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TUTORIAL: G Matte: Fixing poor greenscreens

Posted: Tue, 20th Apr 2004, 12:10pm

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Simon K Jones

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Chromanator Tutorial: Garbage mattes

Fixing poor greenscreens

The whole point of using a greenscreen is to save you having to do any manual compositing. Of course, things do not always go according to plan and some manual fixing may be required after all. Take this shot for example:

This shot is plagued with difficulties, from the poor lighting on the greenscreen to the scaffolding. However, the trickiest problem is that the actor himself protrudes off the edges of the greenscreen. A colour difference key and a simple 4-point garbage matte around the left-hand scaffolding can do the majority of the work:

But that still leaves the area of wall to the top and right. This will need to be removed manually using the garbage matte system. First I draw a mask shape freehand around the right hand side of the picture, ensuring not to lose any of the knee or arm:

This then needs to be animated as the marine slides down the rope. The mask points can be repositioned as required whilst moving through the frames.

  • Multiple points can be selected by dragging with the mouse.

Animating using keyframes is something that becomes easier with practice. You do not need to keyframe every single frame – a useful technique is to start by keyframing every 10th frame, then go through every 5th frame again, gradually adding to the precision as needed, until you have the animation that you need. With practice you will work out your own time-saving techniques.

Additional masks can be added in the gap between the marine's arms, and to the section of wall to the left of his arms, building up the final garbage matte from several individual masks. Careful planning can often reduce the amount of work that is required.

Of course, hopefully you will rarely need to do such precise work, but the point is that Chromanator has the ability and the power should the situation arise – you can even manually key out an entire person from a shot if you so desire.

Last edited Wed, 12th May 2004, 4:19pm; edited 1 times in total.

Posted: Wed, 21st Apr 2004, 6:00pm

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I am completely stunned with the result of this program. Fantastic! smile
Posted: Wed, 28th Apr 2004, 4:18pm

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Force: 220 | Joined: 7th Mar 2004 | Posts: 47

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THis is a great tutorial because most of them tend to be static.. this is the first one I've read that covers moving backgrounds.

I think a bit more detail on how a reference point can move between keyframes, perhaps showing a zoom-in on frame 10 detailing a feew points and then frame 20 and showing how the points are moved. Also a few words about what to do if you need more points halfway through the scene would be great!