Practical Character Physics For Animators

Review : "Practical Character Physics for Animators" Contribution: - Interactive system to enhance character motions, by incorporating physical properties into a existing key frame system. - Tools to enhance realism of balistic motions. - Investigate physical accuracy of manually created animation by skilled animators.

Result Evaluation:

1. Ballistic Paths : The path is created based upon the center of mass m and a constraint. The animator has 2 ways to incorporate this into the animation, either by adjusting the timing of the motion, freezing the initial and final positions or manualy adjust the path and then retime the entire motion.

2. Angular Momentum: An iterative approximation algorithm is suggested to compute the configuration (T_o) of the character given the momentum (h) specified by the user. THe method makes use of the rigid body inertia (J), velocity (v_i), the position (x)

Reproducibility:

The paper should be reproducible to a considerable extent. All the techniques used are backed up with corresponding pseudo code/step-by-step algorithm. The variables are clearly explained in both the cases of ballistic path and angular momentum. An appendix explaining the background on the equations governing the physical properties of an animation is also provided for assistance.

-- Main.sumanm - 17 Nov 2011

a) Contribution This paper contributes an extension of the traditional keyframe-based animation authoring interface that includes visualizations of physically correct motions. The goal is to provide tools and feedback to animators to allow them to produce more physically accurate motions, without removing any of the animator's control.

b) Evaluation The system is evaluated by having professional animators use the tool, and by comparing the physical accuracy of animations produced with and without use of the tool.

c) Reproducibility The tools described in the paper are certainly reproducible, as the algorithms used are given in sufficient detail to recreate them. The success of the tool in terms of usefulness to professional animators is not as certain to be reproducible, since not many details of the authors' specific evaluation methodologies are explained in the paper.

d) Improvements There are a number of spelling and grammar errors present in the paper that require correction in order to improve readability and understandability. Furthermore, the figures and tables are all clumped at the end rather than located near the relevant section of the paper's body, which also makes it more difficult to read and understand exactly what the research is accomplishing.

-- Main.cdoran - 17 Nov 2011

What is the contribution of the paper?

Traditionally, animators use keyframing and inverse kinematics to animate characters. This paper develops an interactive system that allows animators to create physics-based characters which makes the motion more physically plausible.

How are the results evaluated?

The results are evaluated by the physical realism of the animation.

Is the paper reproducible?

It looks to be reproducible, given the equation to compute ballistic paths, and those equations and the algorithm needed to develop the angular-momentum tool.

how could the paper research or paper writing be improved?

Section subsection headers can be more meaningful. For example, section two "Improving Physical Realism" and section three "Keyframe Animations' Physical Accuracy" look similar to each other and they don't seem to summarize the respective sections very well.

Main.shuoshen - 17 Nov 2011

• contribution: It was the first research that incorporated physical properties into an existing key frame-based system by the time the research was conducted. Tools were developed to help animators modify ballistic motions of characters and the physical accuracy of the high quality animations was investigated by measuring important physical properties.

• evaluation: Basically, the tools were tested by animators and the evaluation was based on the effectiveness of how much it helped the animators to create the physical realistic animation.

• reproducibility: I think the tool is reproducible since the basic idea is not hard to understand, and the math details are provided along with appendix. However some of the results seem to be less reproducible since it requires professional animators to create the ballistic motions, which could be uncertain.

• comments: Overall the paper is not hard to follow. However the structure of the paper is not clear when I first read it. I have to go back and forth to get clear about the organization. I also encountered a few issues. For example, the term “COM” was not explained, at the beginning I could only guess it is “center of mass” which could also be “center of momentum”. The paper also has a number of grammar or spelling issues such as “derived form”. But given the factor that this is a preprint version, the author may have corrected these kinds of issues in the final version.

Baoxuan Xu

Practical Character Physics For Animators (Ben Humberston comments)

Contribution

The authors present a group of tools that provides visual feedback about the physical accuracy of an animation during character animation authoring. They provide three specific tools:

• Ballistic trajectory: A simple visualization of a point-particle trajectory through the air; the user may modify the start point, end point, time in air, and initial velocity to achieve a desired path, then modify their existing keyframed animation to better fit the ballistic path

• Angular Momentum: The character's angular momentum during free flight is calculated, and the character's root orientation may be adjusted to preserve the calculated value.

• COP/COM: The projection of a character's 3D center of mass/pressure is shown on a flat 2D plane beneath the character along with the support polygon of the feet.

Additionally, the authors provide observations on the physical errors in animations created by professionals without the use of the paper's tools.

Evaluation

The authors only provide subjective evaluation of their system's usefulness for animators. They put strong emphasis on it's effectiveness as a standardizing tool among multiple animators in a studio, suggesting that the satisfaction experienced by solo animators when using the tool may be quite variable. Additionally, they provide no controlled evaluations of perceived animation quality that compares animations produced with and without their tools.

On the other hand, they do provide quantitative observations of physical errors made by animators without the use of the author's tools. These observations help separate instances where their tool is likely superfluous from those where it could significantly improve the physical accuracy of animations

Reproducibility

The authors provide extensive detail on the mathematical basis of their ballistic path and angular momentum tools. Though the algorithms are relatively simple, they do provide pseudocode in figures. However, because the paper concerns tools provided to human users, the ease-of-use and clarity of the interface shown to the user by the tools is perhaps more important than the technical details; the complete UI system may be difficult to reproduce without more details.

Improvements

• The evaluation of the tools' effectiveness for animators is fairly vague; quantitative results or more controlled testing may have yielded more definite results.

• The angular momentum tool is only capable of modifying the root orientation of the character to correct the value; a more sophisticated system with the option of modifying character joint angles as well may have been useful when a character's rotation speed is fixed

-- BenHumberston - 17 Nov 2011

Practical Character Physics for Animators - David Matheson

(a) Contribution: An interactive system that allows animators to combine their traditional kinematic approach with physical simulation. They focus on visualizing realistic ballistic trajectories and angular momentum. This allows animators to use the algorithms specified to adjust their animations to make them more physically correct.

(b) Evaluation: The results outline how physically realistic a set of animations are before and after using the tools specified in the paper. While they provide quantitative results they don't provide the number of animators/animations evaluated. This paper is evaluating the animators as much as the their purposed tools.

(c) Reproducibility: Yes the tools are reproducible but their claims of how animators use them are not as reproducible.

(d) Improvements in Research/Writing: Overal the paper is well written but it is more like a technical report than a research paper. They go through the math in good detail. In some cases other papers are referenced to explain terminology (such preparatory and recovery phases) when it could have been explained as part of this paper.

* Contribution: The new interface based on keyframe-based animation is pretty good for users to see the parameters and details more clearly. In this sense, user could understand the system very well and the result could be regard as reasonable.

* Result Evaluation: The evaluation include the effectiveness concern, which is not in detail numbers. By scaling the characters, the result are concerned by seeing the stableness and errors.

* Reproducibility: The mathematical processing has been discussed in the paper and the method in itself is pretty straightforward to understand. Thus I think it is reproducible.

* About the improvement: The writing before the result has no pictures to illustrate the work in a viewable way.

-- Main.Jingxian Li - 17 Nov 2011

* What is the contribution of the paper?

This paper describes an implementation of an interactive system that assists an animator in creating more physically plausible character motions as part of the animation process. This system allows an animator to view the motion of a character or object as if it were to obey the laws of physics. The main purpose of system is used to inform the animator of the changes that would need to be made in order to make the motion physically correct.

To summarize, the first contribution is that the incorporation of physical properties into an existing key frame-based system is a new contribution and does not currently exist in any commercial software package or studio animation system. Second, this paper designs tools to help animators modify ballistic motions of characters to enhance the physical realism, which is difficult to achieve using traditional key frame-based means. The last contribution of this paper is to investigate the physical accuracy of the high quality animations that are manually created by professional animators by measuring some important physical properties.

* How are the results evaluated?

First, this paper evaluates the effectiveness and impact. Here, the author uses the comparison between this one with other previous techniques for a set of examples. And then this paper proposes some scenarios of use in which case this algorithm will perform extremely well. And then the author presents how to scale the large and small characters with regard of the previous related method. And at last, the author also points out the limitation of this work.

* Is the paper reproducible?

It depends. For the data, it is obviously reproducible. And the formula used here is in the appendix. But the code is not reproducible because there is even no pseudo-code.

* How could the paper research or paper writing be improved?

The first thing is to fulfill the reproducibility. And this paper should also have more clear titles. I mean the title should be ordered by number, like 1.1, 2.2.1. And another suggestion is that this paper should introduce the outline of the sections before stepping into the main part of algorithm.

-- Main.chuanzhu - 18 Nov 2011

The paper introduces an augmentation to the animator's keyframing toolset by allowing in-place visualization and alignment of body mass trajectories w.r.t. velocity and angular momentum. The augmentation assists in the creating and adjusting of ballistic and ambulatory motion to coincide with proper physics (to a desired degree) without restricting the animator's ability to customize the motion as normal and violate the recommendations of this system for required effects.

The paper was unable to evaluate its method by comparison with existing systems since the technique is novel, but it has also not produced comparisons with real or simulated models to evaluate the possible deviations from the simplified motion models that the artist may have to compensate by manual adjustment. However, the paper did provide a gauge of deviations to the model from artists' manually keyframed animations to promote usefulness of the system.

The system described in the paper appears sufficiently clear and detailed to produce an implementation for reproducibility and verification, but the proposed usefulness for artists has yet to receive empirical studies in the paper and appears to be a projection thus far. Measuring the effectiveness in a more realistic or production environment would be more useful to promote the adoption of the system. -- KevinWoo

(a) What is the contribution of the paper?
The paper describes graphical software tool for animators, which aids in creation of accruate motion invovling ballistic paths, balance and posture. This software helps, but not obliges, to create more realistic motions as well as it can also be used to improve existing keyframed animation by physical realism. The paper also claims that the incorporation of physical properties into existing key frame-based system is a new contribution which is not implemented in any software yet.

(b) How are the results evaluated?
The results are evaluated by comparing of existing keyframe anymations created by professional animators with motions generated by the software for both ballistic and ambulatory motions.

(c) Is the paper reproducible?
Yes, it is. The paper provides very detailed explanation of what the software does. The detalization of the paper even contains mathematical formulas and algorithm listings. Besides, the scince under the hood of the software is not that difficult to reproduce.

(d) How could the paper research or paper writing be improved?
The paper is straightforward and easy to follow. However, the main purpose of the software is repeated several times in the paper, by which author presumably tries to emphasize it. So I find it somehow unneccessary which can reduce the volume of the paper.

-- Main.khamza - 18 Nov 2011