== Pivot Constraint ==

This constraint allows an object or bone to have their rotations applied as if their origin/pivot-point was located elsewhere. The most obvious uses include foot-roll, see-saws, but could also include more complicated rolling-box examples.  

== Usage Examples ==
=== Foot Roll ===
1. Add 'Pivot' Constraint to the bone without any target.
2. Set the 'Y' value of the offset to the length of the bone. Usually this should be negative (if you rig with feet facing 'forwards' along -Y axis). This gives you a pivot point relative to the bone's (preconstraint) location, which should be at the tip of the bone here. Disabling the 'Use Relative Offset' would make this offset be relative to 0,0,0 instead of to the owner/bone-head.
3. Ensure that the 'Pivot When' setting is set to '-X Rot', (default) which means that the pivot will only used when the rotation on the X-Axis is negative to get tip-toe 'roll'. 

=== See Saw ===
1. Add a 'Pivot' constraint too see-saw plank object, this time with a target that you wish to have as the pivot-point. It's possible to do this without too (as before), but is less intuitive.
2. Optionally, if you want the plank slightly raised, set the z-offset value, which should make the pivot-point used to be relative to the target with the z-offset applied.
3. Ensure that 'Pivot When' is set to 'Always', which means that the pivot will always be used, irrespective of the rotation.

== Notes ==
* The 'Pivot When' setting has been integrated in the constraint, since this is something that will often be required for these setups. Having to set up additional drivers to drive the constraint to do this kindof beats the purpose of providing this.

* The 'Offset' functionality is probably not presented as clearly as it could be. We may need to go over this again.

* For foot-roll - if any scaling of the foot is required, simply set up a driver on the y-offset to make this dynamically respond to the "scale" RNA property of the bones (don't use the "Transform Channel" vartype since that won't work correct here). However, this shouldn't be common enough to warrant special treatment.

1 files changed, 113 insertions, 0 deletions

diff --git a/source/blender/blenkernel/intern/constraint.c b/source/blender/blenkernel/intern/constraint.c
index a3f1cb0cb0c..1a8d665a91d 100644
--- a/source/blender/blenkernel/intern/constraint.c
+++ b/source/blender/blenkernel/intern/constraint.c
@@ -3831,6 +3831,118 @@ static bConstraintTypeInfo CTI_SPLINEIK = {
 	NULL /* evaluate - solved as separate loop */
 };
 
+/* ----------- Pivot ------------- */
+
+static void pivotcon_id_looper (bConstraint *con, ConstraintIDFunc func, void *userdata)
+{
+	bPivotConstraint *data= con->data;
+	
+	/* target only */
+	func(con, (ID**)&data->tar, userdata);
+}
+
+static int pivotcon_get_tars (bConstraint *con, ListBase *list)
+{
+	if (con && list) {
+		bPivotConstraint *data= con->data;
+		bConstraintTarget *ct;
+		
+		/* standard target-getting macro for single-target constraints */
+		SINGLETARGET_GET_TARS(con, data->tar, data->subtarget, ct, list)
+		
+		return 1;
+	}
+	
+	return 0;
+}
+
+static void pivotcon_flush_tars (bConstraint *con, ListBase *list, short nocopy)
+{
+	if (con && list) {
+		bPivotConstraint *data= con->data;
+		bConstraintTarget *ct= list->first;
+		
+		/* the following macro is used for all standard single-target constraints */
+		SINGLETARGET_FLUSH_TARS(con, data->tar, data->subtarget, ct, list, nocopy)
+	}
+}
+
+static void pivotcon_evaluate (bConstraint *con, bConstraintOb *cob, ListBase *targets)
+{
+	bPivotConstraint *data= con->data;
+	bConstraintTarget *ct= targets->first;
+	
+	float pivot[3], vec[3];
+	float rotMat[3][3];
+	
+	/* firstly, check if pivoting should take place based on the current rotation */
+	if (data->rotAxis != PIVOTCON_AXIS_NONE) {
+		float rot[3];
+		
+		/* extract euler-rotation of target */
+		mat4_to_eulO(rot, cob->rotOrder, cob->matrix);
+		
+		/* check which range might be violated */
+		if (data->rotAxis < PIVOTCON_AXIS_X) {
+			/* negative rotations (data->rotAxis = 0 -> 2) */
+			if (rot[data->rotAxis] > 0.0f)
+				return;
+		}
+		else {
+			/* positive rotations (data->rotAxis = 3 -> 5 */
+			if (rot[data->rotAxis - PIVOTCON_AXIS_X] < 0.0f)
+				return;
+		}
+	}
+	
+	/* find the pivot-point to use  */
+	if (VALID_CONS_TARGET(ct)) {
+		/* apply offset to target location */
+		add_v3_v3v3(pivot, ct->matrix[3], data->offset);
+	}
+	else {
+		/* no targets to worry about... */
+		if ((data->flag & PIVOTCON_FLAG_OFFSET_ABS) == 0) {
+			/* offset is relative to owner */
+			add_v3_v3v3(pivot, cob->matrix[3], data->offset);
+		}
+		else {
+			/* directly use the 'offset' specified as an absolute position instead */
+			VECCOPY(pivot, data->offset);
+		}
+	}
+	
+	/* get rotation matrix representing the rotation of the owner */
+	// TODO: perhaps we might want to include scaling based on the pivot too?
+	copy_m3_m4(rotMat, cob->matrix);
+	normalize_m3(rotMat);
+	
+	/* perform the pivoting... */
+		/* 1. take the vector from owner to the pivot */
+	sub_v3_v3v3(vec, pivot, cob->matrix[3]);
+		/* 2. rotate this vector by the rotation of the object... */
+	mul_m3_v3(rotMat, vec);
+		/* 3. make the rotation in terms of the pivot now */
+	add_v3_v3v3(cob->matrix[3], pivot, vec);
+}
+
+
+static bConstraintTypeInfo CTI_PIVOT = {
+	CONSTRAINT_TYPE_PIVOT, /* type */
+	sizeof(bPivotConstraint), /* size */
+	"Pivot", /* name */
+	"bPivotConstraint", /* struct name */
+	NULL, /* free data */
+	NULL, /* relink data */
+	pivotcon_id_looper, /* id looper */
+	NULL, /* copy data */
+	NULL, /* new data */ // XXX: might be needed to get 'normal' pivot behaviour...
+	pivotcon_get_tars, /* get constraint targets */
+	pivotcon_flush_tars, /* flush constraint targets */
+	default_get_tarmat, /* get target matrix */
+	pivotcon_evaluate /* evaluate */
+};
+
 /* ************************* Constraints Type-Info *************************** */
 /* All of the constraints api functions use bConstraintTypeInfo structs to carry out
  * and operations that involve constraint specific code.
@@ -3867,6 +3979,7 @@ static void constraints_init_typeinfo () {
 	constraintsTypeInfo[22]= &CTI_SPLINEIK;			/* Spline IK Constraint */
 	constraintsTypeInfo[23]= &CTI_TRANSLIKE;		/* Copy Transforms Constraint */
 	constraintsTypeInfo[24]= &CTI_SAMEVOL;			/* Maintain Volume Constraint */
+	constraintsTypeInfo[25]= &CTI_PIVOT;			/* Pivot Constraint */
 }
 
 /* This function should be used for getting the appropriate type-info when only