I'm not a filmmaker either. Just an avid fan of visual effects work, I read all the behind the scenes stuff I could get my hands on. Matte lines and matte boxes are very different things. A matte line happens when a matte doesn't fit the image properly, the line is the matte itself visible beyond the image edge. The "matte boxes" as some have called them are an artifact of "garbage mattes", so called because they remove light stands, model mounts and other unwanted "garbage" from the frame. If a model ship, say a TIE fighter is filmed with bluescreen, the matting process creates two mattes from it, the "hold out" matte which is the black silhouette of the TIE that allows it to be printed into the background. And the "cover matte" which is the reverse, a black frame with a clear TIE that is used to blacken the bluescreen. The lights and other "garbage" are still there, matted in along with the TIE. The rotoscope animation dept. would create garbage mattes to block it out. This is a black frame with a squarish clear space that surrounds the TIE. This matte usually has a very dense image, very black and opaque. The cover matte made from the bluecreen process isn't quite as dense, it's more of a dark gray by comparison. When the garbage matte is combined with the cover matte, the density difference creates a faint grayish "box" around the TIE in the completed composite.
When the negative of the composite image is printed, the contrast of the projection film stock reduces the grayish effect so it's not obvious on screen. But the video transfer was made from the LC stock, the negative was re-printed onto the new stock. Thus the gray box is more visible.
I hope this makes sense :lol
