Curves and Splines: your secret weapon for precise masking

We’ve probably got the automotive industry to thank for masking tools. In the 1950s and 60s mechanical engineers at Renault needed to accurately define the curvature of car parts and mass produce them. It was easy to draw them on paper, but not to cut them out of sheet metal repeatedly and consistently.

A French engineer working at Renault, Pierre Bézier, changed that. Bézier started his research into CADCAM (Computer Aided Design, Computer Aided Modelling) in 1960 when the discipline was still in its infancy and invented a mathematically efficient way for machines to describe and print curves.

The Bezier Curve as it is now called is a method to graphically represent a curve between two points, with “control points” at either end that allow you to modify the shape of the line. It is possible with these control points to bend the curves produced into any shape in an economical way, defined by mathematical equations.

It’s essentially a way to draw around objects with an array of small curves. Thanks to this innovation, most graphics software (for instance, Photoshop) includes a pen tool for drawing paths with a variant of Monsieur Bézier’s curves.

HitFilm is no exception

When people start to learn masking in HitFilm or other software, it’s usually with the sphere or rectangle tool. However, in certain circumstances you may need to get a more precise mask that is accurate to the form you are cutting out, and that’s when it’s good to try out the Freehand tool, which also uses a variant of the Bézier.

Drawing Bézier curves may seem baffling at first; it’s something that requires some practice to get used to, but once mastered, Bézier curves are a wonderful way to draw, and are the basis of all rotoscoping, too.

HitFilm

In this week’s HitFilm exercise you got a chance to try the ellipse and rectangle mask tools to select actors. If you look at the mask icons by the side of the screen (see the picture above!) you’ll have noticed that underneath the Sphere and Rectangle mask tools there’s a pen icon. That’s the Freehand tool.

To operate it, first check you have a layer selected on the timeline or in the Viewer that you want to draw a mask around. Select the Freehand tool (the Pen icon). If you click in the viewer with it selected, move the cursor and click again, you’ll draw a straight line, useful for cutting out an object composed of straight lines like a box or wall poster. Click a few points then click back on the first point to create the closed mask. If you misplace a click you’ll find you can move it later.

Of course in the real world a lot of objects aren’t made of straight edges, but made of curves or more organic and irregular shapes.

That’s where a more complex way to use this tool comes in handy, and that’s when you click and drag. So start a new mask, and on your second click try dragging the mouse a little with the mouse button pressed down. (if you are using a stylus or trackball do the equivalent) You should see a slight curve appears and two extended ‘handles’ appear either side of your point.

Dragging either handle will change the tautness of the curve on that side. Do a couple more click and drag points and then close the object.

Now this may seem a little difficult and unwieldy at first but with a little practice you’ll see it can help you cut out many objects far closer and more precisely than any ellipse or rectangle. In fact these ‘splines’ – as the lines between the points are called- are the main way to rotoscope. All those VFX-laden Hollywood films owe a debt of gratitude to Pierre Bézier’s wish to find a better way to cut sheet metal at Renault.

Now although you don’t need to use the Freehand tool (Pen icon) for any of our exercises, it may be you want to try it out on your own ideas. Keep your shapes really simple at first, until you get the hang of it.

To recap:

• With the Freehand tool, clicking and dragging creates a curved line, while clicking and releasing creates a linear, straight line.
• The shape of curved points can be further adjusted using the extension handles.
• In addition you can change the properties of any point by right clicking on it and choosing the required type from the menu. These are:
  • Make Curved Locked – both extension handles work in tandem. Adjusting one will adjust the other simultaneously. This is useful for maintaining smooth curves
  • Make Curved Unlocked – the extension handles can be adjusted separately. This is useful for creating sharp corners while retaining a curved line
  • Make Linear – the point forms an angled corner and the line either side is straight, as if you never dragged in the first place.

If you want to find out more, have a look at this extra HitFilm masking tutorial on YouTube.

Remember, if you haven’t got the time to get to grips with this technique right now, it won’t stop you doing this week’s exercise successfully, so you can always come back to this some other time!