Figure 2.

Binary mask creation for tracking an epicardial marker through frames of video. (a) Original subimage of one marker. (b) 4× resampling of image in (a) (bicubic interpolation). (c) Edges of image in (b) detected with Canny edge detection. (d) Morphological dilation closes any gaps in the inner and outer edges of the marker. (e) A flood-fill operation is applied to the center of the image in (d). The area identified by flood-fill is then dilated to compensate for the dilation performed in (d). (f) Subtraction of a dilation of the image in (e) from the image in (d) yields a dilated edge image with the inner ring removed. (g) Flood-fill identifies the outer edge of the marker. The area identified by flood-fill is then dilated to compensate for the dilation performed in (d). (h) Subtraction of the image in (e) from the image in (g) gives a binary mask of the marker. (i) The mask binary mask in (g) is resampled to the image resolution used for motion tracking (8× original camera resolution). The white lines show the inner and outer contours of the resampled binary marker mask, overlaid on an 8× resampling of the original sub-image.