MOTION CAPTUREIN LIFE SCIENCES

Mario Lamontagne

What is Mocap?

The creation of a 3D representation of a live performance.

Understanding Motion Capture for Computer Animation and Video Games by Alberto Menache.

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Applications

Entertainment Medicine Arts / Education Science / Engineering

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Entertainment: Live Action Films

Computer generated characters in live action films (e.g. Battle Droids and many others in Star Wars Prequels, Gullum in The Lord of the Rings, King Kong in King Kong)

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Entertainment: 3D computer animations

Characters in computer animated files (e.g. Polar Express, Monster House)

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Entertainment: Video Games

Video games by Electronic Arts, Gremlin, id, RARE, Square, Konami, Namco, and others, (e.g. Enemy Territory)

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Medicine

Medicine (e.g., gait analysis, rehabilitation) Sports medicine (e.g. injury prevention, performance analyses, performance enhancement)

Gait Analysis Service

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Arts / Education

Dance and theatrical performances Archiving (e.g., Marcel Marceau)

OSU/ACCAD

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Science / Engineering

Computer Science (e.g., human motion database, indexing, recognitions)

Engineering (e.g., Biped robot developments) Ergonomic product design Military (e.g., field exercises, virtual instructors, and role-playing games)

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Mocap animation

Motion capture animation is different from keyframe animation in terms of how motion is created.

Same principles apply to mocap animation & keyframe animation! A combination of motion capture animation and keyframe animation

is often used.

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Keyframe animation

A keyframe is a drawing of a key moment in an animated sequence, where the motion is at its extreme.

Inbetweens fill the gaps between keyframes. Every motion is created by animators.

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Advantages of mocap animation

Faster to create (only if an established production pipeline exists.) Secondary motions and all the subtle motions are captured -> more

realism. Physical interactions between performers and props can be

captured.

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Disadvantages of mocap animation

Cost. Manipulating mocap data is often difficult -> Re-capturing or key

framing a shot with bad data is often easier. Mapping mocap data of a performer to a character with a different

proportion often causes problems.

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Types of mocap equipment

Magnetic systems Mechanical systems Optical systems

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Magnetic systems

Utilize sensors placed on the body to measure the magnetic field generated by a transmitter source.

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Magnetic systems

Require no special lighting condition. Sensors are never occluded. X Require a metal-free environment.

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Mechanical systems

Exoskeleton with angle sensors.

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Mechanical systems

Measure joint angles (no marker ID problems). Sensors are never occluded. X Breakable! X Configuration of sensors is fixed. X Constrains on joints.

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Optical systems

The cameras are equipped with infrared LED's and filters. (Filters enhance the contrast of the image.)

The cameras see reflector markers.

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Optical systems

Higher sampling rate. Larger capture space. X Markers are sometimes occluded -> marker ID problems. X Provide only positional data -> joint angles need to be computed.

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Mocap System at HMBL

Vicon optical system – 10 high-speed MX 13 (up to 1000 fps) Capture volume 3 x 3 2 m.

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PREPARATION

CAPTURE SPACE

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PREPARATION

PLACEMENT OF CAMERAS BASED ON DLT PRINCIPLES• http://kwon3d.com/theory/dlt/dlt.html#dlt

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Direct Linear Transformation (DLT)

Degrees of motion (Freedom)• In 3D, How many degrees of freedom?

> In mechanics, the degree of freedom (DOF) of a mechanical system is the number of independent parameters that define its configuration.

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DLT

Calibration of imaging systems (2D)• To calibrate is to use at least 4 known spatially

defined points.• Scale the film plane to the field of view• x= S u and y = S v

> S is defined by the distance between thecontrol points.

• In 2D, the DLT is defined by the belowequations

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DLT

In 3D, more DoF to account for therefore the equations become more complex and we need more control points (min points are 6)

• x, y and z are digital coordinate of the control points (3D).

• L1…L11 are the DLT parameters relative to each camera.• See Kwon3D.com for more explanation on DLT• See Chen (1994) for the # of control points and accuracy of

the reconstruction.

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PREPARATION

BOUNDARY

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PREPARATION

Static calibration• This calculates the origin or centre of the capture

volume and determines the orientation of the 3D Workspace.

Dynamic calibration• This involves movement of a calibration wand

throughout the whole volume and allows the system to calculate the relative positions and orientations of the cameras. It also linearises the cameras.

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PREPARATION

Calibration procedureTo perform the calibration:• Place the L-Frame on the floor in the centre of the

capture volume• Select System | Live Monitors• Check that each camera is viewing only the four

markers on the L-Frame

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PREPARATION

How good is the calibration?Residual for each Camera

• A residual is a measure of the accuracy of a single camera1. They are larger the greater the distance of the camera from the capture volume and are measured in millimetres.

What are acceptable residual values?

• Residuals should be less than 0.1% of the distance from the camera to the centre of the capture volume. For example in a capture space of dimensions 10_10_3 meters, using MCams with 12.5mm lenses you would expect residuals in the range 1 to 4mm.

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PREPARATION

Waving the wandThere is no particular wand waving technique you

should follow. You should, however, use the suggestions below as a guideline.• Allow the cameras to view the wand in multiple

orientations, e.g., horizontal and vertical• Fill the capture volume completely• Medium pace of movement• Do not point the wand directly at any camera

Human Movement Biomechanics Lab

PREPARATION

What are markers?Markers are the spheres that reflect light from the

strobe back into the camera. Markers come in various shapes and sizes, for example:• 3mm hemispheres or 4mm spheres for facial and hand

capture • 9 or 14mm spheres for full body capture• 12 or 20mm soft markers for capture when performing

moves involving contact• 25, 35 and 50mm spheres for capturing movements in

very large volumes

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Preparation: Markerset

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DATABASE

ECLIPSE

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DATABASE

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