Using kdenlive to edit movies with ubuntu 23.04 - *.mkv (Matroska) file hide rectangular section in video stream

hello community
I have just started using kdenlive to edit movies.
with ubuntu 23.04 a *.mkv file with video stream
includes some textual data I wish to hide. all the data
is included in some static specific rectangular section
of view port (video image rectangle).

how can I define a region to hide with this private data.
I wish to publish the video but prevent the viewers from
seeing the private text data on the video.

how to reencode the *.mkv video so a static rectangular
section of the image will be hidden by smudged/black rectangle
section ?


version is :
Version 23.04.2
os: Ubuntu Mate 23.04 x64

The effect I would use is the Transform effect.
Scale the video to be larger than the monitor area, until it only has the part you want. That will automatically crop the parts you don’t want, you can center the video where you want, and there will be no black areas at the edges.

Use the Obscure effect.

If you need more control over the pixelization do this:

  1. Apply the Alpha Shapes (Mask) effect
  2. Adjust the Size, X/Y position and Transition Width to your needs
  3. Apply the Pixelization effect (to be found in the Stylize effect category)
  4. Adjust the Block Size values to your needs
  5. Apply the Mask Apply effect
1 Like

Thanks bruceraf
unfortunately I need the other way around, The small rectangle in the inter-part
of the video region must be hidden (smudged/covered with black) the surrounding
should remain without any change.

Is there a method to block rectangular “island” in the internal part of view port?
all the rest of the multimedia must pass untouched.

Thanks berndmj
This indeed solve the request I stated above. now after try and succeed
thanks to your recommendation,suddenly noticed I need to change size
and location of the obscured rectangle at specific time to different size
and positioning.
what is the practice to define time region from start time to end time
that the effect is applied, while it is not applied at the rest of the clip? additionally
there will be more than one effect on different times.

your reply was most helpfull

You can use keyframes in the Alpha Shape (Mask) effect to do that.

There can be more than one effect between the (Mask) effect and the Apply Mask effect. Now if you need different effects active (e.g. Obscure for a while, then Gaussian Blur, and then Greyscale) you need to cut the clip every time you want to make that change and then just replace the previous effect with the new one, UNLESS the effect (a) has keyframes AND (b) has settings where the effect does nothing.

Alpha Shapes Mask allows setting key frames interactively
on Kdenlive. position and corners of the rectangle are moving
as interpolated in x,y so user must define 2 key frames one -
after the other so the rectangle of previous section will not -
change/move between frames.

last frame of the section will be with same rectangle as the -
previous section. first frame of next section will be the new