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Displays

important:The basic concept of a display in LVGL is explained in the :ref:`porting` section. So before reading further, please read that section first.

Multiple display support

In LVGL you can have multiple displays, each with their own driver, widgets and color depth.

Creating more displays is easy: just use :cpp:func:`lv_display_create` and add set the buffer and the flush_cb. When you create the UI, use :cpp:expr:`lv_display_set_default(disp)` to tell the library on which display to create objects.

Why would you want multi-display support? Here are some examples:

  • Have a "normal" TFT display with local UI and create "virtual" screens on VNC on demand. (You need to add your VNC driver).
  • Have a large TFT display and a small monochrome display.
  • Have some smaller and simple displays in a large instrument or technology.
  • Have two large TFT displays: one for a customer and one for the shop assistant.

Using only one display

Using more displays can be useful but in most cases it's not required. Therefore, the whole concept of multi-display handling is completely hidden if you register only one display. By default, the last created (and only) display is used.

:cpp:func:`lv_screen_active`, :cpp:func:`lv_screen_load`, :cpp:func:`lv_layer_top`, :cpp:func:`lv_layer_sys`, :c:macro:`LV_HOR_RES` and :c:macro:`LV_VER_RES` are always applied on the most recently created (default) display. If you pass NULL as disp parameter to display related functions the default display will usually be used. E.g. :cpp:expr:`lv_display_trigger_activity(NULL)` will trigger a user activity on the default display. (See below in :ref:`Inactivity <display_inactivity>`).

Mirror display

To mirror the image of a display to another display, you don't need to use multi-display support. Just transfer the buffer received in flush_cb to the other display too.

Split image

You can create a larger virtual display from an array of smaller ones. You can create it as below: 1. Set the resolution of the displays to the large display's resolution. 2. In flush_cb, truncate and modify the area parameter for each display. 3. Send the buffer's content to each real display with the truncated area.

Screens

Every display has its own set of :ref:`screens <objects_screens>` and the objects on each screen.

Be sure not to confuse displays and screens:

  • Displays are the physical hardware drawing the pixels.
  • Screens are the high-level root objects associated with a particular display. One display can have multiple screens associated with it, but not vice versa.

Screens can be considered the highest level containers which have no parent. A screen's size is always equal to its display and their origin is (0;0). Therefore, a screen's coordinates can't be changed, i.e. :cpp:func:`lv_obj_set_pos`, :cpp:func:`lv_obj_set_size` or similar functions can't be used on screens.

A screen can be created from any object type but the two most typical types are :ref:`Base object <lv_obj>` and :ref:`Image <lv_image>` (to create a wallpaper).

To create a screen, use :cpp:expr:`lv_obj_t * scr = lv_<type>_create(NULL)`. NULL indicates no parent.

To load a screen, use :cpp:expr:`lv_screen_load(scr)`. To get the active screen, use :cpp:expr:`lv_screen_active()`. These functions work on the default display. If you want to specify which display to work on, use :cpp:expr:`lv_display_get_screen_active(disp)` and :cpp:expr:`lv_display_load_screen(disp, scr)`. A screen can be loaded with animations too. Read more :ref:`here <objects_load_screens>`.

Screens can be deleted with :cpp:expr:`lv_obj_delete(scr)`, but ensure that you do not delete the currently loaded screen.

Transparent screens

Usually, the opacity of the screen is :cpp:enumerator:`LV_OPA_COVER` to provide a solid background for its children. If this is not the case (opacity < 100%) the display's bottom_layer be visible. If the bottom layer's opacity is also not :cpp:enumerator:`LV_OPA_COVER` LVGL has no solid background to draw.

This configuration (transparent screen and display) could be used to create for example OSD menus where a video is played on a lower layer, and a menu is overlaid on an upper layer.

To properly render the screen the display's color format needs to be set to one with alpha channel.

In summary, to enable transparent screens and displays for OSD menu-like UIs:

Features of displays

Inactivity

A user's inactivity time is measured on each display. Every use of an :ref:`Input device <indev>` (if :ref:`associated with the display <porting_indev_other_features>`) counts as an activity. To get time elapsed since the last activity, use :cpp:expr:`lv_display_get_inactive_time(disp)`. If NULL is passed, the lowest inactivity time among all displays will be returned (NULL isn't just the default display).

You can manually trigger an activity using :cpp:expr:`lv_display_trigger_activity(disp)`. If disp is NULL, the default screen will be used (and not all displays).

Background

Every display has a background color, background image and background opacity properties. They become visible when the current screen is transparent or not positioned to cover the whole display.

The background color is a simple color to fill the display. It can be adjusted with :cpp:expr:`lv_obj_set_style_bg_color(obj, color)`;

The display background image is a path to a file or a pointer to an :cpp:struct:`lv_image_dsc_t` variable (converted image data) to be used as wallpaper. It can be set with :cpp:expr:`lv_obj_set_style_bg_image_src(obj, &my_img)`; If a background image is configured the background won't be filled with bg_color.

The opacity of the background color or image can be adjusted with :cpp:expr:`lv_obj_set_style_bg_opa(obj, opa)`.

The disp parameter of these functions can be NULL to select the default display.

API