How to Replace a Background Using Difference Matting
Avid’s difference matting feature allows users to easily replace a background by removing a stationary background from a moving object. This is done by basically keying it out using the Difference Matte Key. This tool is simple enough to be used by beginners, but it will take some practice to get rid of inconsistencies in the image.
Using Difference Matting
Difference matting is generally used with moving objects since it can be used to remove a background that is not moving. Then, this background can be changed to a different background. The difference matting works by removing the background before the moving subject even enters the scene. So, the user will be manipulating a static background.
The major issue with difference matting is that it will show all the subtle discrepancies between the two images, including differences in lighting and shadows. This makes the final image look rather noisy and clunky. But, this can be fixed up with some time and patience. The Difference Matte Key is meant to be just a temporary replacement for green screen matting. Users will not get as clean of an effect, but it will save them some money as opposed to having a professional agency do the work.
Step 1: Add the Video
The first part of using the Different Matte Key to replace the background is to import the video clip into as a Pal project. Next, change the video resolution to a 1:1 ratio, and set the Aspect Ratio, Pixel Aspect to 601/709, non-square. Next, set the Color Levels to 601/709, the File Field Order to Odd and the Alpha to Invert Existing. The Single Frame Import should be 60 seconds, and check the Autodetect Sequential Files. Now, click OK.
Step 2: Cut the Shot
It is time to start manipulating the frame so clip the shot that is needed onto the V1 (Video Layer 1). Next, freeze the first frame of the clip, and place this onto the V2 option within the first clip.
Step 3: Apply the Difference Matting
Depending on the type of effect that the user wishes to create, there are a number of Transfer/Blending/Apply Mode effects that can be downloaded and added to Avid. These will basically superimpose one image over the other. If users wish to use this effect, they should do it now, and then add the effect to V2.
From here, users should go to Mode and then Difference. The sequence can then be scrubbed so that it resembles a traditional green screen matte. Add the Color Correction option to V2, which removes much of the saturation and improves the contrast.
Step 4: Reduce the Layers
Add a third layer, and then compress V1 and V2. Move V3 to the top. Cut out the original shot, and add it into V2. Add the background to V1. Add the Difference Matte Key to V3, and select the Invert Key option located within the Effect Editor.
Now, add another layer (V4), and cut the source shot into this layer. Add the AniMatte to this layer, and draw a line around the area where the background needs to be changed. This should now change the background, but users may still need to tweak it.