How To: 3D Tracking Concepts

This page is meant as a helpful start for those interested in illustrating the concepts of tracking using 3D modeling.

Step 1: The Computer
Step 2: Learning Blender 3D
Step 3: Plan Before You Illustrate
Step 4: Make Stuff Move
Step 5: Foot Meets Sand … “Collisions”

This is 3-D Modeling …

Construction Simplified

How the mind learns …

Sometimes it does best when it figures it out for itself, and/or when playing.  Playing?  Yup, it’s one of the most engaging, fundamental, and deepest reaching forms of creativity.

For this form of illustration to be adopted for tracking it must provide value to student and instructor … To have value to the tracking world these illustrations must be accurate enough to generate confidence in their similarity to real world processes. They must also work to the best advantage of students, while providing something useful to the instructor which they cannot get elsewhere or more easily.

How the mind learns is an important consideration in creating materials offering the most usefulness.  It would be no surprise if the most used clips demonstrate a concept while also leaving significant bits less obvious and meant for the student to figure out. This can include anything up to and beyond the whole point.  One approach towards this less obvious form could be to illustrate the concept, then remove or alter bits.

Definition: “Zen koan” … a paradoxical anecdote or riddle, used in Zen Buddhism to demonstrate the inadequacy of logical reasoning and to provoke enlightenment. (definition from Google)

Illustrations something like a visual Zen koan, or along the lines of “Coyote Mentoring” might be profoundly helpful.

“Coyote Mentoring” is?

“… a distinctive and highly effective educational approach developed by Jon Young and Wilderness Awareness School over the past 25 years …  This approach develops the capacities of students for learning, problem solving, and full expression, and re-awakens their natural sense of wonder.”


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How This Goal is Being Accomplished:

By building 3D animated illustrations of a translucent foot creating a track.  Physics controls the interactions of the substrate with the foot surface.  These illustrations allow the viewer to watch the process of a track forming in slow motion, without the foot obscuring the internal track details.

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A Place to Start and General Info:

A computer …

3D modeling software …

The capability to simulate real-world physics is needed to create these illustrations. Blender 3D was chosen as the initial software.

For learning Blender a maxed-out graphics computer is not necessary, so if you already have access to a computer that will handle Blender start there.

Blender is open source software and free … A most capable and powerful 3D modeling platform … Well supported … Improved and updated regularly … And has a world-wide community of supportive users.  Pretty amazing actually, especially for free!In the video clips you will be seeing simulations created with Blender using three-dimensional graphical objects. Each ‘sand grain’ a separate object, as are the foot and ‘ground’.

Real in What Ways?

Realistic appearance is wonderful, but to have any practical use the behavior of all objects in these simulations is paramount, they must behave exactly as real world objects do, or so closely that the trained eye can’t tell the difference.

Ultimately the objects must exactly simulate the creation of a track, in all essential details, and appear to as well.  Conformation to this standard is one of the primary goals.  Reaching that goal is seen as a long-term process of refinement, and what you will see here is only the beginning.

In Blender behavior of objects is controlled in two ways:

  – Blender’s animation system can control the movement of any object.

  – Blender’s Rigid Body physics system can control the movements of objects before, during, and after collisions.

This arrangement means animation controls the movement of the foot, while physics controls the creation of the track (the interactions between foot surface, ‘ground’, and ‘sand’).

 In other words … Realistic behavior of both sand and foot can be achieved.

At present, the apparent realism of these clips and the amount of ‘sand’ is limited by the processing power of the computer used, the skill and knowledge of the creator, and to some degree by Blender’s physics system. These clips just begin to express the power of 3D simulation to generate accurate and realistic imagery with physics applied.

How Much Detail and How Accurate Do Simulations Need to Be?

Trackers are detail oriented (a world record understatement?). The more accurate and detailed simulations are the better.

To what level of detail?  The slightest change in shading the eyes can detect?  The line made by a single thin hair as it falls into soft dust?  A slight ridge, no taller than the cross-section of the hair that fell beside it?  Mouse walked across the newly exposed mud … Those holes and ridges left by mouse’s claws?

Accuracy of Blender Software …

Blender is very accurate with room for improvement.

Although Blender was originally intended to be a precise simulation system in terms of how real it appears, each new update is nudging it towards greater precision in how well it simulates the physics of reality.

At present there are a few ways Blender does not yet offer the highest degree of precision.   However, Blender is precise enough for this work, is free, and offers several different systems for simulating the behavior of real world objects with gravity and forces applied.

The primary simulation systems offered by Blender are: Cloth, Fluids, Particles, Rigid Body, Smoke, and Soft Body. There are also “Force Fields” which include charge, magnetism, wind, and others.

An example of imprecision in Blender can be found in the interaction between the animation system and the rigid body simulation, and this is worth knowing more about.

One way to think about what’s needed for the simulation of a track being made is to consider the limits involved in the real world:

The mind and body move the foot … within certain limits.

The physics of the substrate (sand in this case), underlying earth, gravity,  and foot surface … another set of limits.

As force is exerted by the foot against the sand,  the sand will move (to some degree) in reaction … Once that physical process has reached its limits the foot will go no lower, or further back, and all the force is then propelling the body.

Unlike trying to walk in seriously deep snow, it is usually the sand and underlying materials that finally stop the movement of the foot, not the body!  Moving the body depends upon this.

When movement of the foot is controlled by the animation system, and one needs to simulate the sand stopping the foot, a work-around must be used where the animation system is what stops the foot … But in the real world it is the physical limits  of the compacted sand and underlaying earth that stop the foot.  This work around functions sufficiently, but is not a true reflection of real world processes.

Short version? While one can tell Blender’s Rigid Body Physics system to notice and respond to the foot, one cannot, yet, tell Blender’s animation system to notice and respond to the physics of the sand.

How Could Blender Be Improved For This Work?

An example:

Currently … flexure of the foot surface(skin) is controlled by the armature (bones) or the animation system with ‘shape keys’…

(think of shape keys as a copy of the object in its final form, that the animation system will gradually change the original into)…

The heel lands, the foot rolls forward onto a pebble.  The skin in and around that area of contact deforms, dimpling inwards to shape itself around the pebble, because you edited in that deformation.

     … “Hmm, that looks right.”

Better would be …flexure of the foot surface(skin) is controlled by the armature (bones) or the animation system with ‘shape keys’, and the physics system

The heel lands, the foot rolls forward onto a pebble.  The skin in and around that area of contact deforms, dimpling inwards to shape itself around the pebble, because the Animation and Rigid Body Physics systems noticed the contact between pebble and skin, then began deforming the skin appropriately as that contact continued.

    … “Ouch!” you think, “that must have hurt!”

Continuing This Work…

It is suggested that other 3D modeling applications be explored to decide whether one can be more useful for this work than Blender.

   Useful things to look for:

What are the upper and lower limits for the number of physics-enabled objects the software can manage? Will it handle 500,000 Rigid Body sand objects? A million?

Can it simulate soil or clay?

A sand grain weighs a lot less than a foot … how well does the software’s physics engine do with large differences in weight or mass?

When a foot makes a step … what are the parts of that process, and which need to be simulated? As an example; when a foot pushes off from the ground the ball of the foot is pressing backwards and down against the sand … will the software allow the animation system (controlling the movement of the foot) to talk to the physics system (controlling the sand’s interactions with itself, and the foot surface) so the force created by the pressure of the foot can be transferred to the sand, and the animation of the foot will notice and be effected by the resistance of the sand?

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The Computer

Giant Massive Big Computer

If you aren’t familiar with Blender or other 3D modeling software you might benefit by considering the following issues before you jump in. They offer some insights as to why the computer you’ll be using is an important consideration.

Three dimensional modeling is the use of computer graphics to create moveable objects with more than one dimension by using:

points in space (vertices) …
connected by lines (edges) …
with the gaps filled in by planes (faces), think surface here …
the faces colored and textured however you wish …
and then doing stuff with them.

You have the option to make any of the objects move with animation, and can use physics to control some movements and collisions.  You can also add sound and produce video for almost any platform.

All of this is managed by the hardware and software of the computer you are using.  While creating and running one of these  simulations the computer is doing calculations for the movements of thousands or hundreds of thousands of objects, each with physics applied, and all potentially interacting … For each frame in the clip!

Oh yeah, and the computer is also managing all of those calculations for all the points (the vertices) that make up each object  … For as many objects as are moving around and colliding!

So if you have 10,000 objects colliding, and each is made of 10 points (vertices), and each object collides with some other object 10 times, that’s 100,000 points, and 1,000,000 collisions the computer has to think about.

Put simply, even with all possible corners cut, this is an astounding amount of work!

Fortunately, nowadays many off-the-shelf computers are up to the task and for learning Blender a maxed-out graphics computer is not necessary, so if you already have access to a computer that will handle Blender start there.

However,  some computer configurations are significantly more graphics-capable than others, and with large numbers of objects (more than ~10,000) a graphics-capable computer makes using physics as part of illustrating earth and feet interacting go much better and allows much finer detail.

If you decide you want to move further with this work … For example say you want  to create enough realistically sized soil or sand objects (with physics applied) to hold an entire track,  you will need to have, or have access to an appropriate computer … The more graphics-capable the better … Type of graphics card does really matter … Get LOTS of memory … And a bit more!

There are ‘physics cards’ available … worth checking out?

Cooling is another issue to consider … Your computer’s memory will generate heat, and must be appropriately cooled.

Investigate, network, and search out what kind and configuration of computer you need … If you think you’ll continue doing this … get the best you can afford, or build your own.

Places to start searching/networking for computer type and configuration (be prepared to take some time with this) / “About” / “Software” /click on “supported platforms” (it’s in one of the paragraphs) … Or just Google “blender 3d supported platforms”

Google something like “Blender 3D best computer”

Join and ask…

Backup and Storage …

As you get into this work you’ll be creating lots of large files, some will be worth preserving … consider backing up or storing them on a separate drive, or the cloud (and keep in mind … cloud use relies on connectivity).

You might want to keep only the most used or current Blender files on your PC’s hard drive and the rest somewhere else … Yes you can get massive hard drives … And then your operating system has to keep that big drive organized.

Updates: …

Blender is revised and updated at least once a year (author using ver 3.78).   Some users prefer certain versions for specific purposes.  When a new version of Blender becomes available, it is useful to visit the Blender website and read the “What’s New” info.

Occasionally it helps to use an older version, you can keep older versions installed.

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Learning Blender (skip if you already know it) …

Beginning Steps You Might Take:

Start w/ a tutorial or small simple project … Blender has two rendering engines, “Blender Render” and “Cycles Render Engine”.  Start with Cycles … Blender Render will eventually be obsolete.

A well done set of tutorials is: Blender 3D Tutorials by Neal Hirsig

Another well done set: Modeling an Off-Road Wheel, By Zoltan Giber (Not bad! Not too bad at all!).

Get help when you need it.

Use a search engine to find more tutorials, if they used a relatively recent version of Blender all settings and methods should work just fine.

Play with it … What sounds like an interesting and fun thing to create?

Break problems down to bite-size chunks.


As a bench mark: the author had many years work experience illustrating scientific papers for PhD theses and publication in journals, very little training in 3D graphics or simulation, and none in Blender.  From a cold start it took about a month to learn enough to produce simple video clips … 2-3 months to start playing with DC 3’s …

Play w/DC 3's (lrg)

And about 6 months to learn enough to create the first sign of a successful track…

Useful Tip: When using Blender it sometimes happens that something you are working on doesn’t behave as you expect.  Usually this is NOT a bug and is operator error because Blender is so complex.  You most likely are not the first to hit this particular wall … A search with Google or within the Blender community can work wonders.

If you think you have found a genuine bug the Blender developers are quick to listen and resolve, so report it and include a copy of your ‘.blend’ file reduced to the absolute minimum to reproduce the problem.

Suggestions, comments and corrections gladly accepted!

Oh yeah, I sorta forgot to mention …

Giant Massive Big Computer

This is also a fair representation of the Blender controls … Not to worry, many you’ll never need and a few do most of what’s needed for this work.

Help w/ Blender …

Google Website Website Website

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Plan Before You Illustrate … (And include changing your mind as you go)


    How does one find ideas for concepts of tracking to illustrate?

Well, there is any nearby surface: the ground, floors, tree trunks, car hoods, kitchen floor … Oh yes, under water also … The author once found an interesting trail along the bottom of a clear-water Florida pond while snorkeling … A groove between tracks … Deep groove … Large tracks … He followed it till he realized what he was tracking!  (And they said those animals stayed out of clear water ’cause it’s too cold!  Hah!)

Ever try ‘feel-tracking’?  It works.

Go for a walk … especially on a lightly traveled dirt road.

Occasionally some aspect of a track, or of reading a track story in the earth will catch the attention.

An image flashing across the mind.

By looking for something in the literature.

What do you want to illustrate?

What concept of tracking do you find hardest to understand?

How can you best illustrate that?

  Who are you illustrating it for, and what do they need to see?

  How can you make it clearest and most obvious to yourself and to them?

The illustration may seem perfectly clear to you but do you need to make it a little clearer for them?

Timing, Length of Clips, And Slow Motion …

A foot stepping and the track forming in slow motion are easier to observe … And can have a certain elegance.

Roughly how many seconds do you want the scene to play?

Blender does animation using sequential frames (think picture) where a footstep is captured as one tiny bit of movement per frame.

24 frames per second (fps) is a minimum recommended frame rate.  At that rate a pleasantly paced slow-motion footstep takes something like 860 frames (860 images) … 860 frames /24 fps = 35.8 seconds.  This includes a few seconds for comprehension before the track forms, and after the foot leaves.

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Making Stuff Move

The Step…

Relying foot…
Trustworthy earth…
Details remain…

How to simulate this?
So you’re about to build a foot and will want it to take a step. How are you planning to make that first step happen?

Animation is a complex subject … So discussion here is limited to the use of Blender 3D for this project and intended only as a brief overview to offer insights to those unfamiliar with 3D modeling, animation, and Blender.

Examples of animation are cartoons (ever watch the Road Runner & Wile E. Coyote?), movies, etc.

What makes animation work is ‘key framing’ …

‘key’ – adjective: “of paramount or crucial importance” (Source : Google definitions)

‘frame’ – think of a drawing of that foot at one instant in time during a step

The key frames…

This is a good description:
“A frame is a snapshot of the scene at one moment in time. An animation consists of displaying a succession of frames representing successive moments in time; if these are shown sufficiently quickly (at least 24 frames per second), the eye is fooled into seeing smooth movement, instead of a succession of still poses.

Computer animation works in a similar way, except here Blender is your lower-paid assistant. You go to crucial points in the timeline of your animation, position and pose your objects/characters appropriately, and tell Blender that this is a key frame for the relevant transformations (positioning/rotation/scaling) of those objects/characters. Then when you run the animation, Blender will interpolate the specified transformation parameters between key frames, giving you smooth motion over those intervals.”

There are at least three ways to make animated things move …


Key framing the movements of armatures (bones) with mesh objects (skin) attached.

Key framing the movements of mesh objects manually.

Physics combined with gravity

Blender uses a physics engine that provides collision detection, and simulates soft, and rigid body dynamics.

Forces – wind, magnetism, etc.

Problem 1: Making The Foot Move …

A Solution: Use an armature with a mesh foot object attached

To simulate a foot taking a step in Blender, one may begin by constructing and animating an armature (bones) that have a mesh (skin) foot object parented to it.  This way Blender controls the surface of the attached mesh foot so that it moves with the armature(bones) in a way that is visually similar to how the skin moves when connected to muscles, tendons, and bones.

Armature With Mesh Foot Attached
Armature With Mesh Foot Attached

And here is a foot, animated by an armature and set as a Rigid Body object.  The foot was made to look like the mesh it really is.

Another Solution:

A “Collision Proxy”: Create a duplicate of the foot mesh, subdivide it into smaller parts so it works better with the physics engine, parent all the parts to empty objects.  Instead of using an armature, animate the empties and parts of the foot as needed, and then assign the parts of the foot to be Rigid Body Objects.  To get deformations like the expansion of the foot when weight goes onto it, use shape keys.

Unresolved Problem In Blender:  Some illustrations may require greater and more natural control of the deformations of the foot surface.

For the skin to react exactly like the real thing, the surface of mesh objects attached to armatures must be controllable by both an armature and the physics system, or some other way is needed to allow them to deform as real skin would, in response to collisions with other objects … an example would be someone stepping on a small round stone.

One work-around is to use shape keys to control and animate deformation of mesh objects.

So, you have taken that first step, and the foot moves as you wish.  Umm, now exactly how does the track get made?  That topic is covered in the next update: “Step 5: Foot Meets Sand … Collisions”

Introduction To Blender Physics

Blender Rigid Body Physics

The Concepts of Tracking 

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Testing the Substrate … Sand (as an example)

The idea: Test various Blender Rigid Body settings and simulation systems with sand objects to find the settings that hold a track the way you are trying to simulate.

This particular example is aimed at creating some aspects of damp sand behavior. One indicator of success will be when the ‘sand’ holds the track wall after the ‘foot’ leaves.

Sand is like the weather: its behavior is constantly changing and moisture content has a lot to do with that. In the morning a clump of packed wet sand holds its shape while resting on your palm, and dried out by the end of the day it may trickle through your fingers.

This clip is set up so the camera can observe the track wall of 5 various physics settings used to control the sand objects.

One thing to notice: in the first part of the clip as the sand objects flow outwards before the ‘foot’ has stepped, the bottom-most group of sand objects is behaving much like dry sand.

After the ‘feet’ have lifted away, the top group of sand objects is reacting much like wet sand, in that it is holding the track wall.

The behavior of sand objects is critical to these simulations. None of these illustrations will work accurately, seem believable, or convey the correct information unless the sand objects behave realistically, and as described in the next section moisture content, surface tension and cohesiveness are important contributors to sand behavior.

Watch your own work … Does it convince you?

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Foot Meets Sand … Collisions

Discussion about collisions will be kept simple … And essentially they are, sorta … You make a choice … Your foot bangs against the world … The world responds … Something else happens.

Why this is so important…

Tracks are the results of collisions between foot surface and substrate (sand here).  So one aspect of simulating tracks on the computer is just simulating collisions.  Even though Blender does most of the work of managing collisions, a little understanding will save you time and help achieve better results.

What are collisions?

collision –  “3. Physics; the meeting of particles or of bodies in which each exerts a force upon the other, causing the exchange of energy or momentum.”   (Source:

An example; one part of walking is maintaining forward momentum.  When we walk we’re expending energy to push against the ground, and this gives us forward momentum.  To accomplish this we make our foot intentionally collide with the earth in a controlled manner.  This begins the track, and initiates a series of collisions that happen between the foot, sand particles, and among the neighboring sand particles.

How does one simulate a collision on a computer? …

Collision Boundaries.  You assign a boundary to each object that you will want to collide.  Imagine two dots, each surrounded by a circle, or sphere.  As far as Blender is concerned the collision between the two dots happens when they move and their enclosing outer shapes touch. Boundary-based collisions are one way Blender and the computer decide when objects need to move.

And here’s Collision Boundaries at work…

Which of those objects would you use to produce the most realistic sand behavior?  The dark green objects, with their tight collision boundaries maybe?  Because they will give the most precise collisions and look the most natural?

Why not leave out the extra complication of a boundary and just say the collision happens when the two objects themselves touch?

 Think about the letter “M” and a period “.” … How would you tell the computer when the “.” touches the “M”?  Which do you think might be easier to figure out collisions on?  How do you tell the computer where they are?  Is their location at their center?  The upper left corner?

What of you enclosed each one in a sphere and told the computer they touch when the spheres touch?  It turns out that most of the time it works to say they touch when simple enclosing outer shapes intersect (the Collision Boundary) and use their geometric centers as the location of the objects.

For accuracy in this work the sand objects need collision boundaries shaped pretty much exactly like each sand object (look closely at the green sand objects), the foot needs one that is nearly identical to any part of its surface contacting the sand objects, and the ground just needs a simple flat collision boundary matching its upper surface ( in this simplest case).

In Blender the Rigid Body system offers “Convex Hull” or “Mesh” collision boundaries that work best for these objects.

What goes on when a collision is happening?

Nature is always applying the rules of physics to everything.  To the colliding objects as the collision occurs, and to every collision that follows.  We’re doing the same thing.

A simpler version?  Gravity is always on, and when one object collides with another energy is transferred.

Is it really that simple? 

For dry-ish sand, ‘Yes’ with complications.  In other cases, ‘No’.  An example of ‘no’ are the more complex collisions of wet sand … What holds a sand castle together?


“2. Cohering or tending to cohere; well-integrated; unified: a cohesive organization.

3. Physics. of or relating to the molecular force within a body or substance acting to unite its parts.” (Source:

When simulating a foot making a track, there are three essential components, the foot, the substrate, the ground beneath the substrate, and physics (Sorry, that’s four … too much Monty Python).

Topsoil is the most common substrate and comes in many compositions. Sand was chosen as the first substrate to use for this project because it’s a good place to read tracks, can be easily simulated, and the Rigid Body simulation does a good job with one exception, cohesiveness.

Clear examples of cohesiveness are observed in damp sand (grab a handful, squeeze and then open your hand), the disc formed by the toe of a shoe, and a plate formed at the edge of the track wall by pressure against the wall.

Cohesiveness of sand is primarily the result of the surface tension of the water coating the surfaces of two or more sand grains and connecting them. Some aspects of track formation can be simulated to a degree by adjusting the friction and damping settings applied in Blender to the sand objects, but not cohesiveness.

To simulate cohesiveness the simulation system must address this connectivity between soil and/or sand particles in some way.

One solution explored was to switch to one of Blender’s other simulation systems. There is an add-on for Blender’s Particle simulation system that offers hope of a solution, the Molecular Script (by Jean-Francois Gallant).  This add-on directly addresses the cohesiveness issue, is very well done and was recently updated. It requires a more talented computer than the author currently has access to.

One other way to address this problem … Different software.  Houdini by SideFX was explored by contacting the developers and explaining the problem. They indicated Houdini will handle this issue. Unfortunately, after installation, the computer presently used took one bite of Houdini, and said, “That ain’t gonna happen …!”  Ending that discussion.

As this cohesiveness problem is mostly about computer processing power, and solving programming issues, it has already been addressed and resolved. Furthermore, it is likely that other solutions already exist and just need to be found, so this issue is no permanent barrier.

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2 thoughts on “How To: 3D Tracking Concepts

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