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 CompositeLab 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.
