Lack of easy way to change mass during run-time

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What version of the product are you using? On what operating system?
r17, I don't think this has changed since.

Please provide any additional information below.
Would like a `SetDensity` or `SetMassFromDensity` function on either b2Body
or b2Fixture. Setting the mass manually with `SetMassData` is okay, but it
requires us to know how to compute the mass and inertia from only a density
value, which can be confusing.

A case where it would be nice to be able to do this is when a body changes
from one material into another, like in Kirby Super Star, where Kirby
changes into stone, or perhaps when water turns into ice; a user may want
to change its density during run-time.

A code example:
b2BodyDef bd;
b2Body *kirby = world->CreateBody(&bd);
// ... add some fixture data, set density to 0.3 so that kirby is nice and
fluffy
body->SetMassFromDensity(10); // omg, kirby just turned to stone and
suddenly got really heavy so now he can crush his opponents!

Original issue reported on code.google.com by mnbaya...@gmail.com on 22 Sep 2009 at 12:28

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i Need this to, by now, i can't get the density and even change this over the 
Box2D
version on iPhone.

Original comment by bad...@gmail.com on 5 Nov 2009 at 2:36

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Original comment by erinca...@gmail.com on 20 Nov 2009 at 6:43

• Changed state: Accepted

[GoogleCodeExporter]

This has been fixed. You can now call b2Fixture::SetDensity then 
b2Body::ResetMass.

Original comment by erinca...@gmail.com on 20 Nov 2009 at 6:44

• Changed state: Fixed