[Bf-committers] Multiple soft body settings per object

[Luke Magill]

Hi there!

My name is Luke and I've been an amateur Blender user for a few years and I
love it! Recently, I had a cool idea where I wanted to try to emulate
muscles of the human body to create a more realistic style of animation.
This was mostly to satisfy personal curiosity and perhaps to create an
interesting new style of art. However, at some point I hit a snag where I
wasn't able to move forward.

The problem can be summarized as follows: the built in soft body engine of
Blender is great, but what I really needed was to have more than one soft
body setting per object. This was to allow me to sort of pin different
things together, like pinning skin to muscle. Skin and muscle have very
different levels of softness but sometimes the skin and muscle for one
person may need to share points (be connected or glued together). What I
needed was to have one edge connected to a point with a certain push/pull
value and a different edge connected to the same point with a different
push/pull value (this is just an example, the idea is that any soft body
setting for which you can select a vertex group would be able to have
multiple settings).

I couldn't figure out any way to do this or properly emulate this within
blender. True, there are ways to copy data from one point to another but
the soft body forces wouldn't transfer which would leave me with an
unrealistic result.

To solve this problem, I decided to break open the internal C code to try
and add this capability myself. Now before I get into details I want to
tell you a little about myself. I got my bachelor's in computer science 6
years ago and since then I have been a professional web programmer. The
vast majority of my experience with programming has been c#/Java and
scripting languages like PHP/Python, etc. I also do front end stuff like
html, css, and javascript. I don't have a lot of experience with very low
level code like c or c++. However, I'm always eager to try new things and,
to be honest, I was surprised at how approachable the blender code base
was! In any case, please be patient with me if I have any trivial
misconceptions about c coding.

This email/document is meant to be a project proposal and specification. I
will include as many details as I can and I invite everyone and anyone to
provide feedback and criticism.

Feature Description

This section describes the feature without discussing any implementation
details. Implementation will be covered in the next section.

In general, blender soft body settings can be split into two different
groups, general settings and vertex group specific settings. The vertex
group specific settings allow you to select a vertex group (or no vertex
group indicating all vertices) to apply the settings to. All vertices not
part of the specified vertex group will not have the settings, and will not
be part of this animation. This has a disadvantage if you are trying to
simulate heterogenous soft bodies for which part of the body may have
different settings than another part.

To alleviate this problem, I propose allowing MULTIPLE soft body settings
for the vertex-group type of setting. For reference, here is a list of the
different settings associted with vertex groups

Vertex Group Specific Settings:
    Soft Body: Friction, Speed, Mass
    Soft Body Goal:
        Goal Strength: Default, Minimum, Maximum
        Goal Settings: Stiffness, Damping
    Soft Body Edges:
        Settings: Pull, Push, Damp, Plastic, Bending, Length
        Stiff Quads: Stiff Quads, Shear
        Aerodynamics: Simple|Lift Force, Factor
        Collision: Edge, Face

All of the remaining settings are general settings and will not be affected
by this change.

The question naturally arises what will the soft body settings be of a
vertex that is a member of multiple groups? If we imagine trying to emulate
jello that is very jiggly on one side but rigid on the other with a smooth
transition, the most intuitive way to do this would be to have a "rigid"
vertex group and a "soft" vertex group. Then using weight painting, you can
smoothly transition between the two groups to create a nice effect.

To enable this, we institute the following rule: a vertex's specific value
for any setting listed above will be the weighted average of all of the
settings for all of the vertex groups that apply to it, including the "all
group" if there is a setting for that.

Now let's talk about how the UI will enable the user to control this
feature. There are already multiple examples in the blender UI of having a
list of settings that the user is allowed to modify on a per-item basis.
Some examples are render layers, textures and materials. The soft body
settings will work the same way.

First, we will move all of the General soft body settings to the top so
that the user does not get confused thinking that it applies only to a
vertex group. Then, we will have a list selector. For an example list
selector, see the render layers list selector. In the list selector panel
there will be a vertex group selector so that each item in the panel can
have a vertex group. The only change in the UI to the vertex group specific
settings panels will be that they will no longer contain individual vertex
group selectors.

Required Code Changes

This section describes what I will be changing to enable this feature. The
DNA, RNA, and Python UI features will be modeled after existing list type
code that exists within blender, particularly the RenderLayer object.

DNA changes: The settings associated with the aforementioned Vertex Group
Specific Settings will be removed from SoftBody (DNA_object_force.h) and
added to a new struct called SoftBodyVertexSettings. The
SoftBodyVertexSettings will be a linked list, in which the first element
has a *prev and a *next link (based on ListBase linked list). Methods will
be added to softbody.c called SoftBodyVertexSettings_add() and
SoftBodyVertexSettings_remove() to add and remove items to this list. These
_add and _remove functions will be similar to BKE_scene_add_render_layer.

RNA changes: These new DNA changes will be exposed through RNA. In
renderlayer, these are exposed through SCENE_OT_render_layer_add and
rna_RenderLayer_new. I will create two similar functions to expose my new
DNA. All of the RNA which currently modifies the "Vertex Group Specific
Settings" will have to be changed to modify the RNA for the currently
selected setting, rather than trying to modify the SoftBody itself.

Python UI changes: Again relying heavily on RenderLayers as an example,
RENDERLAYER_PT_layers class is responsible for showing the list selector
panel in the UI and RENDERLAYER_UL_renderlayers is resonsible for setting
up the individual items in the selector table. I will have to add similar
classes for soft body and also a vertex group selector to this panel.

Softbody.c changes: The most extensive changes will have to be made to this
file, because this is where the soft body settings are actually used. In
general, the soft body system works by converting a soft body setting and a
mesh (or lattice or curve) into a set of BodyX objects: either BodyPoint,
BodySpring, or BodyFace. These objects contain the information required to
compute each step of the SB animation.

To make soft body have multiple settings, each individual item of these
will have to be expanded to contain more information. To pick an example,
inspring (pull in the UI), is currently only stored in the SB object
itself, like this:
    iks  = 1.0f/(1.0f-sb->inspring)-1.0f ;/* inner spring constants
function */

In the new system, BodySpring would have to have it's own inspring, so this
line would look like this
    iks  = 1.0f/(1.0f-bs->inspring)-1.0f ;/* inner spring constants
function */
(It's somewhat confusing, but sb in this function is SoftBody and bs is the
BodySpring)

In addition to utilising these new fields in computation, I would have to
actually add methods for converting the soft body settings into a set of
BodySprings. This will be done using an averaging algorithm described above.

Filesystem changes: I haven't researched this yet but since I will be
removing items from the DNA I will need to make changes to the file
read/write systems both so that the new setting system can be saved/loaded
but also so that legacy soft body systems can be dealt with in a reasonable
way. I am hoping to write the core parts described above first, as a proof
of concept, before delving into details like how these settings are to be
saved.

Conclusion

Thank you for reading my proposal. At this point I would appreciate
criticism but I'm also kind of fishing for interest in this feature. If I
were to implement this would it be possible to have the change integrated
back to main? I would like to contribute this new functionality if possible.

Thanks!

Luke