Improve plan_arc circle detection (#20440)

Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

Marlin/src/gcode/motion/G2_G3.cpp

@@ -41,13 +41,12 @@
 #endif
 
 /**
- * Plan an arc in 2 dimensions
+ * Plan an arc in 2 dimensions, with optional linear motion in a 3rd dimension
  *
- * The arc is approximated by generating many small linear segments.
- * The length of each segment is configured in MM_PER_ARC_SEGMENT (Default 1mm)
- * Arcs should only be made relatively large (over 5mm), as larger arcs with
- * larger segments will tend to be more efficient. Your slicer should have
- * options for G2/G3 arc generation. In future these options may be GCode tunable.
+ * The arc is traced by generating many small linear segments, as configured by
+ * MM_PER_ARC_SEGMENT (Default 1mm). In the future we hope more slicers will include
+ * an option to generate G2/G3 arcs for curved surfaces, as this will allow faster
+ * boards to produce much smoother curved surfaces.
  */
 void plan_arc(
   const xyze_pos_t &cart,   // Destination position
@@ -77,26 +76,33 @@ void plan_arc(
               rt_Y = cart[q_axis] - center_Q,
               start_L = current_position[l_axis];
 
-  // Angle of rotation between position and target from the circle center.
-  float angular_travel = ATAN2(rvec.a * rt_Y - rvec.b * rt_X, rvec.a * rt_X + rvec.b * rt_Y);
-
   #ifdef MIN_ARC_SEGMENTS
     uint16_t min_segments = MIN_ARC_SEGMENTS;
   #else
     constexpr uint16_t min_segments = 1;
   #endif
 
-  // Do a full circle if angular rotation is near 0 and the target is current position
-  if (!angular_travel || (NEAR_ZERO(angular_travel) && NEAR(current_position[p_axis], cart[p_axis]) && NEAR(current_position[q_axis], cart[q_axis]))) {
+  // Angle of rotation between position and target from the circle center.
+  float angular_travel;
+
+  // Do a full circle if starting and ending positions are "identical"
+  if (NEAR(current_position[p_axis], cart[p_axis]) && NEAR(current_position[q_axis], cart[q_axis])) {
     // Preserve direction for circles
     angular_travel = clockwise ? -RADIANS(360) : RADIANS(360);
   }
   else {
+    // Calculate the angle
+    angular_travel = ATAN2(rvec.a * rt_Y - rvec.b * rt_X, rvec.a * rt_X + rvec.b * rt_Y);
+
+    // Angular travel too small to detect? Just return.
+    if (!angular_travel) return;
+
     // Make sure angular travel over 180 degrees goes the other way around.
     switch (((angular_travel < 0) << 1) | clockwise) {
       case 1: angular_travel -= RADIANS(360); break; // Positive but CW? Reverse direction.
       case 2: angular_travel += RADIANS(360); break; // Negative but CCW? Reverse direction.
     }
+
     #ifdef MIN_ARC_SEGMENTS
       min_segments = CEIL(min_segments * ABS(angular_travel) / RADIANS(360));
       NOLESS(min_segments, 1U);