Display interface¶

To set up a display an lv_disp_buf_t and an lv_disp_drv_t variables have to be initialized.

lv_disp_buf_t contains internal graphic buffer(s).
lv_disp_drv_t contains callback functions to interact with the display and manipulate drawing related things.

Display buffer¶

lv_disp_buf_t can be initialized like this:

    /*A static or global variable to store the buffers*/
    static lv_disp_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) */
    lv_disp_buf_init(&disp_buf, buf_1, buf_2, MY_DISP_HOR_RES*10);

There are 3 possible configurations regarding the buffer size:

One buffer LVGL draws the content of the screen into a buffer and sends it to the display. The buffer can be smaller than the screen. In this case, the larger areas will be redrawn in multiple parts. If only small areas changes (e.g. button press) then only those areas will be refreshed.
Two non-screen-sized buffers having two buffers 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 CPU draw meanwhile. This way the rendering and refreshing of the display become parallel. Similarly to the One buffer, LVGL will draw the display's content in chunks if the buffer is smaller than the area to refresh.
Two screen-sized buffers. In contrast to Two non-screen-sized buffers LVGL will always provide the whole screen's content not only chunks. This way the driver can simply change the address of the frame buffer to the buffer received from LVGL. Therefore this method works the best when the MCU has an LCD/TFT interface and the frame buffer is just a location in the RAM.

You can measure the performance of your display configuration using the benchmark example.

Display driver¶

Once the buffer initialization is ready the display drivers need to be initialized. In the most simple case only the following two fields of lv_disp_drv_t needs to be set:

buffer pointer to an initialized lv_disp_buf_t variable.
flush_cb a callback function to copy a buffer's content to a specific area of the display. lv_disp_flush_ready() 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 which is the last chunk of rendering use lv_disp_flush_is_last().

There are some optional data fields:

hor_res horizontal resolution of the display. (LV_HOR_RES_MAX by default from lv_conf.h).
ver_res vertical resolution of the display. (LV_VER_RES_MAX by default from lv_conf.h).
color_chroma_key a color which will be drawn as transparent on chrome keyed images. LV_COLOR_TRANSP by default from lv_conf.h).
user_data custom user data for the driver. Its type can be modified in lv_conf.h.
anti-aliasing use anti-aliasing (edge smoothing). LV_ANTIALIAS by default from lv_conf.h.
rotated and sw_rotate See the rotation section below.
screen_transp if 1 the screen can have transparent or opaque style. LV_COLOR_SCREEN_TRANSP needs to enabled in lv_conf.h.

To use a GPU the following callbacks can be used:

gpu_fill_cb fill an area in memory with colors.
gpu_blend_cb blend two memory buffers using opacity.
gpu_wait_cb if any GPU function return, while the GPU is still working LVGL, will use this function when required the be sure GPU rendering is ready.

Note that, these functions need to draw to the memory (RAM) and not your display directly.

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 display buffer. It can be used to store the pixels more compactly 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_buf_t can be smaller to hold only the required number of bits for the given area size. set_px_cb is not working with Two screen-sized buffers display buffer configuration.
monitor_cb a callback function tells how many pixels were refreshed in how much time.
clean_dcache_cb a callback for cleaning any caches related to the display

To set the fields of lv_disp_drv_t variable it needs to be initialized with lv_disp_drv_init(&disp_drv). And finally to register a display for LVGL lv_disp_drv_register(&disp_drv) needs to be called.

All together it looks like this:

    lv_disp_drv_t disp_drv;                 /*A variable to hold the drivers. Can be local variable*/
    lv_disp_drv_init(&disp_drv);            /*Basic initialization*/
    disp_drv.buffer = &disp_buf;            /*Set an initialized buffer*/
    disp_drv.flush_cb = my_flush_cb;        /*Set a flush callback to draw to the display*/
    lv_disp_t * disp;
    disp = lv_disp_drv_register(&disp_drv); /*Register the driver and save the created display objects*/

Here 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*/
    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_gpu_blend_cb(lv_disp_drv_t * disp_drv, lv_color_t * dest, const lv_color_t * src, uint32_t length, lv_opa_t opa)
{
    /*It's an example code which should be done by your GPU*/
    uint32_t i;
    for(i = 0; i < length; i++) {
        dest[i] = lv_color_mix(dest[i], src[i], opa);
    }
}

void my_rounder_cb(lv_disp_drv_t * disp_drv, lv_area_t * area)
{
  /* Update the areas as needed. Can be only larger.
   * For example to always have lines 8 px 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.
     * Write 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¶

Display Driver HAL interface header file

Typedefs

typedef struct _disp_drv_t lv_disp_drv_t¶: Display Driver structure to be registered by HAL

typedef struct _disp_t lv_disp_t¶: Display structure.

Note

lv_disp_drv_t should be the first member of the structure.

Enums

enum lv_disp_size_t¶

Values:

enumerator LV_DISP_SIZE_SMALL¶

enumerator LV_DISP_SIZE_MEDIUM¶

enumerator LV_DISP_SIZE_LARGE¶

enumerator LV_DISP_SIZE_EXTRA_LARGE¶

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_buf_init(lv_disp_buf_t *disp_buf, void *buf1, void *buf2, uint32_t size_in_px_cnt)¶

Initialize a display buffer

Parameters

disp_buf -- pointer lv_disp_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 (can be local variable)
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 screen. 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(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

lv_disp_size_t lv_disp_get_size_category(lv_disp_t *disp)¶

Get the size category of the display based on it's hor. res. and dpi.

Parameters: disp -- pointer to a display (NULL to use the default display)
Returns: LV_DISP_SIZE_SMALL/MEDIUM/LARGE/EXTRA_LARGE

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_buf_t *lv_disp_get_buf(lv_disp_t *disp)¶

Get the internal buffer of a display

Parameters: disp -- pointer to a display
Returns: pointer to the internal buffers

uint16_t lv_disp_get_inv_buf_size(lv_disp_t *disp)¶

Get the number of areas in the buffer

Returns: number of invalid areas

void _lv_disp_pop_from_inv_buf(lv_disp_t *disp, uint16_t num)¶

Pop (delete) the last 'num' invalidated areas from the buffer

Parameters: num -- number of areas to delete

bool lv_disp_is_double_buf(lv_disp_t *disp)¶

Check the driver configuration if it's double buffered (both buf1 and buf2 are set)

Parameters: disp -- pointer to to display to check
Returns: true: double buffered; false: not double buffered

bool lv_disp_is_true_double_buf(lv_disp_t *disp)¶

Check the driver configuration if it's TRUE double buffered (both buf1 and buf2 are set and size is screen sized)

Parameters: disp -- pointer to to display to check
Returns: true: double buffered; false: not double buffered

struct lv_disp_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 _disp_drv_t¶

#include <lv_hal_disp.h>

Display Driver structure to be registered by HAL

Public Members

lv_coord_t hor_res¶: Horizontal resolution.

lv_coord_t ver_res¶: Vertical resolution.

lv_disp_buf_t *buffer¶: Pointer to a buffer initialized with lv_disp_buf_init(). LVGL will use this buffer(s) to draw the screens contents

uint32_t antialiasing¶: 1: antialiasing 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¶: Handle if the screen doesn't have a solid (opa == LV_OPA_COVER) background. Use only if required because it's slower.

uint32_t dpi¶: DPI (dot per inch) of the display. Set to LV_DPI from lv_Conf.h by default.

void (*flush_cb)(struct _disp_drv_t *disp_drv, const lv_area_t *area, lv_color_t *color_p)¶: MANDATORY: Write the internal buffer (VDB) to the display. 'lv_disp_flush_ready()' has to be called when finished

void (*rounder_cb)(struct _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 _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 _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 _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 _disp_drv_t *disp_drv)¶: OPTIONAL: Called when lvgl needs any CPU cache that affects rendering to be cleaned

void (*gpu_wait_cb)(struct _disp_drv_t *disp_drv)¶: OPTIONAL: called to wait while the gpu is working

void (*gpu_blend_cb)(struct _disp_drv_t *disp_drv, lv_color_t *dest, const lv_color_t *src, uint32_t length, lv_opa_t opa)¶: OPTIONAL: Blend two memories using opacity (GPU only)

void (*gpu_fill_cb)(struct _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_TRANSP by default. (lv_conf.h)

lv_disp_drv_user_data_t user_data¶: Custom display driver user data

struct _disp_t¶

#include <lv_hal_disp.h>

Display structure.

Note

lv_disp_drv_t should be the first member of the structure.

Public Members

lv_disp_drv_t driver¶: < Driver to the display A task which periodically checks the dirty areas and refreshes them

lv_task_t *refr_task¶

lv_ll_t scr_ll¶: Screens of the display

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

uint8_t del_prev¶: 1: Automatically delete the previous screen when the screen load animation is ready

lv_color_t bg_color¶: Default display color when screens are transparent

const void *bg_img¶: An image source to display as wallpaper

lv_opa_t bg_opa¶: Opacity of the background color or wallpaper

lv_area_t inv_areas[LV_INV_BUF_SIZE]¶: Invalidated (marked to redraw) areas

uint8_t inv_area_joined[LV_INV_BUF_SIZE]¶

uint32_t inv_p¶

uint32_t last_activity_time¶: Last time there was activity on this display