Layers

When the term "layer" is used in LVGL documentation, it may refer to one of several things:

for Widgets, the Order of Creation creates a natural layering of Widgets;
in the context of pixel rendering (drawing), there are Draw Layers;
permanent Screen Layers are part of each Display (lv_display) object, and are covered here

#1 and #2 are covered below.

Order of Creation

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

For example, assume we add a button to a parent Widget named button1 and then another button named button2. Then button1 (along with its child Widget(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 */
lv_obj_delete(label2);

Changing Order

There are four explicit ways to bring a Widget to the foreground:

Use lv_obj_move_foreground(widget) to bring a Widget to the foreground. Similarly, use lv_obj_move_background(widget) to move it to the background.
Use lv_obj_move_to_index(widget, idx) to move a Widget to a given index in the order of children.
- 0: background
- child_num - 1: foreground
- < 0: count from the top, to move forward (up): lv_obj_move_to_index(widget, lv_obj_get_index(widget) - 1)
Use lv_obj_swap(widget1, widget2) to swap the relative layer position of two Widgets.
When lv_obj_set_parent(widget, new_parent) is used, widget will be on the foreground of new_parent.

Draw Layers

Some style properties cause 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 the 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 the layer after another 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 causes LVGL to create a 2 layers with radius height for the top and bottom parts of the Widget.

API

lv_display.h

lv_types.h

lv_obj_draw.h