Hi Hamish,
All you need to do is duplicate your text layer, make that text white, then control click on the layer and add image mask. Then click and drag your moving red rectangle layer onto the Image Mask in the layers palette. It will by default then become invisible, but you can make it visible again by clicking on the little square to the left of the Rectangle layer. Make sure your Rectangle layer is beneath the text and bingo.
Hope this helps
Cheers
Mark
Mac Pro
2 x 3 GHz Quad-Core Intel Xeon
2 GB 667 MHz RAM
Final Cut Studio 2
Motion Graphics | Compositing | Grading
Mac Pro
2 x 3 GHz Quad-Core Intel Xeon
2 GB 667 MHz RAM
Final Cut Studio 2
Motion Graphics | Compositing | Grading