Did you use either a raw AVI or one that uses the correct codec? These are the only two types of video files that you can use in the background.
It is also possible that your AVI file is too long/big and your video card can't handle it.
Yeah that really sucks.
There is a workaround if you are good with a video editor and that is to use a green screen as a background. Here are the steps if you want to know how to do it and want to try:
1. Create a solid bright green color bmp file and load this as the background. Note that if there is any green on the models that you are using this is not going to work. If they do have green (such as Miku's hair) then use a solid color that isn't featured on any of the models. Bright garish colors tend to work best.
If you don't know what the green screen technique is check on YouTube for tutorials. The principles that applies to real world applications work with MMD too.
2. Record your video either as a RAW AVI or using one of the UT codecs. These will be large files, but for raw renders, you want that level of quality.
3. I can't give you specific instructions as video editing software do vary, but you need one that offers chroma key editing functions. This is what allows you to drop out a color from one video to allow you to place another one in the background.
There is one good FREE video editor that can do this, but you'll have to find it on your own as I don't remember the name of it. But it's really popular so should not be hard to find. Also, there are good tutorials on how to use this software on YouTube specifically teaching you how to use the green screen technique.
Good luck with this and if you do go this route, please let me know how it worked out. Honestly, it should be a lot easier to do than trying to mess with MMD's lousy support for background avi. MMD is dated, if only it could have supported MPEG, we would not be needing this discussion.
I was banned from Vimeo because of my "Overall Konko" video even though I have offered to remove it immediately.
You can not post content from the TV, Internet or cinema in his own videos or pictures to include.
For the convert the video to InScreen Videos Is the "Prism Video Converter" from NCH software better.
I use the converter shown because it'll convert almost anything to anything else, but people should just choose the best converter that suits their needs. They're all pretty much the same.