ndg/lib/lvgl/docs/overview/display.md

Displays

Multiple display support

In LVGL you can have multiple displays, each with their own driver and objects. The only limitation is that every display needs to have the same color depth (as defined in LV_COLOR_DEPTH). If the displays are different in this regard the rendered image can be converted to the correct format in the drivers flush_cb.

Creating more displays is easy: just initialize more display buffers and register another driver for every display. When you create the UI, use lv_disp_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.

lv_scr_act(), lv_scr_load(scr), lv_layer_top(), lv_layer_sys(), LV_HOR_RES and 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. lv_disp_trig_activity(NULL) will trigger a user activity on the default display. (See below in 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 drv.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 drv.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 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. lv_obj_set_pos(), 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 Base object and Image (to create a wallpaper).

To create a screen, use lv_obj_t * scr = lv_<type>_create(NULL, copy). copy can be an existing screen copied into the new screen.

To load a screen, use lv_scr_load(scr). To get the active screen, use lv_scr_act(). These functions work on the default display. If you want to specify which display to work on, use lv_disp_get_scr_act(disp) and lv_disp_load_scr(disp, scr). A screen can be loaded with animations too. Read more here.

Screens can be deleted with lv_obj_del(scr), but ensure that you do not delete the currently loaded screen.

Transparent screens

Usually, the opacity of the screen is LV_OPA_COVER to provide a solid background for its children. If this is not the case (opacity < 100%) the display's background color or image will be visible. See the Display background section for more details. If the display's background opacity is also not 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 overlayed on an upper layer.

To handle transparent displays, special (slower) color mixing algorithms need to be used by LVGL so this feature needs to enabled with LV_COLOR_SCREEN_TRANSP in lv_conf.h. As this mode operates on the Alpha channel of the pixels LV_COLOR_DEPTH = 32 is also required. The Alpha channel of 32-bit colors will be 0 where there are no objects and 255 where there are solid objects.

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

• Enable LV_COLOR_SCREEN_TRANSP in lv_conf.h
• Be sure to use LV_COLOR_DEPTH 32
• Set the screen's opacity to LV_OPA_TRANSP e.g. with lv_obj_set_style_local_bg_opa(lv_scr_act(), LV_OBJMASK_PART_MAIN, LV_STATE_DEFAULT, LV_OPA_TRANSP)
• Set the display opacity to LV_OPA_TRANSP with lv_disp_set_bg_opa(NULL, LV_OPA_TRANSP);

Features of displays

Inactivity

A user's inactivity time is measured on each display. Every use of an Input device (if associated with the display) counts as an activity. To get time elapsed since the last activity, use lv_disp_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 lv_disp_trig_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 lv_disp_set_bg_color(disp, color);

The display background image is a path to a file or a pointer to an lv_img_dsc_t variable (converted image data) to be used as wallpaper. It can be set with lv_disp_set_bg_image(disp, &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 lv_disp_set_bg_opa(disp, opa).

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

API

.. doxygenfile:: lv_disp.h
  :project: lvgl