How to leave a trace

 (by Jean-Sebastien Perron)

Watch the video

Brief overview

-Create your scene.

-Animate the car so the tires goes a little under the ground (we need to see them from below).

-Create and set a "creator" that is gonna leave copy of the tires for every frame.

-Render animation from an orthographic camera of the scene from below  (to get a movie of the tire from below the ground).

-Apply the "movie from below" as a texture to the ground object.

-Voila!

The scene

-Create the scene

-Animate the car so the tires goes a little bit under the ground plane.

-Save the scene, we will come back later to it (example : carride.r3d).

-Save the scene again but on a new name this time (example : carride_below_ground.r3d).

Rendering from below the ground

-In the "object properties" set the color of the ground to pure black

-Set the ground illumination to pure white

-Then set the tire color to pure black

-Delete any unnecessary stuff (We only need the ground and the tires)

-Create a camera perpendicular to the ground

-Position it so we see the ground from below

-Set the camera to an orthographic one (we don't want perpective in here)

-This should look like this

Concept

-We need to create a copy of the tires at every frame of the animation.

-By leaving a copy of the tire at every frame of the animation we get a trace that is left behind.

-You do this with a "creator"

-Fallow the hierarchy as on the picture below

-This is the setting for the creator

-Now we get a copy of the tires every frame

-But is this enought?

-If not, we can't set the creator to create more than one copy every frame

-To do this we need a bypass, a cheat

-We will increase the total frame of the animation instead (and rescale the animation also)

-This tutorial animation used : 15 fps, 115 images total.

-For the rendering of the trace : 230 images total to get more copy of the tires at a closer interval.

-Then you render the animation of the tires from below.

-When it's finished (a while later), you need to do the retiming of the "trace video' back to 115 images total.

-The retiming is not necessary, you can keyframe the video instead in the texture properties of the material later.

-This is the result you should get (click to see the video)

Back to the main scene with the video of the trace.

-Reload your main scene.

-Map planar the new "trace texture' to the ground

-Render your animation and that's it.

Other uses possible

-Use it to leave walking steps in the snow for example.

-Use it with a boat on water.

-Use the "trace texture" as a mask between 2 materials : The trace reveal the dirt material on the grass material.

-Use "trace texture" with displacement or bump mapping.

-If you want to stop the "creator" at a certain time set creation intervale to 1000 for the desired period.

-Use the "manual depth rendering" shown here on the tires to get a precise depth of the object penetration on the ground.