GRBL的Bresenham算法的问题

五花肉老乌龟

最近读了一下GRBL的布雷森汉姆直线算法有一个地方不太明白，希望论坛中的大牛给以指导。先看资料

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为什么初始化时counter_x要等于-0.5step_event_count
这个地方不太理解。
下面是整个算法的实现部分有懂的给解释一下。
ISR(TIMER1_COMPA_vect)
{
  if (busy) { return; } // The busy-flag is used to avoid reentering this interrupt

  // Set the direction pins a couple of nanoseconds before we step the steppers
  DIRECTION_PORT = (DIRECTION_PORT & ~DIRECTION_MASK) | (st.dir_outbits & DIRECTION_MASK);

  // Then pulse the stepping pins
  #ifdef STEP_PULSE_DELAY
    st.step_bits = (STEP_PORT & ~STEP_MASK) | st.step_outbits; // Store out_bits to prevent overwriting.
  #else  // Normal operation
    STEP_PORT = (STEP_PORT & ~STEP_MASK) | st.step_outbits;
  #endif

  // Enable step pulse reset timer so that The Stepper Port Reset Interrupt can reset the signal after
  // exactly settings.pulse_microseconds microseconds, independent of the main Timer1 prescaler.
  TCNT0 = st.step_pulse_time; // Reload Timer0 counter
  TCCR0B = (1<<CS01); // Begin Timer0. Full speed, 1/8 prescaler

  busy = true;
  sei(); // Re-enable interrupts to allow Stepper Port Reset Interrupt to fire on-time.
         // NOTE: The remaining code in this ISR will finish before returning to main program.

  // If there is no step segment, attempt to pop one from the stepper buffer
  if (st.exec_segment == NULL) {
    // Anything in the buffer? If so, load and initialize next step segment.
    if (segment_buffer_head != segment_buffer_tail) {
      // Initialize new step segment and load number of steps to execute
      st.exec_segment = &segment_buffer[segment_buffer_tail];

      #ifndef ADAPTIVE_MULTI_AXIS_STEP_SMOOTHING
        // With AMASS is disabled, set timer prescaler for segments with slow step frequencies (< 250Hz).
        TCCR1B = (TCCR1B & ~(0x07<<CS10)) | (st.exec_segment->prescaler<<CS10);
      #endif

      // Initialize step segment timing per step and load number of steps to execute.
      //OCR1A用来设置计数的上限值
      OCR1A = st.exec_segment->cycles_per_tick;
      st.step_count = st.exec_segment->n_step; // NOTE: Can sometimes be zero when moving slow.
      // If the new segment starts a new planner block, initialize stepper variables and counters.
      // NOTE: When the segment data index changes, this indicates a new planner block.
      if ( st.exec_block_index != st.exec_segment->st_block_index ) {
        st.exec_block_index = st.exec_segment->st_block_index;
        st.exec_block = &st_block_buffer[st.exec_block_index];

        // Initialize Bresenham line and distance counters
        st.counter_x = st.counter_y = st.counter_z = (st.exec_block->step_event_count >> 1);
      }
      st.dir_outbits = st.exec_block->direction_bits ^ dir_port_invert_mask;

      #ifdef ADAPTIVE_MULTI_AXIS_STEP_SMOOTHING
        // With AMASS enabled, adjust Bresenham axis increment counters according to AMASS level.
        st.steps[X_AXIS] = st.exec_block->steps[X_AXIS] >> st.exec_segment->amass_level;
        st.steps[Y_AXIS] = st.exec_block->steps[Y_AXIS] >> st.exec_segment->amass_level;
        st.steps[Z_AXIS] = st.exec_block->steps[Z_AXIS] >> st.exec_segment->amass_level;
      #endif

      #ifdef VARIABLE_SPINDLE
        // Set real-time spindle output as segment is loaded, just prior to the first step.
        spindle_set_speed(st.exec_segment->spindle_pwm);
      #endif

    } else {
      // Segment buffer empty. Shutdown.
      st_go_idle();
      #ifdef VARIABLE_SPINDLE
        // Ensure pwm is set properly upon completion of rate-controlled motion.
        if (st.exec_block->is_pwm_rate_adjusted) { spindle_set_speed(SPINDLE_PWM_OFF_VALUE); }
      #endif
      system_set_exec_state_flag(EXEC_CYCLE_STOP); // Flag main program for cycle end
      return; // Nothing to do but exit.
    }
  }


  // Check probing state.
  if (sys_probe_state == PROBE_ACTIVE) { probe_state_monitor(); }

  // Reset step out bits.
  st.step_outbits = 0;

  // Execute step displacement profile by Bresenham line algorithm
  #ifdef ADAPTIVE_MULTI_AXIS_STEP_SMOOTHING
    st.counter_x += st.steps[X_AXIS];
  #else
    st.counter_x += st.exec_block->steps[X_AXIS];
  #endif
  if (st.counter_x > st.exec_block->step_event_count) {
    st.step_outbits |= (1<<X_STEP_BIT);
    st.counter_x -= st.exec_block->step_event_count;
    if (st.exec_block->direction_bits & (1<<X_DIRECTION_BIT)) { sys_position[X_AXIS]--; }
    else { sys_position[X_AXIS]++; }
  }
  #ifdef ADAPTIVE_MULTI_AXIS_STEP_SMOOTHING
    st.counter_y += st.steps[Y_AXIS];
  #else
    st.counter_y += st.exec_block->steps[Y_AXIS];
  #endif
  if (st.counter_y > st.exec_block->step_event_count) {
    st.step_outbits |= (1<<Y_STEP_BIT);
    st.counter_y -= st.exec_block->step_event_count;
    if (st.exec_block->direction_bits & (1<<Y_DIRECTION_BIT)) { sys_position[Y_AXIS]--; }
    else { sys_position[Y_AXIS]++; }
  }
  #ifdef ADAPTIVE_MULTI_AXIS_STEP_SMOOTHING
    st.counter_z += st.steps[Z_AXIS];
  #else
    st.counter_z += st.exec_block->steps[Z_AXIS];
  #endif
  if (st.counter_z > st.exec_block->step_event_count) {
    st.step_outbits |= (1<<Z_STEP_BIT);
    st.counter_z -= st.exec_block->step_event_count;
    if (st.exec_block->direction_bits & (1<<Z_DIRECTION_BIT)) { sys_position[Z_AXIS]--; }
    else { sys_position[Z_AXIS]++; }
  }

  // During a homing cycle, lock out and prevent desired axes from moving.
  if (sys.state == STATE_HOMING) { st.step_outbits &= sys.homing_axis_lock; }

  st.step_count--; // Decrement step events count
  if (st.step_count == 0) {
    // Segment is complete. Discard current segment and advance segment indexing.
    st.exec_segment = NULL;
    if ( ++segment_buffer_tail == SEGMENT_BUFFER_SIZE) { segment_buffer_tail = 0; }
  }

  st.step_outbits ^= step_port_invert_mask;  // Apply step port invert mask
  busy = false;
}

正点原子

百度

五花肉老乌龟

有人知道吗？