(Avisynth) Frozen image when using Overlay or StackHorizontal

[ephienix]

Hi all,
I have been trying to combine two videos. I am using one as the base and trying to get another clip on top of it. When I preview with Windows Media Player from within AvsP, the image is stuck. I can click on various parts of the video and see the result but it is always a static image. I tried to save the result with AvsProxy and Avidemux but it has been calculating for the last 40 minutes. Am I missing something? Here's the script:

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video = AviSource ("F:\leisure\My Pictures\fontainebleau\processing\Targasonne\Chape au_Unnamed_1.avi", pixel_type = "YUY2",audio=false)
video1 = video.Trim(2880,4050)
video3 = video.Trim(7680,8730).Crop(0,0,-500,0)#.FadeIn(60).FadeOut(60)

test = Overlay(video1,video3,x=0)
return test
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Thanks for the input!

Update: Avidemux finally did the work but it took an hour. Is there a way to avoid the long encoding? The thing is, I need to preview the result a few times before saving the file, I can't wait each time.