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 drawing related things.

Draw buffer¶

Draw buffer(s) are simple array(s) that LVGL uses to render the content of the screen. 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 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 needs to be static, global or dynamically allocated and not a local variable destroyed if goes out of the scope.

As you can see the draw buffer can be smaller than the screen. In this case, the larger areas will be redrawn in smaller parts 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 at least 1/10 screen sized.

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

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

In the display driver (lv_disp_drv_t) the full_refresh bit can be enabled to force LVGL to always redraw the whole screen. This works in both one buffer and two buffers modes.

If full_refresh is enabled and 2 screen sized draw buffers are provided, LVGL's display handling works like "traditional" double buffering. This means in flush_cb only the address of the frame buffer needs to be changed to the provided pointer (color_p parameter). This configuration should be used if the MCU has LCD controller periphery and not with an external display controller (e.g. ILI9341 or SSD1963).

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

Display driver¶

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

initialized with lv_disp_drv_init(&disp_drv)
its fields need to be set
it needs to be registered in LVGL with lv_disp_drv_register(&disp_drv)

Note that lv_disp_drv_t also needs to be static, global or dynamically allocated and not a local variable destroyed if goes out of the scope.

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 data fields:

color_chroma_key A color which will be drawn as transparent on chrome keyed images. Set to LV_COLOR_CHROMA_KEY by default from lv_conf.h.
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 needs to enabled in lv_conf.h and requires LV_COLOR_DEPTH 32.
user_data A custom void user data for the driver..

Some other optional callbacks to make 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 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, which is why hardware rotation is also available. In this mode, LVGL draws into the buffer as though your screen now has the width and height inverted. 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.

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

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

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_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 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

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

Display interface¶

Draw buffer¶

Display driver¶

Mandatory fields¶

Optional fields¶

Examples¶

Rotation¶

Further reading¶

API¶