🐛 Fix G2/G3 Arcs stutter / JD speed (#24362)

Marlin/src/gcode/motion/G2_G3.cpp

@@ -197,8 +197,8 @@ void plan_arc(
   // Feedrate for the move, scaled by the feedrate multiplier
   const feedRate_t scaled_fr_mm_s = MMS_SCALED(feedrate_mm_s);
 
-  // Get the nominal segment length based on settings
-  const float nominal_segment_mm = (
+  // Get the ideal segment length for the move based on settings
+  const float ideal_segment_mm = (
     #if ARC_SEGMENTS_PER_SEC  // Length based on segments per second and feedrate
       constrain(scaled_fr_mm_s * RECIPROCAL(ARC_SEGMENTS_PER_SEC), MIN_ARC_SEGMENT_MM, MAX_ARC_SEGMENT_MM)
     #else
@@ -206,19 +206,18 @@ void plan_arc(
     #endif
   );
 
-  // Number of whole segments based on the nominal segment length
-  const float nominal_segments = _MAX(FLOOR(flat_mm / nominal_segment_mm), min_segments);
+  // Number of whole segments based on the ideal segment length
+  const float nominal_segments = _MAX(FLOOR(flat_mm / ideal_segment_mm), min_segments),
+              nominal_segment_mm = flat_mm / nominal_segments;
 
-  // A new segment length based on the required minimum
-  const float segment_mm = constrain(flat_mm / nominal_segments, MIN_ARC_SEGMENT_MM, MAX_ARC_SEGMENT_MM);
+  // The number of whole segments in the arc, with best attempt to honor MIN_ARC_SEGMENT_MM and MAX_ARC_SEGMENT_MM
+  const uint16_t segments = nominal_segment_mm > (MAX_ARC_SEGMENT_MM) ? CEIL(flat_mm / (MAX_ARC_SEGMENT_MM)) :
+                            nominal_segment_mm < (MIN_ARC_SEGMENT_MM) ? _MAX(1, FLOOR(flat_mm / (MIN_ARC_SEGMENT_MM))) :
+                            nominal_segments;
 
-  // The number of whole segments in the arc, ignoring the remainder
-  uint16_t segments = FLOOR(flat_mm / segment_mm);
-
-  // Are the segments now too few to reach the destination?
-  const float segmented_length = segment_mm * segments;
-  const bool tooshort = segmented_length < flat_mm - 0.0001f;
-  const float proportion = tooshort ? segmented_length / flat_mm : 1.0f;
+  #if ENABLED(SCARA_FEEDRATE_SCALING)
+    const float inv_duration = (scaled_fr_mm_s / flat_mm) * segments;
+  #endif
 
   /**
    * Vector rotation by transformation matrix: r is the original vector, r_T is the rotated vector,
@@ -246,29 +245,30 @@ void plan_arc(
    * a correction, the planner should have caught up to the lag caused by the initial plan_arc overhead.
    * This is important when there are successive arc motions.
    */
-  // Vector rotation matrix values
+
   xyze_pos_t raw;
-  const float theta_per_segment = proportion * angular_travel / segments,
+
+  // do not calculate rotation parameters for trivial single-segment arcs
+  if (segments > 1) {
+    // Vector rotation matrix values
+    const float theta_per_segment = angular_travel / segments,
                 sq_theta_per_segment = sq(theta_per_segment),
                 sin_T = theta_per_segment - sq_theta_per_segment * theta_per_segment / 6,
                 cos_T = 1 - 0.5f * sq_theta_per_segment; // Small angle approximation
 
     #if DISABLED(AUTO_BED_LEVELING_UBL)
       ARC_LIJKUVW_CODE(
-      const float per_segment_L = proportion * travel_L / segments,
-      const float per_segment_I = proportion * travel_I / segments,
-      const float per_segment_J = proportion * travel_J / segments,
-      const float per_segment_K = proportion * travel_K / segments,
-      const float per_segment_U = proportion * travel_U / segments,
-      const float per_segment_V = proportion * travel_V / segments,
-      const float per_segment_W = proportion * travel_W / segments
+        const float per_segment_L = travel_L / segments,
+        const float per_segment_I = travel_I / segments,
+        const float per_segment_J = travel_J / segments,
+        const float per_segment_K = travel_K / segments,
+        const float per_segment_U = travel_U / segments,
+        const float per_segment_V = travel_V / segments,
+        const float per_segment_W = travel_W / segments
       );
     #endif
 
-  CODE_ITEM_E(const float extruder_per_segment = proportion * travel_E / segments);
-
-  // For shortened segments, run all but the remainder in the loop
-  if (tooshort) segments++;
+    CODE_ITEM_E(const float extruder_per_segment = travel_E / segments);
 
     // Initialize all linear axes and E
     ARC_LIJKUVWE_CODE(
@@ -282,10 +282,6 @@ void plan_arc(
       raw.e       = current_position.e
     );
 
-  #if ENABLED(SCARA_FEEDRATE_SCALING)
-    const float inv_duration = scaled_fr_mm_s / segment_mm;
-  #endif
-
     millis_t next_idle_ms = millis() + 200UL;
 
     #if N_ARC_CORRECTION > 1
@@ -349,6 +345,7 @@ void plan_arc(
       if (!planner.buffer_line(raw, scaled_fr_mm_s, active_extruder, 0 OPTARG(SCARA_FEEDRATE_SCALING, inv_duration)))
         break;
     }
+  }
 
   // Ensure last segment arrives at target location.
   raw = cart;