Planner anti-stutter by Sebastian Popp

Marlin/src/module/planner.cpp

 
   // Initialize block entry speed. Compute based on deceleration to user-defined MINIMUM_PLANNER_SPEED.
   const float v_allowable = max_allowable_speed(-block->acceleration, MINIMUM_PLANNER_SPEED, block->millimeters);
-  block->entry_speed = min(vmax_junction, v_allowable);
+  // If stepper ISR is disabled, this indicates buffer_segment wants to add a split block.
+  // In this case start with the max. allowed speed to avoid an interrupted first move.
+  block->entry_speed = STEPPER_ISR_ENABLED() ? MINIMUM_PLANNER_SPEED : min(vmax_junction, v_allowable);
 
   // Initialize planner efficiency flags
   // Set flag if block will always reach maximum junction speed regardless of entry/exit speeds.
   // block nominal speed limits both the current and next maximum junction speeds. Hence, in both
   // the reverse and forward planners, the corresponding block junction speed will always be at the
   // the maximum junction speed and may always be ignored for any speed reduction checks.
-  block->flag |= BLOCK_FLAG_RECALCULATE | (block->nominal_speed <= v_allowable ? BLOCK_FLAG_NOMINAL_LENGTH : 0);
+  block->flag |= block->nominal_speed <= v_allowable ? BLOCK_FLAG_RECALCULATE | BLOCK_FLAG_NOMINAL_LENGTH : BLOCK_FLAG_RECALCULATE;
 
   // Update previous path unit_vector and nominal speed
   COPY(previous_speed, current_speed);
      *                                      In that case, the retract and move will be executed together.
      *                                      This leads to too many advance steps due to a huge e_acceleration.
      *                                      The math is good, but we must avoid retract moves with advance!
-     * lin_dist_e > 0                     : Extruder is running forward (e.g., for "Wipe while retracting" (Slic3r) or "Combing" (Cura) moves)
+     * lin_dist_e > 0                       : Extruder is running forward (e.g., for "Wipe while retracting" (Slic3r) or "Combing" (Cura) moves)
      */
     block->use_advance_lead =  esteps && (block->steps[X_AXIS] || block->steps[Y_AXIS])
                             && extruder_advance_k
 
   #endif // LIN_ADVANCE
 
-  const float bnsr = 1.0 / block->nominal_speed;
-  calculate_trapezoid_for_block(block, block->entry_speed * bnsr, safe_speed * bnsr);
-
   // Move buffer head
   block_buffer_head = next_buffer_head;
 

Marlin/src/module/planner.h

     static block_t* get_current_block() {
       if (blocks_queued()) {
         block_t * const block = &block_buffer[block_buffer_tail];
+
+        // If the block has no trapezoid calculated, it's unsafe to execute.
+        if (movesplanned() > 1) {
+          const block_t * const next = &block_buffer[next_block_index(block_buffer_tail)];
+          if (TEST(block->flag, BLOCK_BIT_RECALCULATE) || TEST(next->flag, BLOCK_BIT_RECALCULATE))
+            return NULL;
+        }
+        else if (TEST(block->flag, BLOCK_BIT_RECALCULATE))
+          return NULL;
+
         #if ENABLED(ULTRA_LCD)
           block_buffer_runtime_us -= block->segment_time_us; // We can't be sure how long an active block will take, so don't count it.
         #endif