ofxCvContourFinder
The contour finder allows you to detect objects in a scene by looking contrast caused by the difference between adjoining pixels. For instance, in the image below, the hand is visible and trackable because the contrast between the wall behind it and the arm is quite distinct:
[img src="contour.png"]
You can make contour detection more robust by by the comparing the current image to a background image and subtracting the background from the current image. This enables you to examine the incoming image without the background image data, reducing the amount of data that needs to be inspected.
The contourFinder requires an ofxCvGrayscaleImage be passed to it, so you'll need to create one either the video or camera feed that you're using. An example of working with a camera is shown here:
void contoursApp::setup(){
bLearnBackground = false;
vidGrabber.setVerbose(true);
vidGrabber.initGrabber(320,240);
colorImg.allocate(320,240);
grayImage.allocate(320,240);
grayBg.allocate(320,240);
grayDiff.allocate(320,240);
}
void contoursApp::update(){
vidGrabber.grabFrame();
//do we have a new frame?
if (vidGrabber.isFrameNew()){
colorImg.setFromPixels(vidGrabber.getPixelsRef());
grayImage = colorImg; // convert our color image to a grayscale image
if (bLearnBackground == true) {
grayBg = grayImage; // update the background image
bLearnBackground = false;
}
grayDiff.absDiff(grayBg, grayImage);
grayDiff.threshold(30);
contourFinder.findContours(grayDiff, 5, (340*240)/4, 4, false, true);
}
}
void contoursApp::draw(){
ofSetHexColor(0xffffff);
colorImg.draw(0, 0, 320, 240);
grayDiff.draw(0, 240, 320, 240);
ofRect(320, 0, 320, 240);
contourFinder.draw(320, 0, 320, 240);
ofColor c(255, 255, 255);
for(int i = 0; i < contourFinder.nBlobs; i++) {
ofRectangle r = contourFinder.blobs.at(i).boundingRect;
r.x += 320; r.y += 240;
c.setHsb(i * 64, 255, 255);
ofSetColor(c);
ofRect(r);
}
}
void contoursApp::keyPressed(int key) {
bLearnBackground = true;
}
ofxCvContourFinder methods
getWidth()
float ofxCvContourFinder::getWidth()
Returns the height of the area that detection is being performed upon.
getHeight()
float ofxCvContourFinder::getHeight()
Returns the height of the area that detection is being performed upon.
findContours(...)
int ofxCvContourFinder::findContours(ofxCvGrayscaleImage &input, int minArea, int maxArea, int nConsidered, bool bFindHoles, bool bUseApproximation=true)
input This is an ofxCvGrayscaleImage reference (this is written ofxCvGrayscaleImage&) to a grayscale image that will be searched for blobs. Note that grayscale images only are considered. So if you're using a color image, you'll need to highlight the particular color that you're looking for beforehand. You can do this by looping through the pixels and changing the color values of any pixel with the desired color to white or black, for instance. minArea This is the smallest potential blob size as measured in pixels that will be considered as a blob for the application. maxArea This is the largest potential blob size as measured in pixels that will be considered as a blob for the application. nConsidered This is the maximum number of blobs to consider. This is an important parameter to get right, because you can save yourself a lot of processing time and possibly speed up the performance of your application by pruning this number down. An interface that uses a user's fingers, for instance, needs to look only for 5 points, one for each finger. One that uses a user's hands needs to look only for two points. bFindHoles This tells the contour finder to try to determine whether there are holes within any blob detected. This is computationally expensive but sometimes necessary. bUseApproximation This tells the contour finder to use approximation and to set the minimum number of points needed to represent a certain blob; for instance, a straight line would be represented by only two points if bUseApproximation is set to true.
draw(...)
void ofxCvContourFinder::draw(float x, float y)
Draws the detected contours into the coordintes passed in.
draw(...)
void ofxCvContourFinder::draw(float x, float y, float w, float h)
Draws the detected contours at the point passed in with the height and width, scaling as necessary.
draw(...)
void ofxCvContourFinder::draw(const ofPoint &point)
Draws the detected contours at the point passed in.
draw(...)
void ofxCvContourFinder::draw(const ofRectangle &rect)
Draws the detected contours into the ofRectangle passed in scaling if necessary.
setAnchorPercent(...)
void ofxCvContourFinder::setAnchorPercent(float xPct, float yPct)
Sets the anchor point as a percentage.
setAnchorPoint(...)
void ofxCvContourFinder::setAnchorPoint(int x, int y)
Sets an anchor point for the drawing.
ofxCvContourFinder variables
ofxCvBlob blobs
ofxCvBlob ofxCvContourFinder::blobs