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With my trial and error learning, I managed to track a video in Blender 2.67 and make a cube (the one which is there by default) appear on the "ground" (which is a table).

But how can I make a hole in the ground?

Edit:

I used the two buttons shown below:

Blender motion tracking

The two functions seems to do a bunch of things to the scene, which seems to include setting the background and foreground layers, and adding a Ground object to the scene.

The problem is, what do I need to do to create a hole that appears to sink into the table?

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    $\begingroup$ Have you seen this tutorial? $\endgroup$
    – grc
    Commented Jun 2, 2013 at 4:25
  • $\begingroup$ seems to me like you could skip all the way to 39 minutes in that video: youtube.com/watch?v=CVPcT0dJmoY#t=2391s $\endgroup$
    – zeffii
    Commented Jun 2, 2013 at 7:12
  • $\begingroup$ @grc Just watched it and I think I get it. Still need someone to place a good answer for me to accept. $\endgroup$
    – Alvin Wong
    Commented Jun 2, 2013 at 8:06
  • $\begingroup$ @AlvinWong you are also free to make an answer to your own question if you feel you can explain it. $\endgroup$
    – zeffii
    Commented Jun 2, 2013 at 14:28
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    $\begingroup$ @AlvinWong the question is in danger of being closed, perhaps you might modify the question to how to mask / prevent specified geometry from being rendered. $\endgroup$
    – zeffii
    Commented Jun 2, 2013 at 20:12

3 Answers 3

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Firstly, you need to create the shape of the inside of the hole and make sure the object is on the foreground layer.

After that, you need to crop the generated Ground object to remove an area which matches the opening of the hole.

Here is the result (not realistic, just to show that it works):

Result

This is the way I did it. I guess this might not be the best way to do it, but it works.

I am not familiar with Blender terminology. If someone is willing to post a more detailed or better answer, I would be glad to accept it.

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  • $\begingroup$ presumably this method creates the necessary holdout material or renderlayers for you $\endgroup$
    – zeffii
    Commented Jun 3, 2013 at 7:58
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There are several options, depending on which render engine you use.

for Blender Internal:
For a quick and dirty solution you could use a large plane for the floor, with your sink-hole chopped out of it, and enable transparency on the material associated with the plane, Then enable Mask and set the Alpha to 0.0. The ground plane won't be visible any more in the render, but it will still occlude the geometry that you don't want to see.

for Cycles using Holdout:
Enable Transparent in Properties > Render > Film and give the occluding geometry a Holdout material.

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  • $\begingroup$ Why use a Color Key node? If you set the Cycles render to Transparent it will give you the right alpha already. $\endgroup$
    – brecht
    Commented Jun 2, 2013 at 21:09
  • $\begingroup$ @brecht, Oh! thanks, I'll fix that immediately. $\endgroup$
    – zeffii
    Commented Jun 2, 2013 at 21:37
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What I do the most for this, is to simply model the shape of the hole and put that geometry in a specific layer (alone). Then, composite the alpha channel of that render layer and the video with the "set alpha" node and you get a hole in your video. Then you can composite, the cube you modeled for the hole, and the video with an "alpha over" node.

Example

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