```eval_rst
.. include:: /header.rst 
:github_url: |github_link_base|/overview/display.md
```
# Displays

``` important:: The basic concept of *display* in LVGL is explained in the [Porting](/porting/display) section. So before reading further, please read the [Porting](/porting/display) section first.
```

## 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 is completely hidden if you register only one display.
By default, the lastly 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) screen.
If you pass `NULL` as `disp` parameter to display related function, usually the default display will be used.
E.g. `lv_disp_trig_activity(NULL)` will trigger a user activity on the default screen. (See below in [Inactivity](#Inactivity)).

### Mirror display

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

### Split image
You can create a larger display from 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 display with the truncated area.

## Screens

Every display has its own set of [screens](overview/object#screen-the-most-basic-parent) and the object on the screens.

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.
The screen's size is always equal to its display and size their position is (0;0). Therefore, the screens 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 the [Base object](/widgets/obj) and the [Image](/widgets/core/img) (to create a wallpaper).

To create a screen, use `lv_obj_t * scr = lv_<type>_create(NULL, copy)`. `copy` can be another screen to copy it.

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)`. Screen can be loaded with animations too. Read more [here](object.html#load-screens).

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 it's not the case (opacity &lt; 100%) the display's background color or image will be visible. 
See 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. 

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 screen and displays to create 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

The user's inactivity 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.
To get time elapsed since the last activity, use `lv_disp_get_inactive_time(disp)`. If `NULL` is passed, the overall smallest inactivity time will be returned from all displays (**not 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 background color, a background image and background opacity properties. They become visible when the current screen is transparent or not positioned to cover the whole display.

Background color is a simple color to fill the display. It can be adjusted with `lv_disp_set_bg_color(disp, color)`;

Background image is a path to a file or a pointer to an `lv_img_dsc_t` variable (converted image) to be used as wallpaper. It can be set with `lv_disp_set_bg_color(disp, &my_img)`;
If the background image is set (not `NULL`) 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 refer it to the default display.



## API

```eval_rst

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

```
