How to edit friction of ongoing contact?

[horttanainen]

I am using box2d for a platformer game that mixes physics with crisp movement control. My current approach is copied from this article:
https://bhopkins.net/pages/mmphysics/

In the article the author states:

Note that to ensure the friction is set properly between the player and a floor, it needs to be set in every PreSolve collision callback between the hitbox and a floor fixture. If you only modify the hitbox fixture's friction, the collision may use an outdated friction coefficient (e.g. if you were not walking when you landed on the floor, and then start walking without recontacting it).

I can confirm that just modifying the friction does not modify the friction for the ongoing collision with the floor. The player keeps sliding across the floor as it were ice. I then followed the authors advise on modifying the contact friction in the presolve callback but I seem to be doing it wrong.

My preSolve callback is as follows. I am using zig here:

pub const Context = struct { playerBodyShapeId: box2d.b2ShapeId, isMoving: *bool };

pub fn preSolve(shapeIdA: box2d.b2ShapeId, shapeIdB: box2d.b2ShapeId, manifold: [*c]box2d.b2Manifold, context: ?*anyopaque) callconv(.C) bool {
    const ctx: *Context = @alignCast(@ptrCast(context));
    if (box2d.B2_ID_EQUALS(shapeIdA, ctx.playerBodyShapeId) or box2d.B2_ID_EQUALS(shapeIdB, ctx.playerBodyShapeId)) {
        var p1 = manifold.*.points[0];
        var p2 = manifold.*.points[1];

        p1.tangentImpulse = if (ctx.isMoving.*) config.player.movementFriction else config.player.restingFriction;
        p2.tangentImpulse = if (ctx.isMoving.*) config.player.movementFriction else config.player.restingFriction;
    }
    return true;
}

Should I modify the tangentImpulse of manifold points p1 and p2 or should I be doing something else? Does this seem correct?

[horttanainen]

Okay it seems impossible to edit the ongoing contact friction in box2d v3. Is there a way to tell box2d to reset the ongoing contact?

[horttanainen]

Also, I logged the tangetImpulse and normalImpulse of manifold points 1 and 2 and they seem to be always 0 regardless of the frictions of the contacting shapes.

EDIT:

It seems I have old version of box2d v3 installed. I will see if the problem persist after updating box2d

[erincatto]

You can set the friction on the shape and this will update the contact friction. You don't need to do this in the presolve. See b2FrictionCallback on main.

[horttanainen]

That works! Thank you very much!