How to determine the exact pixel size of a Chart's plotting area?

How-to
#1

Description

I have a working chart where I plot a real-time stream of values. I currently set the chart to have 100 points, but would to set it to the exact plot area width in pixel to have max resolution data. Same go for the setting the Y range. How do I determine the exact size of the plot area?

What MCU/Processor/Board and compiler are you using?

STM32F401

What LVGL version are you using?

LVGL_VERSION_MAJOR 7
LVGL_VERSION_MINOR 5
LVGL_VERSION_PATCH 0
LVGL_VERSION_INFO “dev”

What do you want to achieve?

My chart having number of points which is exactly the width of the plot area in pixels.

What have you tried so far?

Looked for an lv function that returns the size of Chart’s plot area.

Code to reproduce

This is how I initialize the chart styles:

static void init_chart_styles() {
  // Background style
  lv_style_init(&chart_styles.bg);
  lv_style_set_radius(&chart_styles.bg, LV_STATE_DEFAULT, 0);
  lv_style_set_bg_color(&chart_styles.bg, LV_STATE_DEFAULT,
                        LV_COLOR_BLACK);  // BLACK

  lv_style_set_text_font(&chart_styles.bg, LV_STATE_DEFAULT,
                         &font_montserrat_alphanum_12);
  lv_style_set_text_color(&chart_styles.bg, LV_STATE_DEFAULT, LV_COLOR_SILVER);

  lv_style_set_border_width(&chart_styles.bg, LV_STATE_DEFAULT, 0);
  lv_style_set_outline_width(&chart_styles.bg, LV_STATE_DEFAULT, 0);

  lv_style_set_pad_top(&chart_styles.bg, LV_STATE_DEFAULT, 10);
  lv_style_set_pad_bottom(&chart_styles.bg, LV_STATE_DEFAULT, 10);
  lv_style_set_pad_right(&chart_styles.bg, LV_STATE_DEFAULT, 0);
  lv_style_set_pad_left(&chart_styles.bg, LV_STATE_DEFAULT, 25);

  // Series style
  lv_style_init(&chart_styles.series);
  lv_style_set_size(&chart_styles.series, LV_STATE_DEFAULT, 0);
  lv_style_set_line_width(&chart_styles.series, LV_STATE_DEFAULT, 2);

  // Series bg style
  lv_style_init(&chart_styles.series_bg);
  lv_style_set_line_dash_gap(&chart_styles.series_bg, LV_STATE_DEFAULT, 0);
  lv_style_set_line_color(&chart_styles.series_bg, LV_STATE_DEFAULT,
                          LV_COLOR_MAKE(0x00, 0x40, 0x00));  // dark green
}

And this is how I create the chart:

void create_chart(const Screen& screen, Chart* chart) {
  init_styles_if_needed();

  lv_obj_t* lv_chart = lv_chart_create(screen.lv_screen, NULL);

  lv_obj_set_size(lv_chart, 420, 220);

  lv_obj_align(lv_chart, NULL, LV_ALIGN_CENTER, 0, 0);
  lv_chart_set_type(lv_chart, LV_CHART_TYPE_LINE);

  lv_chart_set_update_mode(lv_chart, LV_CHART_UPDATE_MODE_SHIFT);

  // TODO: set this to actual number of X pixels.
  lv_chart_set_point_count(lv_chart, 100);

  /*Add two data series*/
  lv_chart_series_t* lv_series1 =
      lv_chart_add_series(lv_chart, LV_COLOR_YELLOW);
  lv_chart_series_t* lv_series2 =
      lv_chart_add_series(lv_chart, LV_COLOR_ORANGE);

  lv_obj_add_style(lv_chart, LV_CHART_PART_BG,
                   &chart_styles.bg);  // apply background style
  lv_obj_add_style(lv_chart, LV_CHART_PART_SERIES,
                   &chart_styles.series);  // Apply part series style.
  lv_obj_add_style(lv_chart, LV_CHART_PART_SERIES_BG,
                   &chart_styles.series_bg);  // apply series background style

  lv_chart_set_y_tick_length(lv_chart, 0, 0);
  lv_chart_set_y_tick_texts(lv_chart, "-2\n-1\n0\n1\n2", 3,
                            LV_CHART_AXIS_DRAW_LAST_TICK);

  chart->lv_chart = lv_chart;
  chart->ser1.lv_chart = lv_chart;
  chart->ser1.lv_series = lv_series1;
  chart->ser2.lv_chart = lv_chart;
  chart->ser2.lv_series = lv_series2;
}

Screenshot and/or video

If possible, add screenshots and/or videos about the current state.

#2

… being able to control or query the the plot area size in pixels also helps in the general case, making sure that the pitch of chart points is a multiple of a whole pixel. This provides better consistency and smoothness of the display.

#3

Just to confirm: in X if you have only 3 points from the stream you want those 3 points to span through the whole width?

#4

It goes along these lines

n = chart size in points.
perform every 1 sec {
capture from ADC n points.
update the chart to show the newly captured n points (line mode)
}

That is, on every update I write all the chart points. My question is, how can I make n equal to the chart’s width in pixels such each of the n displayed points will span exactly one pixel.

Preferably I would like to size the chart that it has plot area of n = 200 pixels and then capture and display 200 points. Alternatively, query the chart for whatever size in pixels it has and then set that exact number of points.

Edit: my goal is to display a captured signal at max screen resolution such that the horizontal spacing between data points is exactly 1 pixels. Number of data points can be adapted to the actual width in pixels of the chart plot area.

#5

Hi @kisvegabor, let me rephrase the question.

Let’s say that I drew the chart below, how to I set the number of series points to have max possible resolution on the current display? (I presume is one data point per screen pixel)

signal_good

#6

I see now.

You can use this:

lv_area_t a;
lv_chart_get_series_area(chart, &a);
int n = lv_area_get_width(&a);
#7

Thanks @kisvegabor. This is what I was looking for.