If we converted a 3D movie to watch on 3D TV and we decided to watch in 2D TV, we need to convert it back from 3D to 2D.
We need a fast computer like i5 or i7 to run efficiently, it would take 90+ minutes to convert 1080p 3D to 2D. All we need is a software called AVIDEMUX.
Download execute and downloaded all the software needed, the avidemux will download and extract them for us automatically, remember to check the avisynth (default was unchecked). When done, just launch the program and open the 3D mkv file we want to convert to 2D. If we encoded the 3D with horizontal (side-by-side), the crop will be different from 3D vertical (top-bottom) but the same logic.
Watch this video
3D mkv frame has 2 images, the goal is to crop and resize to 1 image with the frame we want. This sample 3D mkv has frames of 1920×1080 and horizontal display (side-by-side), which means we have to crop horizontally 1/2 = 1920/2 = 960 either side, the logic is keep the left and crop the right. So if you have 1280×720 horizontal, you will have to crop horizontally 1280/2 = 640. So we need to select video output section, select mpeg4-AVC(x264), select filters for crop and resize.
We will have an image of 960×1080 in the preview, we need to resize it back to 1920×1080 or a new frame size 1280×720 as in the video.
Now select audio output and select AC3 (5.1) (smaller size than DTS), choose the bitrate and click save movie to output the new 2D MKV video. This was done on i5 2500k running at OC 4200Mhz and it took more than 90 minutes, if you have AMD or i3, think about 3 hours or more.