Physics File Format
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There has been some discussion on how to share 2D scenes between the various Physics frameworks, activities, etc.
Here is a proof of concept, presented here for the purposes of discussion. It was developed for the Mac OS X application Physix written by Jminor The format is XML and is meant to map directly to concepts, names and features of Box2D. It was inspired by a similar format made by kne? as part of the Elements project.
Here is a simple example:
<!-- Physix.app v1.0 -->
<world lowerBound="-10,-10" upperBound="14,18" gravity="0,-9.81">
A square and circle balanced on a teeter-totter.
<body name="window"><box size="4,3" sensor="1"/></body>
<body name="ground" position="0,0" static="1">
Make a hollow box that is 4 wide and 8 tall. The upper half is off screen,
so objects can go off the top, but they can't get away entirely.
<box offset="0,4" size=".1,8"/>
<box offset="2,0" size="4,.1"/>
<box offset="4,4" size=".1,8"/>
<box offset="2,8" size="4,.1"/>
</body>
<body name="pivot" position="2,0" static="1">
<box size="1,1" rotation="45"/>
</body>
<body name="platform" position="2,.75">
<box size="2,.2" density="2" restitution=".5" friction=".15"/>
</body>
<revoluteJoint body1="pivot" body2="platform" anchor="2,.65"/>
<body name="box" position="1.5,3" rotation="0">
<box size=".355,.355" rotation="0" density="1" restitution=".5" friction=".15"/>
</body>
<body name="circle" position="2.5,3">
<circle radius=".2" density="1" restitution=".15" friction=".5"/>
</body>
</world>
The defined tags, attributes and structure are as follows. Note that most attributes are given default values if they are not present in the XML. Most defaults are 0. All rotations are in degrees. All 2D vectors are specified as two numbers separated by a comma (see examples for clarity). Booleans are 0 or 1. Distances are measured in meters.
- world
- sleep = 0 or 1 indicating whether settled objects are allowed to sleep to save computation
- lowerBound and upperBound = 2D vectors defining the lower and upper corners of the simulation's bounds (different from the visible window, see below)
- attribute: gravity = 2D vector defining the value of gravity in meters per second per second (default = 0,-9.81 which is roughly Earth's gravity)
- children: body, *joint or repeat elements
- body
- name = the name for this body. This is used when specifying the anchors for a joint.
- static = boolean defining whether this body is fixed in place or a dynamic part of the simulation.
- position = 2D vector for the origin/center of this body.
- rotation = rotation in degrees.
- children: 1 or more shapes (box, circle, polygon)
- box, circle or polygon
- density = density (kilograms per square meter?) of this shape (default = 1)
- restitution = measure of how springy this shape is (usually between 0 and 1) (default = 0.15)
- friction = measure of how rough or slippery this shape is (0 = very slick, 1 = very rough) (default = 0.5)
- layers = which collision layers does this shape occupy (default = "A")
- mask = which collision layers does this shape interact with (default = "ABCDEFGHIJKLMNOP")
- sensor = boolean specifying whether this shape is used purely for sensing collisions or not (default = 0)
- box also has these attributes
- rotation = rotation in degrees.
- size = 2D vector of width, height.
- offset = 2D vector offset relative to the parent body (default 0,0)
- circle also has these attributes
- radius = distance from center to edge of the circle
- polygon also has these attributes
- vertices = list of 2D vectors specifying the points in this polygon. The list is a string of 2D vectors separated by spaces. (As of Box2D 2.0, the vertices should be in clockwise order and must be convex.)
- offset = 2D vector offset relative to the parent body (default 0,0)
- repeat = the children of this element are duplicated a number of times with an offset and/or rotation between each repetition
- count = the number of repetitions (default = 1)
- offset = 2D vector defining the offset applied between each repetition
- center = 2D vector defining the base offset applied to the first repetition (in other words: the origin of the children)
- rotation = rotation in degrees between each repetition
...insert examples...