CG Jam

Using a garbage matte

Garbage Matting

Clean up unwanted set pieces from your Greenscreen clip.

Setting up Greenscreen sets can get expensive, so it’s common practice to just set up enough greenscreen to cover the action. This means there are often bits of the set that we are unable to key, such as lighting, cameras and maybe even stage hands.

In this lesson I show you how to easily apply a garbage matte and cut out all the needless junk from the clip so we can aquire a good key.

Hit the project link below to get the clip titled “match move test-wide” and unzip the file to your hard drive.

Hollywood camerawork

  1. open Blender and switch to the movie clip editor, and open the clip you just downloaded.
  2. In the properties menu (N on the keyboard) add a new object and rename it “mask” then add a single tracking marker to the girl and track the clip.
  3. Switch the mode to mask, by pressing tab and draw a mask around the girl, make sure to keep the set objects outside the mask. Parent the mask to the tracking marker so that the mask remains close to the girl through out the clip.
  4. Step through the clip and adjust the mask accordingly. It should remain just outside the girl while keeping set objects on the outside.
  5. Once you have finished, switch the editor window to the node editor, enable compositing nodes and check the use nodes tick-box. and check the backdrop box also. Delete the render layer node.
  6. Add an image node and open the clip, set the node to images sequence and adjust the frame length to match the clip length. and add a viewer node so you can see what you are doing.
  7. Add a scale node and set it to render size, then add the keying node. sample a section of green from the movie clip.
  8. Add a mask node to the setup. From the ADD menu (shift A) select input then mask. and connect the output from the mask node into the Garbage socket on the keying node. Select the mask we created earlier from the menu on the mask node.
  9. Now with just we can adjust the clip black/clip white on the keying node to get close to a good key. once you are close, connect a color ramp to the keying node using the MATTE output and bring the white slider down until the mask becomes a solid silhouette.
  10. drop in a blur node after the ramp and push the x/y values up by just 2 or 3 pixels to clean up the edges. now add a mix node and connect the blur node
  11. Place a mix node in after the blur node and connect the blur output into the fac socket and pull the noodle from the image output of the keying node and connect it into the bottom image socket of the mix node.
  12. Insert a new image node for the background, you can use any image for this as it is just a test to check the Matte. connect the image to a scale node, again set to render size and connect that node into the top socket of the mix node.
  13. If all has gone well, your girl should be standing in a still scene as the camera turns around her.if its a bit grainy and the background is still showing in places go back th the ramp and tweak the settings slightly. Once your done, connect the final mix node to the composite node and you can render your little clip

And that’s that, slightly more complicated than last time but still a simple exercise. in the next lesson we will use what we have so far and track the clip ready to place it into a virtual scene. so save your work, have a cup of tea and a 5 minute break and we’ll get straight back into it :)

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