How to mask triangle out of the video strip in VSE? Im struggling with masking portions out of my video in VSE. I need to mask two triangles at each side of the video, because when positioned next to each other in 90 degrees angle (via transform layers), the corners will overlay each other and ulimately obstruct the view of the other video. 

So in another words I need to have 4 same videos but cut the corners of each. I think this can be solved by some masking but I did not find the answer yet.

Thanks, Mike

I need to mask red parts, but how?
Use create a Mask (in an UV/Image Editor set to Mask Mode) covering the content you want to keep. You can Ctrl+Click to add Points to create a Mask and Alt+C to Toggle Cyclic so that the first and last point are connected. Use V to Set Handle Types if needed.
Set your Transform Effect Strips Blend type Properties (N) each to Alpha Over. Oh and you'd better mute (H) your underlying Video Strips when you use an Effect Strip on top to reduce unwanted side effects.
Use that Mask going to the Modifiers Properties of your Video Strip in the VSE, click Add Strip Modifier, set it to Mask instead of Strip (Type of input data) and select your Mask created in Step 1.