i < pointsArray.Length; i++)
{
int noPoints = pointsArray.Length,
ki = (i + 1) % noPoints, ik = (i >= 1) ? (i - 1) : (noPoints - 1 + i) % noPoints;
Point pointCurrent = pointsArray[i],
pointNext = pointsArray[ki],
pointPrev = pointsArray[ik];
// get normals pointing in and out
PointF pointNormalOut = NormalAtPoint(pointPrev, pointCurrent, pointNext, false),
pointNormalIn = NormalAtPoint(pointPrev, pointCurrent, pointNext, true);
// get points away from normal
Point pointOut = new Point(
(int)Math.Round(pointNormalOut.X * _normalOffset) + pointCurrent.X,
(int)Math.Round(pointNormalOut.Y * _normalOffset) + pointCurrent.Y),
pointIn = new Point(
(int)Math.Round(pointNormalIn.X * _normalOffset) + pointCurrent.X,
(int)Math.Round(pointNormalIn.Y * _normalOffset) + pointCurrent.Y);
LineSegment2D lineOut = new LineSegment2D(pointCurrent, pointOut),
lineIn = new LineSegment2D(pointCurrent, pointIn);

// sample along the normals
float[,] sampleIn = _featureMap.Sample(lineIn);
float[,] sampleOut = _featureMap.Sample(lineOut);
float maxByte = 0.0f, sample = 0.0f;
int j = 0;
bool inOut = false;
// run through the normal pointing out to find the best fit
for (int k = 0; k < sampleOut.Length; k++)
{
sample = sampleOut[k, 0] + _multiplierCoeff * (float)Math.Pow(_inertiaCoeff, k);
if (sample > maxByte)
{
maxByte = sample;
j = k;
inOut = false;
}
}

// run through the normal pointing in to find the best fit
for (int k = 0; k < sampleIn.Length; k++)
{
sample = sampleIn[k, 0] + _multiplierCoeff * (float)Math.Pow(_inertiaCoeff, k);
if (sample > maxByte)
{
maxByte = sample;
j = k;
inOut = true;
}
}

// if feature on point found exceeds a threshold add it to the contour
if (maxByte >= _featureThreshold)
{
int x, y;
double length, xLength, yLength;
if (!inOut)
{
xLength = lineOut.P1.X - lineOut.P2.X;
yLength = lineOut.P1.Y - lineOut.P2.Y;
length = lineOut.Length;
x = (int)Math.Round((float)j / (float)sampleOut.Length * pointNormalOut.X * _normalOffset);
y = (int)Math.Round((float)j / (float)sampleOut.Length * pointNormalOut.Y * _normalOffset);
}
else
{
xLength = lineIn.P1.X - lineIn.P2.X;
yLength = lineIn.P1.Y - lineIn.P2.Y;
length = lineIn.Length;
x = (int)Math.Round((float)j / (float)sampleIn.Length * pointNormalIn.X * _normalOffset);
y = (int)Math.Round((float)j / (float)sampleIn.Length * pointNormalIn.Y * _normalOffset);
}
pointsFitted.Add(new Point(pointCurrent.X + x, pointCurrent.Y + y));
}
}
_objectContour.Clear();
_objectContour.PushMulti(pointsFitted.ToArray(), BACK_OR_FRONT.BACK);
return _objectContour;
}

/// <summary>
/// Calulcate the normal at given point
/// </summary>
/// <param name="_prevPoint">Previous point</param>
/// <param name="_currentPoint">Current point</param>
/// <param name="_nextPoint">Next point</param>
/// <param name="_inOut">In or out flag</param>
/// <returns>Normal at point</returns>
public static PointF NormalAtPoint(
Point _prevPoint,
Point _currentPoint,
Point _nextPoint,
bool _inOut = true)
{
PointF normal;
float dx1 = _currentPoint.X - _prevPoint.X,
dx2 = _nextPoint.X - _currentPoint.X,
dy1 = _currentPoint.Y - _prevPoint.Y,
dy2 = _nextPoint.Y - _currentPoint.Y;
if (_inOut)
normal = new PointF((dy1 + dy2) * 0.5f, -(dx1 + dx2) * 0.5f);
else
normal = new PointF(-(dy1 + dy2) * 0.5f, (dx1 + dx2) * 0.5f);
return NormalizePoint(normal);
}

/// <summary>
/// Normalize a given point so its _noBinsAngle equals to one
/// </summary>
/// <param name="_point">Point to normalize</param>
/// <returns>Normalized point</returns>
public static PointF NormalizePoint(PointF _point)
{
float length = (float)Math.Sqrt(_point.X * _point.X + _point.Y * _point.Y);
if (length > 0.0f)
return new PointF(_point.X / length, _point.Y / length);
return new PointF(0.0f, 0.0f);
}


## Monday, February 27, 2012

### HAAR xml file

Because many people have asked for it, I believe that it will make your life easier I give you my trained hand HAAR cascade xml file.
It's trained on about 20k positives and 20k negatives and works on any orientation.
Watch for high false positive rates. It also works with the cuda version of OpenCV.

The xml download. In a later post I will show you how to make haar cascade perform even better.

## Monday, January 30, 2012

### Recognising Fingertips

Pulling an all nighter is always rewarding , so Friday night I made my first attempt at detecting the fingertips on the hand. Most people use the "convex irregularities" method. I really didn't like that method. It seems sloppy and doesn't detect all fingers. I prefer the kcosines method as described in "Vision-Based Finger Action Recognition by Angle Detection and Contour Analysis".

These are the results of my first attempt.

This week I'll concentrate on making tracking and contour extraction more robust because as you can see at some points the contours break up. I guess selecting my wooden office desk as a testing area proved to be quite a challenge.

## Wednesday, January 25, 2012

### Extracting and stabilizing contours

It's been a busy week. I'm now at the stage of contour extraction. Using my adaptive skin classifier and the samples gathered from the detector I build a histogram model for the hand and extract the contours around it. As you can see it is quite robust and works under different illumination. All these are possible with the assumption that the detector doesn't return a false positive. While HAAR cascades are fairly good at the job they don't have a 0% percent of false positives so I intend to add a Fourier hand validator.

## Thursday, January 12, 2012

The detector I trained back in November doesn't work for me anymore, so I'm attempting to train another one using the train cascades executable provided in the OpenCV framework.

I gathered 70k negatives , 30k positives and I run exactly the same code and boom crash burn. All I got was the message "Train dataset for temp stage can not be filled. Branch training terminated."

After 2 hours of screwing around all possible combinations and reading the same problems on the net I decided to look at the code (damn messy C++ code). After that I decided to reduce the number of negatives to the same number of positives and voila it worked.

I've come to believe that OpenCV was originally written by a couple of uber programmers and then left to be wrecked by drunken monkeys.

## Wednesday, January 11, 2012

### Building an adaptive skin classifier

Building an adaptive skin classifier is quite some work.
I've seen some examples on the net. The good ones are not real time and the ones that are  simply don't cut it.
A small lighting variation, a slightly complex background and the classifier is lost.

Many examples I've seen use hard coded variables. That is obviously wrong.
I've also seen many that use the RGB color space which is also wrong.
I strongly recommend the HSV color space because it is slightly more lighting invariant or at least normalized RGB.

I also recommend using more features than the color channels. You also have to experiment with different bin sizes in order to get real time performance.

I recently found out that arithmetic accuracy is also important because of all the normalization operations.

During segmentation is important to postpone thresholding  because of the mass loss of information. I prefer using the probability map during tracking and later threshold to extract the contours.

## Saturday, January 7, 2012

### Well I'm getting closer...

I'm usually very critical of the stuff I make but today I feel quite satisfied with the results.

## Tuesday, January 3, 2012

### The constraints of histogram tracking

While in the last months I have implemented and tested a couple of histogram based tracking algorithms I only recently realized their inherent constraints. If you use it for hand tracking and you have your arms naked or your face exposed it is very easy for the tracker to get confused because of the coarse quantization of the histograms.
The worst part is that there is little you can do :
• I tried using a different color space such as HSV. While it is far better that RGB for tracking skin and coping for small lighting variances it is still not enough and the tracker often gets confused especially when the hand goes out of view.
• I tried incorporating different features such as edge magnitude (bad idea) and edge orientation (much better) and it had the effect of better localizing the detector.
Integral histograms are still an option but there are far too slow for a real time application like the one I'm working on.