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TUTORIAL: Masks: Basic use - creating depth

Posted: Thu, 22nd Apr 2004, 11:07am

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Simon K Jones

Force: 27955 | Joined: 1st Jan 2002 | Posts: 11683

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Chromanator Tutorials: Masks

Basic use - creating depth

Compositing works by layering several layers on top of one another. This can often lead to a rather flat shot, with little interaction between the layers, which in turn can make the shot less convincing. Masking can be used to blend layers together and hide the joins.

In this example I will be using a shot of a fireball racing towards the ground. Upon impact, it bursts into flames. Due to the framing of the shot, where the fireball hits the ground is partially behind a large wooden crate. However, as the fire effects have been composited on top of the background plate, the flame bleeds over the top of the crate:

I need to tell Chromanator that part of the fire should be obscured by the crate. To do this, I'll use a simple mask.

Positioning the mask
The first thing is to work out which frames require the masking. A quick scrub through the timeline shows that the flames are only ‘behind' the crate for a certain period. The default mask object can be found in the Media Browser, and I now drag it onto the timeline and adjust its length accordingly.

  • The standard editing tools work for masks in the same way as they do for any other media object.

Drawing the mask
Now I switch to the Object View by selecting the mask and clicking the Object tab. The Object View is very different when viewing a mask object, as you do not have access to all the standard tools. You may, however, recognise the toolbox from the garbage matte interface.

After adding a new section with the button, I can draw the mask directly onto the canvas. In this case I draw carefully around the crate, making sure not to go over the edges.

The mask turns blue when it is closed. It can be locked by clicking the Animate tab. The mask shape now needs to be repositioned as the camera moves, by scrubbing through the timeline and moving the control points.

  • Masking is always easier with a stationary camera/object.
  • You do not necessarily have to keyframe every single frame. However, the more complex and fast the movement is, the more detail you will have to put into the animation.

Linking the mask
Now that the mask has been drawn, I need to specify which objects it affects. Even though it is the shape of the crate, it is actually the fire that needs to be affected by the mask.

Masks can be ‘linked' to as many objects as you wish. Simply select the mask and the objects, then choose ‘Link Mask to Media' from the objects' menu.

  • Multiple timeline objects can be selected by holding down Ctrl (Apple key on Macs) whilst clicking on them.

After objects have been linked to a mask their colours will alter temporarily when clicked upon to reflect the links.

  • If you click on the mask, all the linked objects will change to the same colour as the mask.
  • If you click on a linked object, the mask will change to the same colour as the object.

The mask is represented on the canvas as a semi-transparent, animated shape that is the same colour as the mask on the timeline.

  • You can change the colour using the object colour wheel to the bottom left of the timeline.

When you render the project, the mask will remove the flame whenever it passes behind the mask shape, creating the illusion that it is moving behind the crate.

Click here to watch an example video.