In LVGL "layers" can be interpreted in various ways:

  1. The order of widget creation naturally creates a layering of widgets

  2. Permanent screen-sized layers can be also used

  3. For some draw operations LVGL renders a widget and all its children into a buffer (a.k.a. layer) first

Order of creation

By default, LVGL draws new objects on top of old objects.

For example, assume we add a button to a parent object named button1 and then another button named button2. Then button1 (along with its child object(s)) will be in the background and can be covered by button2 and its children.

/*Create a screen*/
lv_obj_t * scr = lv_obj_create(NULL, NULL);
lv_screen_load(scr);          /*Load the screen*/

/*Create 2 buttons*/
lv_obj_t * btn1 = lv_button_create(scr, NULL);     /*Create a button on the screen*/
lv_button_set_fit(btn1, true, true);               /*Enable automatically setting the size according to content*/
lv_obj_set_pos(btn1, 60, 40);                      /*Set the position of the button*/

lv_obj_t * btn2 = lv_button_create(scr, btn1);     /*Copy the first button*/
lv_obj_set_pos(btn2, 180, 80);                     /*Set the position of the button*/

/*Add labels to the buttons*/
lv_obj_t * label1 = lv_label_create(btn1, NULL);   /*Create a label on the first button*/
lv_label_set_text(label1, "Button 1");             /*Set the text of the label*/

lv_obj_t * label2 = lv_label_create(btn2, NULL);   /*Create a label on the second button*/
lv_label_set_text(label2, "Button 2");             /*Set the text of the label*/

/*Delete the second label*/

Change order

There are four explicit ways to bring an object to the foreground:

Screen-like layers

Top and sys layers

LVGL uses two special layers named layer_top and layer_sys. Both are visible and common on all screens of a display. They are not, however, shared among multiple physical displays. The layer_top is always on top of the default screen (lv_screen_active()), and layer_sys is on top of layer_top.

The get these layers use lv_layer_top() and lv_layer_sys().

These layers work like any other widget, meaning the can be styles, scrolled, and any kind of widgets can be created on them.

The layer_top can be used by the user to create some content visible everywhere. For example, a menu bar, a pop-up, etc. If the click attribute is enabled, then layer_top will absorb all user clicks and acts as a modal.

lv_obj_add_flag(lv_layer_top(), LV_OBJ_FLAG_CLICKABLE);

The layer_sys is also used for similar purposes in LVGL. For example, it places the mouse cursor above all layers to be sure it's always visible.

Bottom layers

Similarly top and sys. layer bottom layer is also screen size but it's located below the active screen. It's visible only if the active screen's background opacity is < 255.

The get the bottom layer use lv_layer_bottom().

Draw layers

Some style properties make LVGL to allocate a buffer and render a widget and its children there first. Later that layer will be merged to the screen or its parent layer after applying some transformations or other modifications.

Simple layer

The following style properties trigger the creation of a "Simple layer":

  • opa_layered

  • bitmap_mask_src

  • blend_mode

In this case widget will be sliced into LV_DRAW_SW_LAYER_SIMPLE_BUF_SIZE sized chunks.

If there is no memory for a new chunk, LVGL will try allocating layer when an other chunk is rendered and freed.

Transformed layer

When the widget is transformed a larger part of the widget needs to rendered to provide enough data for transformation. LVGL tries to render as small area of the widget as possible, but due to the nature of transformations no slicing is possible in this case.

The following style properties trigger the creation of a "Transform layer":

  • transform_scale_x

  • transform_scale_y

  • transform_skew_x

  • transform_skew_y

  • transform_rotate

Clip corner

The clip_corner style property also makes LVGL to create a 2 layers with radius height for the top and bottom part of the widget.