I really hate video tutorials. There are very few which are well made and concise. Most of them have a very low information density. Also with the video format one cannot just skim it like one can with a text. I much prefer text with pictures. Also I prefer papers over talks as I can access them with in the way that I like best.
This particular time I had a video where I wanted to obscure some regions. You know this effect from TV shows where they pixelate some face or company logo. The problem is that the object moves on the screen and therefore the pixelated area needs to track the object. This can either be done by hand or automatically.
Using my video editing program of choice, Kdenlive, I have tried this. Quickly I realized that I needed some tutorial. There is a bit in the official documentation about “auto mask”. There is not a lot of explanation, rather just a video. In that video the author shows how automatic tracking works. One basically just selects a region and let the thing do some work. It just did not work out for me so well.
Therefore I wanted something with keyframes, where I can manually adjust the box every few seconds. I let Google search for “kdenlive pixelate” and found only video tutorials. So I took a look into the first one, “Kdenlive Lesson 20 - Blur and Obscure”. The author spends the first minutes to talk about sub-clip selection, which I already knew and shouldn't be the topic. Then he shows how one can pixelate a fixed section of the screen, which does not help me. And only after spending four minutes I heard him say that there is a way to track stuff on screen, but he does not know and is going to look into at some future time. Five minutes wasted.
So I looked into the next one, Kdenlive Tutorial: How To Pixelate And Keyframe A Video Clip. It starts off really slow, with the author introducing himself, advertising all his other tutorials. I don't care for any of these things. He takes some clip, and adds the pixelation filter. But it covers the whole screen, not just part. And then he shows keyframes to change the amount of pixelation, using it to fade into pixelation. That uses keyframes and pixelation, but does not help me at all. Another few minutes wasted.
Then I tried the third one, “Kdenlive: How To Auto Track And Obscure A Moving Face Or Object”. This shows the “auto mask” feature, which I already tried. I only realized that after spending a few minutes into the video.
I then tried to use the “transform” filter with the fixed box pixelation, but I could not figure out how to make one filter apply to a previous one. Then I remembered a blog post from somebody that one just creates a black clip and uses the “transform” filter on that with keyframes.
It turns out that it is surprisingly easy but tedious:
Create a “color clip” which is just black. It will show up in the “project bin”.
Drag it down into an empty track. You might need to create one first by right-clicking on the very left of an existing layer and using the context menu.
Search for the “transform” filter and drag it down onto the clip. You may need to select the “composition track” to be the one with the actual video.
Make sure that you have “distort” selected, otherwise the black box would retain its aspect ratio and would not be flexible enough.
Use the “+” button to create new key frames where you want them. Click on keyframe markers to select them.
Then drag and scale the rectangle in the video screen such that it covers the area that you want. You might need to add more keyframes in the middle to compensate camera movement.
This explanation took just a page of text, one screenshot and I would find it much more informative than a five minute video. Unfortunately the video tutorials seem really popular for video editing or general computer stuff.