Lv_obj_invalidate_area not doing anything

How-to
1

What do you want to achieve?

I want to manually update parts of an lv_canvas

The canvas is an overview waveform and I want to add a 2px wide white line indicating the current play position

What have you tried so far?

lv_obj_invalidate works so far, but eats up processing time I need to spare, hence why I want to use lv_obj_invalidate_area

Code to reproduce

The function below draws the white line in the new position and restores the waveform lines in the old position

static uint16_t oldX = 0xFFFF; // Initialize to invalid position
// New function to update position efficiently
FASTRUN void updatePlaybackPosition(uint16_t newX)
{
  if(oldX == newX) return; // No change, skip update
  // Restore the old position by redrawing the waveform data
  if (oldX < chartWidth && oldX + 1 < chartWidth) {
    // Clear the old indicator position (both pixels)
    for (uint8_t px = 0; px < 2; px++) {
      uint16_t xPos = oldX + px;
      drawFastVLine16BitOverview(xPos, 0, overviewChartHeight, 0x0000, 
                                 overviewCanvasBuffer, chartWidth);
      // Redraw the waveform at old position
      for (uint8_t i = 0; i < 3; i++) {
        uint16_t height = overViewWaveSampleData[i][xPos];
        uint16_t startY = overviewChartHeight - height;
        
        drawFastVLine16BitOverview(xPos, startY, height, waveformColors[i], 
                                   overviewCanvasBuffer, chartWidth);
      }
    }
    
    // Invalidate old area
    lv_area_t area;
    area.x1 = oldX;
    area.y1 = 0;
    area.x2 = oldX + 1;
    area.y2 = overviewChartHeight - 1;
    lv_obj_invalidate_area(static_waveform_canvas, &area);
  }
  
  // Draw the new position indicator (2px wide)
  if (newX < chartWidth && newX + 1 < chartWidth) {
    uint16_t indicatorColor = 0xFFFF; // White, or use your preferred indicator color
    
    for (uint8_t px = 0; px < 2; px++) {
      uint16_t xPos = newX + px;
      
      // Draw full-height indicator line
      drawFastVLine16BitOverview(xPos, 0, overviewChartHeight, indicatorColor, 
                                 overviewCanvasBuffer, chartWidth);
    }
    
    // Invalidate new area
    lv_area_t area;
    area.x1 = newX;
    area.y1 = 0;
    area.x2 = newX + 1;
    area.y2 = overviewChartHeight - 1;
    lv_obj_invalidate_area(static_waveform_canvas, &area);
  }
  
  // Update stored position
  oldX = newX;
}

Screenshot and/or video

The bottom waveform:

IMG_6777
IMG_67774007×2576 1.92 MB

Environment

  • MCU/MPU/Board: Custom Micromod Teensy with SDRAM
  • LVGL version: v8.4