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Example of red bread crumbs being dropped behind an object in a video.Suppose I have a movie showing an object (such as a bird) moving around. I would like to use the video editor to add a line behind the object showing the trajectory. Does anyone have suggestions on how to accomplish this? I've tried using the UV/Image editor to add a dynamically changing mask but am unable to figure out how to apply a color to this dynamic mask. Perhaps this is the wrong approach? Any help is appreciated, thank you!

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  • $\begingroup$ docs.blender.org/manual/en/latest/editors/movie_clip_editor/… $\endgroup$ – batFINGER Dec 29 '18 at 9:19
  • $\begingroup$ Thanks for pointing me in the right track. However when I research motion tracking, most applications I've seen seem to use motion to overlay an object that moves with the tracked object. Is there a way to render the tracked trajectory in the output video? I'm trying to create a trail of breadcrumbs that are dropped behind a moving object in a video. $\endgroup$ – clum Dec 30 '18 at 18:11
  • $\begingroup$ I edited the original question to show an image showing what I'm trying to accomplish. $\endgroup$ – clum Dec 30 '18 at 18:17
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Combining colored elements is easiest in the compositor.

I would also suggest using 3D to create the colored line as Blender has good visual tools in 3D. Just use a Shadeless or emitting material to make it monochromatic.

Then mask out the part of the bird, which shouldn't be covered by the line.

enter image description here

Render it with transparency and use a mix node in the compositor. Check the invert button in the mask options (in the image editor) to invert the mask.

enter image description here

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  • $\begingroup$ Wow, this is really cool, thanks for sharing. I clarified my original question to better show what I'm trying to achieve. Can your technique be used to create a bread crumb trail that is left behind the object as it moves through the video? $\endgroup$ – clum Dec 30 '18 at 18:19
  • $\begingroup$ @clum I don't see how the answer isn't applicable anymore. You create the "trail" in the 3D editor. You create a mask covering the whole image (instead of the wing in my example). You animate the mask so that it gives view to the path of the bird. At the start the 3D Render Layer will be fully masked and gradually revealed. $\endgroup$ – Leander Dec 30 '18 at 20:02

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