Skel2D

2D skeletal animation tool

https://urraka.github.io/skel2d/

This is an experimental animation tool in which you define a skeleton and animations in a custom format while getting a live preview of the end result rendered with webgl.

sample: https://urraka.github.io/skel2d/#2327f320712b6f6e13be34c8a4e32747

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Documentation

Index

• Setting up the skeleton
  • Defining a skeleton hierarchy
  • Bone properties
  • Bone flags
  • Setting bone properties/flags
  • Adding slots/attachments
  • Attachment types
  • Slot properties
  • Attachment properties
  • Path attachments
• Animation
  • Defining a new animation
  • Animation properties
  • Animation timelines
  • Easing functions
• Skins
  • Defining skins
• Draw order
  • Defining the draw order

Defining a skeleton hierarchy

Skeleton with 2 child bones called bone1 and bone2:

skeleton
	bone1
	bone2

Skeleton with a child bone called bone1, which has a child bone called bone2:

skeleton
	bone1
	bone1.bone2

Same as the previous one:

skeleton
	bone1.bone2

As many levels as needed:

skeleton
	grandpa.parent.child.grandchild

Notes:

• Indentation must be with tabs.
• Only alphanumeric characters plus - and _ are allowed for bone names.
• A bone name can't start with a number.
• A bone name can't be skeleton.

Bone properties

• l: length
• r: rotation
• x: position-x
• y: position-y
• i: scale-x
• j: scale-y
• color (format is #RGB or #RRGGBB optionally followed by an opacity value separated by a comma, i.e #F00,0.5 or #FF0000,0.5)

Bone flags

• flip-x: flip on x axis
• flip-y: flip on y axis
• no-rot: don't inherit rotation from parents
• no-scale: don't inherit scale from parents

Setting bone properties/flags

A red bone with length=100, position=10,-10, scale=1.5x, flipped on x axis which doesn't inherit rotation:

skeleton
	parent.bone #F00 l100 r45 x10 y-10 i1.5 j1.5 flip-x no-rot

Notes:

• There can't be whitespace between a property and its value.
• The properties belong to the child-most bone defined in the line (in the example above the properties apply to bone, parent is unaffected).

Adding slots/attachments

The concept of slots and attachments is taken from Spine. Basically, each bone can have slots and each slot can have attachments (but only one active/visible attachment).

Currently, an attachment can be a sprite, a path or a shape.

This would define a bone with a slot named slot which has two attachments named attachment1 and attachment2. attachment1 will be the default active attachment for slot because it's the first one:

skeleton
	bone
		@slot[attachment1]
		@slot[attachment2]

Note: having multiple attachments in one slot is only useful when doing animations, where you can switch a slot's active attachment.

If a bone has only one slot with one attachment it can be annoying to give them names, so by default a slot will take the owner bone name and attachments will take the owner slot name. Example of how this rule works:

skeleton
	parent.bone
		@               # slot named "bone" with attachment named "bone"
		@[attachment]   # slot named "bone" with attachment named "attachment"
		@slot           # slot named "slot" with attachment named "slot"

Attachment types

By default, an attachment will be of type none which isn't very useful. This is a list of available attachment type keywords:

• :sprite
• :rect
• :circle
• :ellipse
• :path

This would define a slot with a rectangle attachment:

skeleton
	bone
		@ :rect

Slot properties

Slot properties must be placed before the attachment type keyword.

Currently, slots have only one property which is its color. Example:

skeleton
	bone
		@ #F00 :rect

The slot color will be multiplied with attachment colors. By default it's #FFF so it won't affect the attachment. What makes the slot color different from whatever color the attachment has is that it can be animated.

Attachment properties

Attachment properties must be placed after the attachment type keyword.

All attachment types share the same transform properties as bones for defining position, scale and rotation (x, y, i, j, r).

Shape and path attachments (that is, all of them except sprites) share some properties to define the stroke and fill styles:

• t: thickness for the stroke (default: 1)
• m: miter limit (default: 10)
• f: fill color (default: #000)
• s: stroke color (default: #000)
• miter-join or bevel-join or round-join: line join style (default miter-join)
• butt-cap or square-cap or round-cap: line cap style (default: butt-cap)

The following are properties specific to some attachment types:

• w: width (used by rects and ellipses, default: 0)
• h: height (used by rects and ellipses, default: 0)
• d: diameter for a circle or for rect corners (for making rounded rects, default: 0)

Example: a 100x80 rect with a line width of 5, filled with blue, rotated 90 degrees and round line joins:

skeleton
	bone
		@ :rect w100 h80 t5 f#00F r90 round-join

Sprites only have one additional property (apart from transform properties) which is a string with the name of the image:

skeleton
	bone
		@ :sprite "image-name.png"

Note: currently, strings are limited to no whitespace.

Path attachments

Paths are defined by the following commands:

• M <point>: move to (only supported as the first command, if ommited it will default to 0,0)
• L <point>: line to
• Q <ctrl-point> <end-point>: quadratic curve to
• B <ctrl-point1> <ctrl-point2> <end-point>: bezier curve to
• C: close path
• : <bone>: switch default bone binding (doesn't count as a command and can be used before M)

Each point given as a parameter to a command is bound to a bone to which it is relative to. By default, that bone is the one that owns the attachment, but it can be any bone in the skeleton. This allows paths to be changed by animating bones.

The format to define a point is x,y or x,y:bone to bind the point to a bone other than the default.

Simple path example, a red triangle with black outline:

skeleton
	bone
		@ :path f#F00
			M 0,0       # M could be ommited here because it's the default 0,0
			L 25,50
			L 50,0
			C

A bezier curve:

skeleton
	bone
		@ :path
			B 50,100 100,-100 150,0

Same bezier curve but with points bound to different bones:

skeleton
	bone
		@ :path
			B 0,0:cp1 0,0:cp2 0,0:end
	bone.cp1 x50 y100
	bone.cp2 x100 y-100
	end x150 y0

Using the default bone binding switch (this is the same red triangle as above but the top vertex can be controled by bone.child):

skeleton
	bone x150
		@ :path f#F00
			M 0,0
			: child
			L 0,0
			: bone
			L 50,0
			C
	bone.child x25 y50

Notes:

• When referencing a bone, it's enough to give the shortest unambiguous name for it. If the name of a bone that is a direct child of skeleton happened to be ambiguous, the skeleton keyword can be used as the parent bone to disambiguate. For example, if there are two bones bone and bone.bone, the former can be referenced as skeleton.bone and the latter as bone.bone.
• Currently, path rendering is quite limited and filling will only work with simple covex shapes.

Defining a new animation

Animations are defined with the anim keyword. A name to identify it can be given with a string:

anim "name"
	# ...

Animations consist of timelines. There are bone and slot timelines. Each timeline is used to animate a single animatable property from either a bone or a slot.

The following lists all the animatable bone and slot properties:

skeleton
	bone
		@slot

anim "name"
	bone
		r    # rotation
		x    # position-x
		y    # position-y
		i    # scale-x
		j    # scale-y
		s    # flip-x
		t    # flip-y
	@slot
		@    # current attachment
		r    # color (red component)
		g    # color (green component)
		b    # color (blue component)
		a    # color (alpha component)
		c    # color

Notes:

• Slot names must have a leading @.
• Bone and slot names can be the shortest unambiguous name that uniquely identifies them (same rule explained in path attachments notes).
• If a slot is given a color timeline (c), the individual color component timelines won't take effect.

Animation properties

An animation has the following properties:

• fps: frames per second (default: 20)
• frame: defines the starting frame (default: 0)
• step: number of frames to advance on each step (default: 5)
• easing: function used for interpolation between frames (default: li)

These properties have a specific syntax to define them. Here's an example that would define the default values:

anim "name" 20fps 0:5:li
	# ...

As shown above, frame, step and easing are defined together separated by a colon. It's not necessary, however, to give a value to all of them. These are all valid ways of setting these properties:

Syntax	Description
0: or 0::	sets frame
:5: or :5	sets step
::li or :li	sets easing
0:5: or 0:5	sets frame and step
0::li	sets frame and easing
:5:li or 5:li	sets step and easing
0:5:li	sets frame, step and easing

These properties (frame, step and easing) can be overriden on each animation item:

anim "name" 0:4
	bone :2:     # change step to 2
		# ...

Animation timelines

The purpose of timelines is to define a list of key frames for a given property. The syntax to define them can be thought of as a list of commands. For example:

anim "name"
	bone
		x 0 -> 50 --> 0

The above can be read as:

1. Add key frame with value 0
2. Advance one step (5 frames by default)
3. Add key frame with value 50
4. Advance two steps (10 frames)
5. Add key frame with value 0

Most of the animatable properties take a numeric value, but there are some exceptions. Flip timelines (s and t) take a boolean value (true or false). Attachment timelines (@) take an attachment name from the list of attachments available for the slot that is being animated. Finally, color timelines (c) take a color value with the same format described in bone properties.

The following illustrates the syntax for the commands available:

-1.5      # add key frame with value -1.5 (numeric timelines)
+-1.5     # add key frame incrementing the previous key frame by -1.5 (numeric timelines)
*-1.5     # add key frame multiplying the previous key frame by -1.5 (numeric timelines)
true      # add key frame with value "true" (flip timelines)
false     # add key frame with value "false" (flip timelines)
name      # add key frame with value "name" (attachment timelines)
>         # advance 0 steps (noop)
->        # advance 1 step
---->     # advance 4 steps (number of hyphens defines the number of steps to advance)
0:5:li>   # set frame to 0, set step to 5, set easing to li and advance 0 steps
0:3:li->  # set frame to 0, set step to 3, set easing to li and advance 1 step (3 frames)
+1:>      # advance 1 *frame* (and 0 steps)
+1::li>   # advance 1 *frame*, set easing to li (and advance 0 steps)
{         # begin loop
}[2]      # end loop (loop 2 times)

Notes:

• The syntax that sets frame, step and easing is identical to the one described in animation properties, except that it must be followed by "advance zero or more steps" (> preceded by zero or more -) and that it can set frame relatively by incrementing the current frame by x (+x:>).
• Adding multiple key frames without advancing in time will result in only one key frame with the value of the last one.
• Setting frame to go back in time won't work.
• Setting the step value will take immediate effect, i.e. :10:-> will advance 10 frames. It also takes effect on all the following commands (until changed again).
• Setting easing will change the function used to interpolate the previous key frame with the next one, and it will also change the default easing function for all the following frames (until changed again).
• Whitespace is important. All the commands illustrated must be separated by whitespace and there must not be any whitespace within a command (i.e. {0 -> 1 -> 0}[2] is invalid, must be { 0 -> 1 -> 0 }[2]).
• Color component timelines (r, g, b and a) take values between 0 and 1. However, they are not restricted to it.

A more elaborate example:

skeleton
	bone l50
		@[red] :circle d30 t0 f#F00
		@[blue] :circle d30 t0 f#00F
		@line :path
			L 0,0:handle
	bone.handle

anim "test" 20fps
	bone 40:sio
		x 0 -> 200 -> 0
		y :10:> 0 :so-> 20 :li--> 20 :si-> 0 :so-> -20 :li--> -20 :si-> 0
		s false -> true -> false
		i :10:> 0.1 -> 1 --> 1 -> 0.1 -> 1 --> 1 -> 0.1
	handle :5:
		x { -15 :si-> 0 :so-> 15 :si-> 0 :so-> -15 }[4]
		y { 0 :so-> 15 :si-> 0 :so-> -15 :si-> 0 }[4]
	@bone :40:
		@ red -> blue -> red

See it in action: http://urraka.github.io/skel2d/#3a69c6d2c25e67f9950a

Easing functions

Easing functions change the way a key frame is interpolated with the next one for a smooth transition. The default is a linear interpolation (li). Currently there are only a few functions available:

Identifier	Description	Function
li	linear	y = x
si	sin-in	y = 1 - sin(pi/2 + x * pi/2);
so	sin-out	y = sin(x * pi/2)
sio	sin-in-out	y = 0.5 + sin(x * pi - pi/2) / 2

These functions are used to convert a value t between 0 and 1 into a value t'. The result is always used in a linear interpolation. So given two key frame values a and b, the function used to interpolate them will be a + (b - a) * t' (or a + (b - c) * f(t) where f is the easing function).

Defining skins

Skins are used to change the visuals of the skeleton by redefining its attachments.

Example:

skeleton
	bone
		@ :circle d10 f#F00
		@slot :rect w10 h10
		@other[small] :ellipse w20 h10
		@other[big] :ellipse w50 h20

skin "skin-name"
	@bone :circle d10 f#00F        # change to blue fill color
	@slot                          # remove (attachment of type none)
	@other[small] :ellipse w10 h5  # make it smaller

Notes:

• The last attachment is left untouched so it will appear as defined in the skeleton.
• All properties must be redefined when overriding an attachment, even if they don't change.

The naming rules are the same as described in path attachments notes. The following would have the same effect as the previous example:

skeleton
	bone
		@ :circle d10 f#F00
		@slot :rect w10 h10
		@other[small] :ellipse w20 h10
		@other[big] :ellipse w50 h20

skin "skin-name"
	@skeleton.bone.bone[bone] :circle d10 f#00F
	@skeleton.bone.slot[slot]
	@skeleton.bone.other[small] :ellipse w10 h5

Defining the draw order

By default, skeleton attachments are drawn in the order in which their owner slots are defined. This can be changed by defining a draw order:

order
	@slot1
	@slot2
	# ...

It's simply a list of slots from the skeleton. The ones on top will be drawn first, so they will appear in the back of following slots. Unlisted slots will be pushed at the end of the list in the order they were defined in the skeleton.