Display interface

To register a display for LVGL, a lv_disp_draw_buf_t and a lv_disp_drv_t variable have to be initialized.

  • lv_disp_draw_buf_t contains internal graphic buffer(s) called draw buffer(s).

  • lv_disp_drv_t contains callback functions to interact with the display and manipulate low level drawing behavior.

Draw buffer

Draw buffer(s) are simple array(s) that LVGL uses to render the screen content. Once rendering is ready the content of the draw buffer is sent to the display using the flush_cb function set in the display driver (see below).

A draw buffer can be initialized via a lv_disp_draw_buf_t variable like this:

/*A static or global variable to store the buffers*/
static lv_disp_draw_buf_t disp_buf;

/*Static or global buffer(s). The second buffer is optional*/
static lv_color_t buf_1[MY_DISP_HOR_RES * 10];
static lv_color_t buf_2[MY_DISP_HOR_RES * 10];

/*Initialize `disp_buf` with the buffer(s). With only one buffer use NULL instead buf_2 */
lv_disp_draw_buf_init(&disp_buf, buf_1, buf_2, MY_DISP_HOR_RES*10);

Note that lv_disp_draw_buf_t must be a static, global or dynamically allocated variable. It cannot be a local variable as they are destroyed upon end of scope.

As you can see above, the draw buffer may be smaller than the screen. In this case, larger areas are redrawn in smaller segments that fit into the draw buffer(s). If only a small area changes (e.g. a button is pressed) then only that area will be refreshed.

A larger buffer results in better performance but above 1/10 screen sized buffer(s) there is no significant performance improvement. Therefore it's recommended to choose the size of the draw buffer(s) to be at least 1/10 screen sized.

Buffering modes

There are several settings to adjust the number draw buffers and buffering/refreshing modes.

You can measure the performance of different configurations using the benchmark example.

One buffer

If only one buffer is used LVGL draws the content of the screen into that draw buffer and sends it to the display. LVGL then needs to wait until the content of the buffer is sent to the display before drawing something new in it.

Two buffers

If two buffers are used LVGL can draw into one buffer while the content of the other buffer is sent to the display in the background. DMA or other hardware should be used to transfer data to the display so the MCU can continue drawing. This way, the rendering and refreshing of the display become parallel operations.

Full refresh

In the display driver (lv_disp_drv_t) enabling the full_refresh bit will force LVGL to always redraw the whole screen. This works in both one buffer and two buffers modes. If full_refresh is enabled and two screen sized draw buffers are provided, LVGL's display handling works like "traditional" double buffering. This means the flush_cb callback only has to update the address of the framebuffer (color_p parameter). This configuration should be used if the MCU has an LCD controller peripheral and not with an external display controller (e.g. ILI9341 or SSD1963) accessed via serial link. The latter will generally be too slow to maintain high frame rates with full screen redraws.

Direct mode

If the direct mode flag is enabled in the display driver LVGL will draw directly into a screen sized frame buffer. That is the draw buffer(s) needs to be screen sized. It this case flush_cb will be called only once when all dirty areas are redrawn. With direct_mode the frame buffer always contains the current frame as it should be displayed on the screen. If 2 frame buffers are provided as draw buffers LVGL will alter the buffers but always draw only the dirty areas. Therefore the the 2 buffers needs to synchronized in flush_cb like this:

  1. Display the frame buffer pointed by color_p

  2. Copy the redrawn areas from color_p to the other buffer.

The get the redrawn areas to copy use the following functions _lv_refr_get_disp_refreshing() returns the display being refreshed disp->inv_areas[LV_INV_BUF_SIZE] contains the invalidated areas disp->inv_area_joined[LV_INV_BUF_SIZE] if 1 that area was joined into an other one and should be ignored disp->inv_p number of valid elements in inv_areas

Display driver

Once the buffer initialization is ready a lv_disp_drv_t display driver needs to be:

  1. initialized with lv_disp_drv_init(&disp_drv)

  2. its fields need to be set

  3. it needs to be registered in LVGL with lv_disp_drv_register(&disp_drv)

Note that lv_disp_drv_t also needs to be a static, global or dynamically allocated variable.

Mandatory fields

In the most simple case only the following fields of lv_disp_drv_t need to be set:

  • draw_buf pointer to an initialized lv_disp_draw_buf_t variable.

  • hor_res horizontal resolution of the display in pixels.

  • ver_res vertical resolution of the display in pixels.

  • flush_cb a callback function to copy a buffer's content to a specific area of the display. lv_disp_flush_ready(&disp_drv) needs to be called when flushing is ready. LVGL might render the screen in multiple chunks and therefore call flush_cb multiple times. To see if the current one is the last chunk of rendering use lv_disp_flush_is_last(&disp_drv).

Optional fields

There are some optional display driver data fields:

  • physical_hor_res horizontal resolution of the full / physical display in pixels. Only set this when not using the full screen (defaults to -1 / same as hor_res).

  • physical_ver_res vertical resolution of the full / physical display in pixels. Only set this when not using the full screen (defaults to -1 / same as ver_res).

  • offset_x horizontal offset from the the full / physical display in pixels. Only set this when not using the full screen (defaults to 0).

  • offset_y vertical offset from the the full / physical display in pixels. Only set this when not using the full screen (defaults to 0).

  • color_chroma_key A color which will be drawn as transparent on chrome keyed images. Set to LV_COLOR_CHROMA_KEY from lv_conf.h by default.

  • anti_aliasing use anti-aliasing (edge smoothing). Enabled by default if LV_COLOR_DEPTH is set to at least 16 in lv_conf.h.

  • rotated and sw_rotate See the Rotation section below.

  • screen_transp if 1 the screen itself can have transparency as well. LV_COLOR_SCREEN_TRANSP must be enabled in lv_conf.h and LV_COLOR_DEPTH must be 32.

  • user_data A custom void user data for the driver.

  • full_refresh always redrawn the whole screen (see above)

  • direct_mode drive directly into the frame buffer (see above)

  • user_data A custom void user data for the driver..

Some other optional callbacks to make it easier and more optimal to work with monochrome, grayscale or other non-standard RGB displays:

  • rounder_cb Round the coordinates of areas to redraw. E.g. a 2x2 px can be converted to 2x8. It can be used if the display controller can refresh only areas with specific height or width (usually 8 px height with monochrome displays).

  • set_px_cb a custom function to write the draw buffer. It can be used to store the pixels more compactly in the draw buffer if the display has a special color format. (e.g. 1-bit monochrome, 2-bit grayscale etc.) This way the buffers used in lv_disp_draw_buf_t can be smaller to hold only the required number of bits for the given area size. Note that rendering with set_px_cb is slower than normal rendering.

  • monitor_cb A callback function that tells how many pixels were refreshed and in how much time. Called when the last chunk is rendered and sent to the display.

  • clean_dcache_cb A callback for cleaning any caches related to the display.

LVGL has built-in support to several GPUs (see lv_conf.h) but if something else is required these functions can be used to make LVGL use a GPU:

  • gpu_fill_cb fill an area in the memory with a color.

  • gpu_wait_cb if any GPU function returns while the GPU is still working, LVGL will use this function when required to make sure GPU rendering is ready.

Examples

All together it looks like this:

static lv_disp_drv_t disp_drv;          /*A variable to hold the drivers. Must be static or global.*/
lv_disp_drv_init(&disp_drv);            /*Basic initialization*/
disp_drv.draw_buf = &disp_buf;          /*Set an initialized buffer*/
disp_drv.flush_cb = my_flush_cb;        /*Set a flush callback to draw to the display*/
disp_drv.hor_res = 320;                 /*Set the horizontal resolution in pixels*/
disp_drv.ver_res = 240;                 /*Set the vertical resolution in pixels*/

lv_disp_t * disp;
disp = lv_disp_drv_register(&disp_drv); /*Register the driver and save the created display objects*/

Here are some simple examples of the callbacks:

void my_flush_cb(lv_disp_drv_t * disp_drv, const lv_area_t * area, lv_color_t * color_p)
{
    /*The most simple case (but also the slowest) to put all pixels to the screen one-by-one
     *`put_px` is just an example, it needs to implemented by you.*/
    int32_t x, y;
    for(y = area->y1; y <= area->y2; y++) {
        for(x = area->x1; x <= area->x2; x++) {
            put_px(x, y, *color_p);
            color_p++;
        }
    }

    /* IMPORTANT!!!
     * Inform the graphics library that you are ready with the flushing*/
    lv_disp_flush_ready(disp_drv);
}

void my_gpu_fill_cb(lv_disp_drv_t * disp_drv, lv_color_t * dest_buf, const lv_area_t * dest_area, const lv_area_t * fill_area, lv_color_t color);
{
    /*It's an example code which should be done by your GPU*/
    uint32_t x, y;
    dest_buf += dest_width * fill_area->y1; /*Go to the first line*/

    for(y = fill_area->y1; y < fill_area->y2; y++) {
        for(x = fill_area->x1; x < fill_area->x2; x++) {
            dest_buf[x] = color;
        }
        dest_buf+=dest_width;    /*Go to the next line*/
    }
}


void my_rounder_cb(lv_disp_drv_t * disp_drv, lv_area_t * area)
{
  /* Update the areas as needed.
   * For example it makes the area to start only on 8th rows and have Nx8 pixel height.*/
   area->y1 = area->y1 & 0x07;
   area->y2 = (area->y2 & 0x07) + 8;
}

void my_set_px_cb(lv_disp_drv_t * disp_drv, uint8_t * buf, lv_coord_t buf_w, lv_coord_t x, lv_coord_t y, lv_color_t color, lv_opa_t opa)
{
   /* Write to the buffer as required for the display.
    * For example it writes only 1-bit for monochrome displays mapped vertically.*/
   buf += buf_w * (y >> 3) + x;
   if(lv_color_brightness(color) > 128) (*buf) |= (1 << (y % 8));
   else (*buf) &= ~(1 << (y % 8));
}

void my_monitor_cb(lv_disp_drv_t * disp_drv, uint32_t time, uint32_t px)
{
  printf("%d px refreshed in %d ms\n", time, ms);
}

void my_clean_dcache_cb(lv_disp_drv_t * disp_drv, uint32)
{
  /* Example for Cortex-M (CMSIS) */
  SCB_CleanInvalidateDCache();
}

Rotation

LVGL supports rotation of the display in 90 degree increments. You can select whether you'd like software rotation or hardware rotation.

If you select software rotation (sw_rotate flag set to 1), LVGL will perform the rotation for you. Your driver can and should assume that the screen width and height have not changed. Simply flush pixels to the display as normal. Software rotation requires no additional logic in your flush_cb callback.

There is a noticeable amount of overhead to performing rotation in software. Hardware rotation is available to avoid unwanted slow downs. In this mode, LVGL draws into the buffer as if your screen width and height were swapped. You are responsible for rotating the provided pixels yourself.

The default rotation of your display when it is initialized can be set using the rotated flag. The available options are LV_DISP_ROT_NONE, LV_DISP_ROT_90, LV_DISP_ROT_180, or LV_DISP_ROT_270. The rotation values are relative to how you would rotate the physical display in the clockwise direction. Thus, LV_DISP_ROT_90 means you rotate the hardware 90 degrees clockwise, and the display rotates 90 degrees counterclockwise to compensate.

(Note for users upgrading from 7.10.0 and older: these new rotation enum values match up with the old 0/1 system for rotating 90 degrees, so legacy code should continue to work as expected. Software rotation is also disabled by default for compatibility.)

Display rotation can also be changed at runtime using the lv_disp_set_rotation(disp, rot) API.

Support for software rotation is a new feature, so there may be some glitches/bugs depending on your configuration. If you encounter a problem please open an issue on GitHub.

Further reading

API

@description Display Driver HAL interface header file

Typedefs

typedef struct _lv_disp_draw_buf_t lv_disp_draw_buf_t

Structure for holding display buffer information.

typedef struct _lv_disp_drv_t lv_disp_drv_t

Display Driver structure to be registered by HAL. Only its pointer will be saved in lv_disp_t so it should be declared as static lv_disp_drv_t my_drv or allocated dynamically.

typedef struct _lv_disp_t lv_disp_t

Display structure.

Note

lv_disp_drv_t should be the first member of the structure.

Enums

enum lv_disp_rot_t

Values:

enumerator LV_DISP_ROT_NONE
enumerator LV_DISP_ROT_90
enumerator LV_DISP_ROT_180
enumerator LV_DISP_ROT_270

Functions

void lv_disp_drv_init(lv_disp_drv_t *driver)

Initialize a display driver with default values. It is used to have known values in the fields and not junk in memory. After it you can safely set only the fields you need.

Parameters

driver -- pointer to driver variable to initialize

void lv_disp_draw_buf_init(lv_disp_draw_buf_t *draw_buf, void *buf1, void *buf2, uint32_t size_in_px_cnt)

Initialize a display buffer

Parameters
  • draw_buf -- pointer lv_disp_draw_buf_t variable to initialize

  • buf1 -- A buffer to be used by LVGL to draw the image. Always has to specified and can't be NULL. Can be an array allocated by the user. E.g. static lv_color_t disp_buf1[1024 * 10] Or a memory address e.g. in external SRAM

  • buf2 -- Optionally specify a second buffer to make image rendering and image flushing (sending to the display) parallel. In the disp_drv->flush you should use DMA or similar hardware to send the image to the display in the background. It lets LVGL to render next frame into the other buffer while previous is being sent. Set to NULL if unused.

  • size_in_px_cnt -- size of the buf1 and buf2 in pixel count.

lv_disp_t *lv_disp_drv_register(lv_disp_drv_t *driver)

Register an initialized display driver. Automatically set the first display as active.

Parameters

driver -- pointer to an initialized 'lv_disp_drv_t' variable. Only its pointer is saved!

Returns

pointer to the new display or NULL on error

void lv_disp_drv_update(lv_disp_t *disp, lv_disp_drv_t *new_drv)

Update the driver in run time.

Parameters
  • disp -- pointer to a display. (return value of lv_disp_drv_register)

  • new_drv -- pointer to the new driver

void lv_disp_remove(lv_disp_t *disp)

Remove a display

Parameters

disp -- pointer to display

void lv_disp_set_default(lv_disp_t *disp)

Set a default display. The new screens will be created on it by default.

Parameters

disp -- pointer to a display

lv_disp_t *lv_disp_get_default(void)

Get the default display

Returns

pointer to the default display

lv_coord_t lv_disp_get_hor_res(lv_disp_t *disp)

Get the horizontal resolution of a display

Parameters

disp -- pointer to a display (NULL to use the default display)

Returns

the horizontal resolution of the display

lv_coord_t lv_disp_get_ver_res(lv_disp_t *disp)

Get the vertical resolution of a display

Parameters

disp -- pointer to a display (NULL to use the default display)

Returns

the vertical resolution of the display

lv_coord_t lv_disp_get_physical_hor_res(lv_disp_t *disp)

Get the full / physical horizontal resolution of a display

Parameters

disp -- pointer to a display (NULL to use the default display)

Returns

the full / physical horizontal resolution of the display

lv_coord_t lv_disp_get_physical_ver_res(lv_disp_t *disp)

Get the full / physical vertical resolution of a display

Parameters

disp -- pointer to a display (NULL to use the default display)

Returns

the full / physical vertical resolution of the display

lv_coord_t lv_disp_get_offset_x(lv_disp_t *disp)

Get the horizontal offset from the full / physical display

Parameters

disp -- pointer to a display (NULL to use the default display)

Returns

the horizontal offset from the full / physical display

lv_coord_t lv_disp_get_offset_y(lv_disp_t *disp)

Get the vertical offset from the full / physical display

Parameters

disp -- pointer to a display (NULL to use the default display)

Returns

the horizontal offset from the full / physical display

bool lv_disp_get_antialiasing(lv_disp_t *disp)

Get if anti-aliasing is enabled for a display or not

Parameters

disp -- pointer to a display (NULL to use the default display)

Returns

true: anti-aliasing is enabled; false: disabled

lv_coord_t lv_disp_get_dpi(const lv_disp_t *disp)

Get the DPI of the display

Parameters

disp -- pointer to a display (NULL to use the default display)

Returns

dpi of the display

void lv_disp_set_rotation(lv_disp_t *disp, lv_disp_rot_t rotation)

Set the rotation of this display.

Parameters
  • disp -- pointer to a display (NULL to use the default display)

  • rotation -- rotation angle

lv_disp_rot_t lv_disp_get_rotation(lv_disp_t *disp)

Get the current rotation of this display.

Parameters

disp -- pointer to a display (NULL to use the default display)

Returns

rotation angle

lv_disp_t *lv_disp_get_next(lv_disp_t *disp)

Get the next display.

Parameters

disp -- pointer to the current display. NULL to initialize.

Returns

the next display or NULL if no more. Give the first display when the parameter is NULL

lv_disp_draw_buf_t *lv_disp_get_draw_buf(lv_disp_t *disp)

Get the internal buffer of a display

Parameters

disp -- pointer to a display

Returns

pointer to the internal buffers

void lv_disp_drv_use_generic_set_px_cb(lv_disp_drv_t *disp_drv, lv_img_cf_t cf)
struct _lv_disp_draw_buf_t
#include <lv_hal_disp.h>

Structure for holding display buffer information.

Public Members

void *buf1

First display buffer.

void *buf2

Second display buffer.

void *buf_act
uint32_t size
lv_area_t area
int flushing
int flushing_last
uint32_t last_area
uint32_t last_part
struct _lv_disp_drv_t
#include <lv_hal_disp.h>

Display Driver structure to be registered by HAL. Only its pointer will be saved in lv_disp_t so it should be declared as static lv_disp_drv_t my_drv or allocated dynamically.

Public Members

lv_coord_t hor_res

Horizontal resolution.

lv_coord_t ver_res

Vertical resolution.

lv_coord_t physical_hor_res

Horizontal resolution of the full / physical display. Set to -1 for fullscreen mode.

lv_coord_t physical_ver_res

Vertical resolution of the full / physical display. Set to -1 for fullscreen mode.

lv_coord_t offset_x

Horizontal offset from the full / physical display. Set to 0 for fullscreen mode.

lv_coord_t offset_y

Vertical offset from the full / physical display. Set to 0 for fullscreen mode.

lv_disp_draw_buf_t *draw_buf

Pointer to a buffer initialized with lv_disp_draw_buf_init(). LVGL will use this buffer(s) to draw the screens contents

uint32_t direct_mode

1: Use screen-sized buffers and draw to absolute coordinates

uint32_t full_refresh

1: Always make the whole screen redrawn

uint32_t sw_rotate

1: use software rotation (slower)

uint32_t antialiasing

1: anti-aliasing is enabled on this display.

uint32_t rotated

1: turn the display by 90 degree.

Warning

Does not update coordinates for you!

uint32_t screen_transp
uint32_t dpi

Handle if the screen doesn't have a solid (opa == LV_OPA_COVER) background. Use only if required because it's slower.

void (*flush_cb)(struct _lv_disp_drv_t *disp_drv, const lv_area_t *area, lv_color_t *color_p)

DPI (dot per inch) of the display. Default value is LV_DPI_DEF. MANDATORY: Write the internal buffer (draw_buf) to the display. 'lv_disp_flush_ready()' has to be called when finished

void (*rounder_cb)(struct _lv_disp_drv_t *disp_drv, lv_area_t *area)

OPTIONAL: Extend the invalidated areas to match with the display drivers requirements E.g. round y to, 8, 16 ..) on a monochrome display

void (*set_px_cb)(struct _lv_disp_drv_t *disp_drv, uint8_t *buf, lv_coord_t buf_w, lv_coord_t x, lv_coord_t y, lv_color_t color, lv_opa_t opa)

OPTIONAL: Set a pixel in a buffer according to the special requirements of the display Can be used for color format not supported in LittelvGL. E.g. 2 bit -> 4 gray scales

Note

Much slower then drawing with supported color formats.

void (*monitor_cb)(struct _lv_disp_drv_t *disp_drv, uint32_t time, uint32_t px)

OPTIONAL: Called after every refresh cycle to tell the rendering and flushing time + the number of flushed pixels

void (*wait_cb)(struct _lv_disp_drv_t *disp_drv)

OPTIONAL: Called periodically while lvgl waits for operation to be completed. For example flushing or GPU User can execute very simple tasks here or yield the task

void (*clean_dcache_cb)(struct _lv_disp_drv_t *disp_drv)

OPTIONAL: Called when lvgl needs any CPU cache that affects rendering to be cleaned

void (*gpu_wait_cb)(struct _lv_disp_drv_t *disp_drv)

OPTIONAL: called to wait while the gpu is working

void (*drv_update_cb)(struct _lv_disp_drv_t *disp_drv)

OPTIONAL: called when driver parameters are updated

void (*gpu_fill_cb)(struct _lv_disp_drv_t *disp_drv, lv_color_t *dest_buf, lv_coord_t dest_width, const lv_area_t *fill_area, lv_color_t color)

OPTIONAL: Fill a memory with a color (GPU only)

lv_color_t color_chroma_key

On CHROMA_KEYED images this color will be transparent. LV_COLOR_CHROMA_KEY by default. (lv_conf.h)

void *user_data

Custom display driver user data

struct _lv_disp_t
#include <lv_hal_disp.h>

Display structure.

Note

lv_disp_drv_t should be the first member of the structure.

Public Members

struct _lv_disp_drv_t *driver

< Driver to the display A timer which periodically checks the dirty areas and refreshes them

lv_timer_t *refr_timer

The theme assigned to the screen

struct _lv_theme_t *theme
struct _lv_obj_t **screens

Screens of the display Array of screen objects.

struct _lv_obj_t *act_scr

Currently active screen on this display

struct _lv_obj_t *prev_scr

Previous screen. Used during screen animations

struct _lv_obj_t *scr_to_load

The screen prepared to load in lv_scr_load_anim

struct _lv_obj_t *top_layer

See

lv_disp_get_layer_top

struct _lv_obj_t *sys_layer

See

lv_disp_get_layer_sys

uint32_t screen_cnt
uint8_t del_prev

1: Automatically delete the previous screen when the screen load animation is ready

lv_opa_t bg_opa

Opacity of the background color or wallpaper

lv_color_t bg_color

Default display color when screens are transparent

const void *bg_img

An image source to display as wallpaper

void (*bg_fn)(lv_area_t*)

A function to handle drawing

lv_area_t inv_areas[LV_INV_BUF_SIZE]

Invalidated (marked to redraw) areas

uint8_t inv_area_joined[LV_INV_BUF_SIZE]
uint16_t inv_p
uint32_t last_activity_time

Last time when there was activity on this display