1# Displays
2
3``` important:: The basic concept of a *display* in LVGL is explained in the [Porting](/porting/display) section. So before reading further, please read the [Porting](/porting/display) section first.
4```
5
6## Multiple display support
7
8In 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`).
9If the displays are different in this regard the rendered image can be converted to the correct format in the drivers `flush_cb`.
10
11Creating more displays is easy: just initialize more display buffers and register another driver for every display.
12When you create the UI, use `lv_disp_set_default(disp)` to tell the library on which display to create objects.
13
14Why would you want multi-display support? Here are some examples:
15- Have a "normal" TFT display with local UI and create "virtual" screens on VNC on demand. (You need to add your VNC driver).
16- Have a large TFT display and a small monochrome display.
17- Have some smaller and simple displays in a large instrument or technology.
18- Have two large TFT displays: one for a customer and one for the shop assistant.
19
20### Using only one display
21Using 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.
22By default, the last created (and only) display is used.
23
24`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.
25If you pass `NULL` as `disp` parameter to display related functions the default display will usually be used.
26E.g. `lv_disp_trig_activity(NULL)` will trigger a user activity on the default display. (See below in [Inactivity](#Inactivity)).
27
28### Mirror display
29
30To 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.
31
32### Split image
33You can create a larger virtual display from an array of smaller ones. You can create it as below:
341. Set the resolution of the displays to the large display's resolution.
352. In `drv.flush_cb`, truncate and modify the `area` parameter for each display.
363. Send the buffer's content to each real display with the truncated area.
37
38## Screens
39
40Every display has its own set of [screens](overview/object#screen-the-most-basic-parent) and the objects on each screen.
41
42Be sure not to confuse displays and screens:
43
44* **Displays** are the physical hardware drawing the pixels.
45* **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.
46
47Screens can be considered the highest level containers which have no parent.
48A 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.
49
50A screen can be created from any object type but the two most typical types are [Base object](/widgets/obj) and [Image](/widgets/core/img) (to create a wallpaper).
51
52To create a screen, use `lv_obj_t * scr = lv_<type>_create(NULL, copy)`. `copy` can be an existing screen copied into the new screen.
53
54To 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](object.html#load-screens).
55
56Screens can be deleted with `lv_obj_del(scr)`, but ensure that you do not delete the currently loaded screen.
57
58### Transparent screens
59
60Usually, the opacity of the screen is `LV_OPA_COVER` to provide a solid background for its children. If this is not the case (opacity &lt; 100%) the display's background color or image will be visible.
61See the [Display background](#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.
62
63This 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.
64
65To 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`.
66The Alpha channel of 32-bit colors will be 0 where there are no objects and 255 where there are solid objects.
67
68In summary, to enable transparent screens and displays for OSD menu-like UIs:
69- Enable `LV_COLOR_SCREEN_TRANSP` in `lv_conf.h`
70- Set the screen's opacity to `LV_OPA_TRANSP` e.g. with `lv_obj_set_style_bg_opa(lv_scr_act(), LV_OPA_TRANSP, LV_PART_MAIN)`
71- Set the display opacity to `LV_OPA_TRANSP` with `lv_disp_set_bg_opa(NULL, LV_OPA_TRANSP);`
72
73## Features of displays
74
75### Inactivity
76
77A user's inactivity time is measured on each display. Every use of an [Input device](/overview/indev) (if [associated with the display](/porting/indev#other-features)) counts as an activity.
78To 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**).
79
80You 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**).
81
82### Background
83Every 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.
84
85The background color is a simple color to fill the display. It can be adjusted with `lv_disp_set_bg_color(disp, color)`;
86
87The 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)`;
88If a background image is configured the background won't be filled with `bg_color`.
89
90The opacity of the background color or image can be adjusted with `lv_disp_set_bg_opa(disp, opa)`.
91
92The `disp` parameter of these functions can be `NULL` to select the default display.
93
94
95
96## API
97
98```eval_rst
99
100.. doxygenfile:: lv_disp.h
101  :project: lvgl
102
103```
104