workaround [#30480] Knife tool flicker

the problem was numeric precision when in ortho mode the start/end points for the view vector would be 2000 apart which caused trouble for the intersection test.

source/blender/blenlib/BLI_math_geom.h

@@ -73,6 +73,7 @@ void  closest_to_plane_v3(float r[3], const float plane_co[3], const float plane
 
 float line_point_factor_v3(const float p[3], const float l1[3], const float l2[3]);
 float line_point_factor_v2(const float p[2], const float l1[2], const float l2[2]);
+void  limit_dist_v3(float v1[3], float v2[3], const float dist);
 
 /******************************* Intersection ********************************/
 

source/blender/blenlib/intern/math_geom.c

@@ -1320,6 +1320,25 @@ float line_point_factor_v2(const float p[2], const float l1[2], const float l2[2
 	return(dot_v2v2(u, h)/dot_v2v2(u, u));
 }
 
+/* ensyre the distance between these points is no greater then 'dist'
+ * if it is, scale then both into the center */
+void limit_dist_v3(float v1[3], float v2[3], const float dist)
+{
+	const float dist_old = len_v3v3(v1, v2);
+
+	if (dist_old > dist) {
+		float v1_old[3];
+		float v2_old[3];
+		float fac = (dist / dist_old) * 0.5f;
+
+		copy_v3_v3(v1_old, v1);
+		copy_v3_v3(v2_old, v2);
+
+		interp_v3_v3v3(v1, v1_old, v2_old, 0.5f - fac);
+		interp_v3_v3v3(v2, v1_old, v2_old, 0.5f + fac);
+	}
+}
+
 /* Similar to LineIntersectsTriangleUV, except it operates on a quad and in 2d, assumes point is in quad */
 void isect_point_quad_uv_v2(const float v0[2], const float v1[2], const float v2[2], const float v3[2], const float pt[2], float r_uv[2])
 {

source/blender/editors/mesh/bmesh_tools.c

@@ -4447,7 +4447,7 @@ static int mesh_bevel_exec(bContext *C, wmOperator *op)
 	BMIter iter;
 	BMEdge *eed;
 	BMOperator bmop;
-	float factor = RNA_float_get(op->ptr, "percent"), fac = factor /*, dfac */ /* UNUSED */, df, s;
+	float factor = RNA_float_get(op->ptr, "percent"), /*, dfac */ /* UNUSED */, df, s;
 	int i, recursion = RNA_int_get(op->ptr, "recursion");
 	const int use_even = RNA_boolean_get(op->ptr, "use_even");
 	const int use_dist = RNA_boolean_get(op->ptr, "use_dist");
@@ -4484,9 +4484,8 @@ static int mesh_bevel_exec(bContext *C, wmOperator *op)
 
 	mul_vn_fl(w, recursion, 1.0f / (float)ftot);
 
-	fac = factor;
 	for (i = 0; i < recursion; i++) {
-		fac = w[recursion - i - 1] * factor;
+		float fac = w[recursion - i - 1] * factor;
 
 		if (!EDBM_InitOpf(em, &bmop, op,
 		                  "bevel geom=%hev percent=%f lengthlayer=%i use_lengths=%b use_even=%b use_dist=%b",

source/blender/editors/mesh/knifetool.c

@@ -1178,6 +1178,16 @@ static void knife_find_line_hits(knifetool_opdata *kcd)
 	mul_m4_v3(kcd->ob->imat, v3);
 	mul_m4_v3(kcd->ob->imat, v4);
 
+	/* numeric error, 'v1' -> 'v2', 'v2' -> 'v4' can end up being ~2000 units apart in otho mode
+	 * (from ED_view3d_win_to_segment_clip() above)
+	 * this gives precision error in 'knife_edge_tri_isect', rather then solving properly
+	 * (which may involve using doubles everywhere!),
+	 * limit the distance between these points */
+	if (kcd->is_ortho) {
+		limit_dist_v3(v1, v3, 200.0f);
+		limit_dist_v3(v2, v4, 200.0f);
+	}
+
 	BLI_smallhash_init(ehash);
 
 	/* test two triangles of sceen line's plane */

source/blender/python/mathutils/mathutils_Quaternion.c

@@ -1101,7 +1101,7 @@ static PyObject *quat__apply_to_copy(PyNoArgsFunction quat_func, QuaternionObjec
 	}
 }
 
-/* axis vector suffers from precission errors, use this function to ensure */
+/* axis vector suffers from precision errors, use this function to ensure */
 static void quat__axis_angle_sanitize(float axis[3], float *angle)
 {
 	if (axis) {