Body: Gravity Scaling

[TheColorRed]

Is there a way to set the gravity scale?

If not, I would like to suggest something like:

• gravityScale: 2 would make it have twice the gravity (falls twice as fast)
• gravityScale: 1 would be the default (runs as it currently does)
• gravityScale: 0 would make it so gravity doesn't affect the body (at least visually)
• gravityScale: -1 would make it have an inverse of the default gravity (like a balloon with helium)
• gravityScale: -2 would make it have twice the inverse of the default gravity (rises twice as fast)

I am no expert on the math, but wouldn't adding * gravityScale to these lines in the update be the solution? https://github.com/liabru/matter-js/blob/63b02cfbeebe93d28082504f5e89e93f98739743/src/body/Body.js#L581-L583

[TheColorRed]

I have added the code for this, and it seems to work the way I was thinking.

Here are the new sources:

https://github.com/TheColorRed/matter-js/blob/master/src/body/Body.js#L51 https://github.com/TheColorRed/matter-js/blob/master/src/body/Body.js#L592-L594 https://github.com/TheColorRed/matter-js/blob/master/src/body/Body.js#L433-L441

• box 1: gravityScale: -2
• box 2: gravityScale: -1
• box 3: gravityScale: 0
• box 4: gravityScale: 1 (default)
• box 5: gravityScale: 2

The box with a gravityScale: 0 is still affected by other bodies, as seen in this screen capture:

Here are two boxes that have different gravity scales that collide with one another (the x offset of each are slightly different otherwise the one with more gravityScale will push the other one down/up with it):

[liabru]

Interesting idea, but I'd prefer to avoid calling this gravity and instead generalise it more. I think it may be best implemented as a plugin and potentially tied into #498 and the concepts in the manipulation examples somehow. Maybe a matter-kinematic plugin would be a neat, I'll think about it.

[TheColorRed]

I used the term gravityScale because I was basing this off of Unity3d's Rigidbody2d, which uses that name/term and applies the same effect. Also in Unity3d, only dynamic bodies have gravity scaling whereas Kinematic/Static bodies don't.

off-topic (kinda): I think that it could be a good idea to remove body.isStatic and replace it with an enum, like body.type and accepts one of the following:

• Body.Dynamic (default)
• Body.Static
• Body.Kinematic (when and if added)

[kindid]

Did anything come of this? I'm interested because I'm tying to implement a "float" animation state for a character. She jumps up, under normal gravitational waves but when she reaches her zenith she opens a "parachute" (actually an umbrella!) and floats down. Now, air friction seems to apply in both dimensions and this I do not want - I want her to start floating along the x axis. The only way I have affected this so far is by changing gravity - naturally this works but also affects all other bodies in the world.

I may well just borrow your code and fork a copy of matter for my own use.

[liabru]

@kuiash I suggest just disabling the built in air friction and gravity and then applying your own forces as needed.

[kindid]

I've added in the gravity scale and it's doing just what I want it too.