The Trace tool is for tracing contours manually. It can also be selected temporarily or permanently by pressing
T or Shift + T, respectivelyon the keyboard.
Contours created using this tool will be added to the Sliced Contours Fragment marked for editing in the image composition tree. Similarly, countours replacing parts of existing contours will affect the same fragment.
Note that you need to have a Sliced Contours Object in the image composition tree and must select it for editing.
Draw using the left button of the mouse to create a contour that follows the edge points extracted from the image selected to be analyzed in the image composition tree.
Use the right button of the mouse to draw contours free-hand.
When drawing with either the left or right button of the mouse, holding the Alt key makes the contour follow existing contours. The Alt key can be pressed and released at any time while drawing, adjusting the alignment behavior as needed.