Tuesday, 1 July 2008

19 Common CG Animation Pitfalls (and counting) ~ Causes and Solutions


Jeremy Cantor - Animation Supervisor - Sony Pictures Imageworks - May 4th, 2002

1) Fearing the technology

Cause: We fear what we don’t understand. Fearing a computer, or believing it is more powerful than it really is comes from not having a fundamental understanding of how a computer works.

Solution: Learn enough about the computer so that you understand its strengths and limitations. Remember that the computer is not a magical device. It can’t think. It just runs programs. It can’t do anything a person can’t do; it can just do things faster. It is only a tool. A very sophisticated pencil. Assuming the user has learned how to speak the computer’s language, the user is the master of the tool, not the other way around.

2) Motion is too robotic…linear.

Cause: Letting the computer simply write linear f-Curves that have no slow-in or slow-out.

Solution: Understand how f-Curves translate into motion. Learn & apply the fundamental principles of animation. Manually insert additional ease-in/ease-out keyframes when appropriate.

(My earliest computer animations were done with software that created linear keyframes and did not have an f-Curve editor. I could not tweak the slope of the curves to create ease-in & ease-out. I had to rely on the application of traditional animation principles and create additional keys to achieve such results.)

3) …or the opposite: Motion is too spliney…watery….”computery”

Cause: Allowing computer to do too much “unsupervised” work. CG software usually creates smooth f-Curves automatically when you set keyframes. “Watery” motion comes from just leaving f-Curves in their default spline shapes. This is why rubber is the easiest thing to animate in CG. Spline f-Curves result in rubbery motion by default.

Solution: Don’t trust the computer to make properly shaped f-Curves. Study how f-Curves translate into movement & manipulate their shapes/slopes accordingly.

4) Characters not displaying a proper sense of weight

Cause: Not understanding the basic principles of timing, slow-in/slow-out, squash/stretch, gravity, etc.

Solution: Don’t let f-Curves just remain in in their default shape after keyframes have been set. The slope of your curves translates to how the forces are acting upon your animated objects. If an object is falling, for instance, make sure your Y-translation curves are accelerating (gravity does not apply itself as a constant force, rather it causes objects to accelerate as they fall). Understand the differences in how characters of varying mass will move. It takes more energy to initiate, stop or reverse the motion of a heavy object or character than it does to do such to a light object or character. Think of the difference in the force it requires to set a bowling ball in motion as opposed to that which is required to initiate the motion of a balloon. And similarly, the force it takes to slow, stop or reverse the motion of such objects.

5) Characters seem off balance.

Cause: Not paying attention to the proper location of character’s center of gravity. Simple physics: A static object’s center of gravity must be directly above or below the point (or average of the points) of suspension, otherwise the object will fall. When a biped character lifts one leg, he must shift his center of gravity over the supporting foot in order to maintain balance. (Variations to this rule apply when the object is in motion, however).

Solution: Pay attention to general physics of center-of-gravity. Study posing and the concept of contraposto. Use yourself as a guide. Study the shifting location of your center of gravity when you transfer your weight from one foot to the other. When you walk. When you run. When you hang from your hands. Etc.

6) Isolated body part movement. Lack of overlap.

Cause: Because it is so easy to animate individual body parts separately in CG, there is a tendency to create movement where separate body parts don’t seem to be working together, or where one part comes to a complete stop before another part begins moving (no overlap). Such inorganic motion also, of course, results from not learning and understanding basic animation principles.

Solution: Study and understand the fundamental principles of animation. Don’t allow all of your keys to remain lined up on the same frames (unless there is a deliberate reason to do so). Work locally but think globally. Always remember that even when you’re focusing on a single limb, it is connected to the rest of the body and all of the parts need to work together, not individually.

7) Twinning (unnatural motion symmetry)

Cause: Twinning is when opposite body parts move as exact mirrors of one another. When the left arm motion starts and stops on exactly the same frames as the right arm. This is usually not desired for natural looking animation (although there are certainly times when it is appropriate) This happens when the animator gets lazy and animates multiple body parts simultaneously, or simply copies/mirrors motion from one limb to another & then leaving the resulting twinned motion as is.

Solution: To avoid twinning, after simultaneously animating multiple body parts or copying/mirroring motion, be sure to go in and add keyframe offsets or other naturalistic variations to the movement.

8) Repetitive or metronomic movement

Cause: Relying too much on the computer’s ability to copy and paste motion. Leaving cycles as is.

Solution: As always, remember that you control the computer, not the other way around. Don’t just blindly copy or cycle movement. Each step in a walk will often be (at least) slightly different from the one before it. Add some naturalistic variation and imperfections (unless of course, repetitive, robotic motion is the desired effect).

9) Squash/Stretch used on inappropriate objects (ie Bowling Balls)

Cause: Learning but not truly understanding the fundamental principles of animation.

Solution: Apply an artistic eye and understand when it is appropriate to apply squash & stretch & when it is not. It is certainly okay to add squash & stretch to a bowling ball, but only if doing so is the result of an aesthetic choice to deliberately bend the rules. It is not acceptable to do such if it is the result of simply applying the fundamental animation principles blindly. It is not enough to simply memorize the principles of animation. You must truly understand them so you can apply them appropriately (or deliberately ignore them if the animation at hand calls for such disobedience in order to most effectively tell your story.)

10) Volume changing when Sqashing/Stretching

Cause: Squashing & stretching an object in CG is a 2 step process. You must scale the object in one axis then oppositely scale it appropriately in the other axes. Neglecting this second step causes the object to appear to shrink when squashing & grow when stretching. Volume changing during squash/stretch is also the result of not truly learning & understanding this fundamental animation principle.

Solution: Learn & understand this principle & don’t forget the second step of scaling in the other axes.

11) Linear wrist/ankle movement (the “marionette look”)

Cause: A wrist does not move from here to there via translation of the wrist itself, rather, such movement is the result of elbow & shoulder (and clavicle…and back…etc) rotations. Therefore the resulting trajectory of a wrist will tend to follow an arc. A wrist can certainly move in a straight line, but that requires simultaneous compensatory adjustments in the shoulder & elbow joint. Such linear movement does occur in such instances as when throwing a straight punch, but the natural tendency is an arc. When animating limbs with IK, the resulting motion often looks like the character is a marionette with its wrists on puppet strings. This is the result of simply animating the trajectory of the IK handles in straight lines.

Solution: One solution is to animate your character’s limbs with FK, which will result in arc motion by default. However, it is often desirable to use IK, so, when doing so, remember to (usually) make the motion an arc. Simply setting an initial translation keyframe at point A then a destination key at point B will result in a linear trajectory and a “marionette” look. Intermediate keyframes are often required to create an arc trajectory. Linear trajectory is okay, assuming that is the intended result. Just remember that such motion is not the normal tendency of a jointed character.

12) Frozen holds

Cause: In traditional cel animation, it if often desirable to completely freeze a character’s motion for dramatic effect. CG animators can sometimes forget that this is one of the few traditional techniques that does not always translate successfully into 3D. Because of the additional dimensionality, the ultra perfect perspective, texture mapping & super-accurate shadow casting (etc) displayed in a 3D CG scene, the viewer tends to have a “higher” expectation of reality. And since very few real-live characters ever actually freeze completely, when a 3D character does so, it can look unnatural and the action of the scene can die completely.

Solution: Use “moving holds” instead, where your character maintains a small degree of motion. Just enough so that the scene doesn’t entirely stop dead, but not too much or the pose will no longer be a “hold”. Perhaps he continues moving ever-so-slightly along his previous trajectory. Perhaps he takes a breath or scratches his ribs. Some animators will put their character’s central pivot point on a very small figure-eight path, so that he will sway just a little bit.

13) Character motion starts & stops exactly in synch with camera cuts

Cause: When an animated scene is made up of several shots, the simplest screen direction for each complete action (or group of actions) to be perfectly book-ended by a camera cut. This creates a scene that looks as if a director had yelled “action” at the very beginning of each shot (just after the camera had started rolling) and then “stop” just before the end of each shot. This is rarely considered good screen direction, as the camera does not appear to be operated by a human being, rather it has the unnaturally cold, perfect & predictable feel of a computer that always miraculously knows exactly when to cut.

Solution: It is usually more visually appealing when the illusion of a human camera operator is created. A human camera operator will suffer from human error. He will invariably end up following just behind the action on occasion, or sometimes actually anticipating it. Overshooting will occur once in a while. Etc. Maintain some degree of overlap between your animation and the camera cuts. Addition of such real world “imperfections” can help to make your scene feel more natural.

14) Arbitrary poses & motions

Cause: The difference between “animating” a character and simply “moving” a character is that “animation” implies life…and character…and purpose. Because of the power of the computer, it’s very easy and often tempting to simply add more & more to your scene simply because you can.

Solution: Remember that every motion of a thinking character must have a purpose. Movement for movement’s sake doesn’t communicate anything & only contributes to unnecessarily increasing the length of your performance and reducing the clarity of the story being told. Ask yourself what is the reason for each pose & motion in your performance. More is not always better. Most often, “elegant simplicity” is the key to telling your story most effectively. The well known acronym KISS means: “Keep it simple, stupid!”

15) Geometry intersections

Cause: Not paying enough attention to the details of your scene. Since CG involves working with virtual objects that are intangible, there is only visual feedback to inform the user when objects intersect one another. Therefore it is sometimes easy to overlook such errors.

Solution: Pay attention.

16) Relying too much on automated processes

Cause: Expecting the computer to do too much of the work for you. With expressions, constraint systems and various other software “bells & whistles”, it is possible to create a variety of automated motions in your characters, such as automatic lagging ponytail bounces that occur whenever the head moves. Such processes usually look automated. Too perfect. Unnatural.

Solution: You should do the work, not the computer. There are certainly occasions where automated processes are effective time savers, but it is very important to implement controls into your character setups that allow you to override or completely disable such processes. This way, even though certain things are happening automatically, you still have ultimate control over them.

17) Unnatural facial animation – not enough shapes, not animating enough parts of the face

Cause: Using too few morph target shapes and animating too few parts of the face. Unless an extremely simplified style is the desired effect, facial animation usually requires a good number of target shapes and plenty of detail in order to effectively create a natural feel to the motion.

Solution: Make enough morph target shapes. Don’t just animate the mouth and the eyebrows. Add appropriate motion to cheeks, eyes, forehead & even ears. Add some corresponding head movement. Apply squash and stretch.

18) Too much camera movement.

Cause: Disregarding the notion that just because you can do something, doesn’t necessarily mean you should. CG software gives you complete control over the motion of your camera. You can add all sorts of crazy camera motion that is extremely difficult or downright impossible in the real world. Because of this power, there is often a temptation to overdo it. Too much camera motion can confuse the action and distract the viewer, and in extreme cases, cause dizziness and queasiness. Sometimes it is certainly appropriate for the camera’s motion to have “character” but it shouldn’t steal the action from the scene (unless the camera is being used as the primary storyteller of that particular shot, like in situations where we are “looking through a character’s eyes”. But such staging should be used sparingly).

Solution: Keep cameral motion to a minimum. Study films and notice that cameras usually don’t move all that much. Sometimes big, sweeping camera motions are appropriate. However, just make sure that you are adding such exaggerated camera motion to help tell the story, and not simply because you can.

19) Motion blur turned up too high

Cause: The motion blur button in your CG software package is a fun toy and, much like the ability to animate your camera, there is an initial tendency to play with it too much.

Solution: Remember, motion blur is an effect that is more sensed than seen. With most motions, you can only see the blurring when you freeze-frame. Watch live action films & you’ll notice that you can only really see blurring when there is extremely fast motion happening. If you can see the motion blur during the normal motions of your characters, it is turned up too high.

The bottom line

a) Study and truly understand the fundamental rules of animation before you start breaking them.

b) Don’t rely on the computer to do too much of the work for you. Remember that the computer is just a tool. You are the artist.
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