Detect paper as series of points with OpenCV - iphone

Question

I'm attempting to detect a piece of paper in a photo on the iPhone using OpenCV. I'm using the code from this question: OpenCV C++/Obj-C: Detecting a sheet of paper / Square Detection
Here's the code:
- (void)findEdges {
image = [[UIImage imageNamed:#"photo.JPG"] retain];
Mat matImage = [image CVMat];
find_squares(matImage, points);
UIImageView *imageView = [[[UIImageView alloc] initWithImage:image] autorelease];
[imageView setFrame:CGRectMake(0.0f, 0.0f, self.frame.size.width, self.frame.size.height)];
[self addSubview:imageView];
[imageView setAlpha:0.3f];
}
- (void)drawRect:(CGRect)rect {
[super drawRect:rect];
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetRGBStrokeColor(context, 1.0, 0.0, 0.0, 0.8);
CGFloat scaleX = self.frame.size.width / image.size.width;
CGFloat scaleY = self.frame.size.height / image.size.height;
// Draw the detected squares.
for( vector<vector<cv::Point> >::const_iterator it = points.begin(); it != points.end(); it++ ) {
vector<cv::Point> square = *it;
cv::Point p1 = square[0];
cv::Point p2 = square[1];
cv::Point p3 = square[2];
cv::Point p4 = square[3];
CGContextBeginPath(context);
CGContextMoveToPoint(context, p1.x * scaleX, p1.y * scaleY); //start point
CGContextAddLineToPoint(context, p2.x * scaleX, p2.y * scaleY);
CGContextAddLineToPoint(context, p3.x * scaleX, p3.y * scaleY);
CGContextAddLineToPoint(context, p4.x * scaleX, p4.y * scaleY); // end path
CGContextClosePath(context);
CGContextSetLineWidth(context, 4.0);
CGContextStrokePath(context);
}
}
double angle( cv::Point pt1, cv::Point pt2, cv::Point pt0 ) {
double dx1 = pt1.x - pt0.x;
double dy1 = pt1.y - pt0.y;
double dx2 = pt2.x - pt0.x;
double dy2 = pt2.y - pt0.y;
return (dx1*dx2 + dy1*dy2)/sqrt((dx1*dx1 + dy1*dy1)*(dx2*dx2 + dy2*dy2) + 1e-10);
}
void find_squares(Mat& image, vector<vector<cv::Point> >& squares) {
// blur will enhance edge detection
Mat blurred(image);
medianBlur(image, blurred, 9);
Mat gray0(blurred.size(), CV_8U), gray;
vector<vector<cv::Point> > contours;
// find squares in every color plane of the image
for (int c = 0; c < 3; c++)
{
int ch[] = {c, 0};
mixChannels(&blurred, 1, &gray0, 1, ch, 1);
// try several threshold levels
const int threshold_level = 2;
for (int l = 0; l < threshold_level; l++)
{
// Use Canny instead of zero threshold level!
// Canny helps to catch squares with gradient shading
if (l == 0)
{
Canny(gray0, gray, 10, 20, 3); //
// Dilate helps to remove potential holes between edge segments
dilate(gray, gray, Mat(), cv::Point(-1,-1));
}
else
{
gray = gray0 >= (l+1) * 255 / threshold_level;
}
// Find contours and store them in a list
findContours(gray, contours, CV_RETR_LIST, CV_CHAIN_APPROX_SIMPLE);
// Test contours
vector<cv::Point> approx;
for (size_t i = 0; i < contours.size(); i++)
{
// approximate contour with accuracy proportional
// to the contour perimeter
approxPolyDP(Mat(contours[i]), approx, arcLength(Mat(contours[i]), true)*0.02, true);
// Note: absolute value of an area is used because
// area may be positive or negative - in accordance with the
// contour orientation
if (approx.size() == 4 &&
fabs(contourArea(Mat(approx))) > 1000 &&
isContourConvex(Mat(approx)))
{
double maxCosine = 0;
for (int j = 2; j < 5; j++)
{
double cosine = fabs(angle(approx[j%4], approx[j-2], approx[j-1]));
maxCosine = MAX(maxCosine, cosine);
}
if (maxCosine < 0.3)
squares.push_back(approx);
}
}
}
}
}
Here's the input image:
Here's the result:
What am I doing wrong?