Lvgl Overlay - Element on transparent screen

Question

I have a simple lvgl example which runs on an embedded device. It should work like an Overlay on top of an existing image. In this case, I only want to render the text of the label thus the label and the screen should have a transparent background.

My approach is to set

lv_obj_set_style_bg_opa(screen, LV_OPA_TRANSP, 0);

on the screen and

lv_style_set_bg_opa(&label_style, LV_OPA_TRANSP);

on the label.

This does not render anything (I get a fully transparent window with no content). Removing the lv_obj_set_style_bg_opa(screen, LV_OPA_TRANSP, 0); will result in having a green screen with a red label having a transparent background.

#include "lvgl/lvgl.h"
#include <unistd.h>
#include <pthread.h>
#include <time.h>
#include <stdio.h>
#include <stdlib.h>

static const char * getenv_default(const char * name, const char * dflt)
{
    return getenv(name) ?: dflt;
}

static lv_obj_t * example_label;

int main(void)
{
    lv_init();

    const char * device = getenv_default("LV_LINUX_FBDEV_DEVICE", "/dev/fb0");
    lv_display_t * disp = lv_linux_fbdev_create();
    lv_linux_fbdev_set_file(disp, device);

    lv_obj_t * screen = lv_obj_create(NULL);
    lv_screen_load(screen);

    //lv_obj_set_style_bg_opa(screen, LV_OPA_TRANSP, 0);
    lv_obj_set_style_bg_color(screen, lv_color_make(0, 255, 0), 0);

    static lv_style_t label_style;
    lv_style_init(&label_style);
    lv_style_set_text_color(&label_style, lv_color_hex(0xFF0000));
    lv_style_set_bg_opa(&label_style, LV_OPA_TRANSP);
    lv_style_set_bg_color(&label_style, lv_color_hex(0x0000FF));
    lv_style_set_text_font(&label_style, &lv_font_montserrat_32);
    example_label = lv_label_create(screen);
    lv_label_set_text(example_label, "Hello World");
    lv_obj_add_style(example_label, &label_style, LV_PART_MAIN | LV_STATE_DEFAULT);
    lv_obj_align(example_label, LV_ALIGN_BOTTOM_MID, 0, 0);

    while(1) {
        lv_timer_handler();
        usleep(5000);
    }

    return 0;
}

In lv_conf.h I've been playing around with

#define LV_COLOR_DEPTH 32

#define LV_USE_LINUX_FBDEV      1
#if LV_USE_LINUX_FBDEV
    #define LV_LINUX_FBDEV_BSD           0
    //#define LV_LINUX_FBDEV_RENDER_MODE   LV_DISPLAY_RENDER_MODE_DIRECT
    #define LV_LINUX_FBDEV_RENDER_MODE LV_DISPLAY_RENDER_MODE_FULL
    //#define LV_LINUX_FBDEV_BUFFER_COUNT  1
    #define LV_LINUX_FBDEV_BUFFER_COUNT  2
    #define LV_LINUX_FBDEV_BUFFER_SIZE   60
#endif

It seems like there was an LV_COLOR_SCREEN_TRANSP option at some point but but yet no more.

I'm using the lvgl linux port from https://github.com/lvgl/lv_port_linux and tried v9.0 and master.

I also managed to get the same example working in QT via framebuffer so the device (hardware and software) definitely supports this.

Answer 1

It seems like setting lv_display_set_color_format(NULL, LV_COLOR_FORMAT_ARGB8888); right after lv_screen_load(screen); does the job. So no special settings in lv_conf.h are needed.