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Given two joints that define a bone, I would like to know how to decide where, between those two joints, I should draw the sprite. This should be a fairly simple thing to solve, but there is one thing that I am not sure about.

After I've determined the rotation of the sprite (which is the absolute angle the joints form with the x-axis), I also need to determine the origin point from where I need to start drawing the transformed image.

So how should I position the sprite between the two joints? Should I make the center of the image be the midpoint between the two joints, or should I make one the of the joints be the origin? Do these things matter that much (could the wrong positioning make the sprite move oddly during the animation)?

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Are you using the "render a node/scene graph while concatenating matrices" approach or something else? –  Torious Jun 9 '12 at 20:56

3 Answers 3

My 2D animation skeleton looks like this:

animation of moving the nodes of an arm on an animation skeleton

The large hollow circles represent constraints that the nodes (filled circles) have to other nodes. For example, I make sure that the elbow stays within some given distance of the shoulder.

To actually draw a sprite between nodes n1 and n2, this is the function call I use in Love2D:

love.graphics.draw(sprite.img -- image
    , n1.pos.x -- x coordinate
    , n1.pos.y -- y coordinate
    , (math.pi/2)-v.dir(n1.pos, n2.pos) -- orientation (radians)
    , (n2.pos - n1.pos):len() / len -- x scale
    , 1 -- y scale
    , sprite.orig.x -- x offset
    , sprite.orig.y -- y offset

Your framework likely has a similar function.

I use the two nodes' positions to figure out the rotation and scale that the sprite should have. The vector library I use implements these with simple trigonometric functions and Pythagoras' theorem, respectively.

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It's quite simple, you calc

vec2 dir = vec2_end_joint - vec2_start_joint
angle = dir.Angle();
//set origin of sprite if required here.
sprite.draw(vec2_start_joint, angle, calculated_scale_if_required);

calculated_scale_if_required - is sprite size and your dir.Magnitude ratio

Or you can use matrix multiplication, like glRotate glScale, to make direct kinematics calculations and use your bones hierarchy.

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It sounds like for each bone you are currently defining the texture to be rendered - you might as well allow for more flexibility and include a texture origin offset in your bone structure as well (relative to either of the joints you choose). That way you can be sure the rendered texture will be consistently placed.

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