Canny Edge Detection Canny Edge Detection n 5

Canny Edge Detection

Canny Edge Detection n 5 STEPS n n n Apply Gaussian filter to smooth the image in order to remove the noise Find the intensity gradients of the image Non-maximum suppression is an edge thinning technique. Apply double threshold to determine potential edges Track edge by hysteresis: Finalize the detection of edges by suppressing all the other edges that are weak and not connected to strong edges.

Gaussian Filter n n Gaussian Smoothing In 2 -D, an isotropic (i. e. circularly symmetric) Gaussian has the form:

Gaussian Filter n The figure shows a suitable integer-valued convolution kernel that approximates a Gaussian with a σ of 1. 0.

Intensity Gradients n Example

Intensity Gradients n Example

Intensity Gradients n Example

Intensity Gradients n Open. CV

Non-maximum suppression n Example

Thresholding n n Edges are found by thresholding the output of NONMAX_SUPRESSION If the threshold is too high: n Very few (none) edges n n High MISDETECTIONS, many gaps If the threshold is too low: n Too many (all pixels) edges n High FALSE POSITIVES, many extra edges

Thresholding n Use a high threshold to start edge curves and a low threshold to continue them.

Edge Detection With Hysteresis Low threshold Hysteresis (high and low threshold) High threshold

Edge Tracking Hysteresis thresholding [Canny ’ 86] Weak edges Strong edges reinforce weak edges Weak edge removed We call a pixel an edge if it is strong. We also call a pixel an edge if it is weak but is connected to an edge. A pixel is connected to an edge if it is in a direction perpendicular to the edge normal

References n n http: //www. ics. uci. edu/~majumder/DIP/cl asses/Edge. Detect. pdf http: //cronos. rutgers. edu/~meer/TEACH/A DD/cannyedge. pdf http: //www. cs. hunter. cuny. edu/~ioannis/ CSc 83020_lec 06_Edge. Detection 2. ppt http: //homepages. inf. ed. ac. uk/rbf/HIPR 2/ gsmooth. htm