all MPEGs available
Question 1a)
Question 1b)
Question 3: extension to system
Insights gained
Other extensions (minor)
Source

links to all MPEGs available

• 2 pyramids, base to base
• 2 pyramids, base to base, slow-motion beginning
• ball dropped onto a flat surface
• ball rolling
• ball rolling towards observer
• ball bouncing in slow motion
• a cube rolling down the slope
• 2 (10point) cones, base to base
• a 10x10 sail
• 8x8 flag, attached at top two corners
• 8x8 flag, attached at the side corners
• 7x7 flag, attached at top two corners, in strong wind
• flag with unconstrained springs, network file
• flag with constrained springs (same object as previous), network file
• a pendulum once started will keep oscillating since it has no way to dissipate the energy: the pivot (the fixed point) has no angular friction, network file
• same pendulum with air dragadded in; this feature will allow all hanging objects to finally come to rest (hair examples below use it, for example), network file
• a strand of hair, demonstrating use of hidden flexion springs, network file
• a patch of hair blowing in the wind, network file
• top view of an attractor/deflector particle rolling in a patch of grass, network file
• a perspective view of the above

First, here's a basic 8-by-8 flag, attached at the two top corners:
MPEG, network file

Then there is the same 8-by-8 flag, but attached by the side corners:
MPEG, network file

Finally, here is a 7-by-7 flag, also attached by the sides, but in stronger wind:
MPEG, network file

Just for fun, here is a sail:
MPEG, network file

In fact, most of these 'network' files can be much more easily generated with a utility I wrote for just this purpose:

• flag_maker

The syntax of this command is:
flag_maker <cols> <rows> <dcol> <drow> <mass> <kp> <kd> <height> where,

• <cols> is the number of columns
• <rows> is the number of rows
• <dcol> is the distance between columns
• <drow> is the distance between rows
• <mass> is the mass of each particle
• <kp> is the k_p to be assigned to each spring
• <kd> is the k_d to be assigned to each spring
• <height> is the height of the highest row

For starters, here's a general network file for the ball:
network.ball

Like the flag, this file was generated automatically by another utility I wrote:

• ball_maker

It's syntax is:
ball_maker <arcs> <slices> <radius> <mass> <height> <kp> <kd>
where,

• <arcs> is the number of arcs (into how many parts a slice should be divided)
• <slices> is the number of slices (how many longitudinal lines there are)
• <radius> is the radius of the sphere
• <mass> is the mass of each particle
• <height> is the height of the highest of the center of the sphere
• <kp> is the k_p to be assigned to each spring
• <kd> is the k_d to be assigned to each spring

Here are some MPEGs of the ball from different angles:

• ball dropped onto a flat surface
• ball rolling
• ball rolling towards observer
• ball bouncing in slow motion

Here are some MPEGs of other objects generated with the ball_maker program:

• 2 pyramids, base to base
• 2 pyramids, base to base, slow-motion beginning
• 2 (10point) cones, base to base

Instead of the research paper I extended the mass-spring system from question 1 so that it is able to imitate fabrics better, using the Deformation Constraint technique (Xavier Provot, Deformation Constraints in a Mass-Spring Model to Describe Rigid Cloth Behaviour, Graphics Interface '95, pp.147-154).

The flag_maker utility turns on this ability using the 'cloth_model' command in the network file.

Here's an MPEG of a flag hanging down without deformation constraint:
unrestrained flag MPEG, its network file
While here is an MPEG of the same flag hanging down WITH deformation constraint (20% max deformation):
constrained flag MPEG, its network file

In fact, all of the flag network files mentioned in Question 1 used this deformation control.

To select a different constraint (maximum percentage deformation), just modify the ELASTICITY_MAX constant.

• picking just the right k_p and k_d values can be your worst nightmare
• the stiffer the springs used, the smaller DTsim should be or the system becomes unstable (the DTsim is too coarse for the tremendous instantaneous velocities and accelerations)
• oh... THAT's why they put braces in bridges...

Here's a list of new commands that are now allowed in network files:

• 'ground <nx> <ny> <nz> <dz>': defines the ground plane by providing the normal vector (nx,ny,nz), as well as the z value at x=0,y=0
• 'wind <x> <y> <z>': provides a wind vector (the magnitude is the strength)
• 'cloth_model': turns on deformation constraint (see question 3)
• 'record_on': turns on frame-to-ppm saving
• 'dt_sim <x>': sets DTsim to x (although not updated in the TclTk interface)
• 'dt_disp <x>': sets DTdisp to x
• 'drag_coeffs <b> <n>': : sets the drag coefficients (F_drag = b * vel^n)
• 'bounce_alpha <alpha>': : sets the ratio of energy kept in a bounce
• 'friction_kcoeff <c>': : sets the kinetic friction coefficient

The 'spring' command has been extended to have an additional optional parameter at the end; if this parameter starts with 'n' or 'N', the spring will not be painted (although its effect can be seen). This is very handy for flexion springs which would otherwise obscure the true shape of the object.

The 'masspt' command has been extended to have an additional optional parameter (specified before the optional velocity component). It specifies an attraction (or repulsion if negative) coefficient which is used to attract or repulse all other particles. I've modelled the attraction somewhat on gravitational pull with the exception that the mass of the attractor is irrelavent (instead, the above mentioned parameter is used). This is so that one would be free to set its mass to whatever is necessary to achieve the desired effect (for example one could want to make the attractor really light so it rolls very far on the ground, and yet have it attract very strongly).

Two more utilities have been added on: hair_maker and grass_maker. Both have similar syntax to flag_maker and hence their full syntax is not given here. hair_maker generates a patch of hair, while grass_maker generates a patch of grass on the ground plane.

Here's my complete source directory, including the lib directory. Since I modified almost all files, it's much easier just to download the whole source tree. This includes the source for the two utilities I mentioned above (flag_maker and ball_maker).

mac.tgz

The problem with ball_maker has been fixed...