LittlevGL + FreeType

custom_font.c

#include "custom_font.h"

int rows = 20;

int load_custom_font(void)
{
    printf("load custom font\n");
    int error;
    error = FT_Init_FreeType(&library);
    if (error) {
        printf("error init freetype\n");
        return EXIT_FAILURE;
    }

    error = FT_New_Face(library, "./NotoSansCJKtc-Regular.otf", 0, &face);
    if (error == FT_Err_Unknown_File_Format) {
        printf("unkown file format\n");
        return EXIT_FAILURE;
    } else if (error) {
        printf("error loading font\n");
        return EXIT_FAILURE;
    }

    int height = rows * 64;
    error = FT_Set_Char_Size(face, 0, height, 72, 72);
    if (error) {
        printf("couldnt get char size\n");
        return EXIT_FAILURE;
    }

    return EXIT_SUCCESS;
}

const uint8_t * get_ttf_bitmap(const lv_font_t * font, uint32_t unicode_letter)
{
    return font_buffer;
}

int16_t get_ttf_width(const lv_font_t * font, uint32_t unicode_letter)
{
    int error;
    error = FT_Load_Char(face, unicode_letter, FT_LOAD_RENDER);
    if (error) {
        printf("Could not load char in width\n");
        return -1;
    }

    if (face->glyph->bitmap.buffer == NULL) return NULL;

    FT_Bitmap bitmap = face->glyph->bitmap;

    char * tmp_buffer;
    tmp_buffer = malloc(rows * bitmap.width * sizeof(tmp_buffer));
    for (int row = 0; row < rows; row++) {
        for (int col = 0; col < bitmap.width; col++) {
            int idx = (row * bitmap.width) + col;
            if (row < (rows - bitmap.rows)) {
                tmp_buffer[idx] = 0x00;
            } else {
                int bufferIdx = idx - (bitmap.width * (rows - bitmap.rows));
                tmp_buffer[idx] = bitmap.buffer[bufferIdx];
            }
        }
    }

    font_buffer = tmp_buffer;
    free(tmp_buffer);
    return bitmap.width;
}

custom_font.h

#ifndef CUSTOM_FONT
#define CUSTOM_FONT

#include <stdint.h>
#include "./lvgl/lv_misc/lv_font.h"
#include <ft2build.h>
#include FT_FREETYPE_H

static FT_Library library;
static FT_Face face;
static char * font_buffer;

int load_custom_font(void);
const uint8_t * get_ttf_bitmap(const lv_font_t * font, uint32_t unicode_letter);
int16_t get_ttf_width(const lv_font_t * font, uint32_t unicode_letter);

static lv_font_t custom_font = 
{
  .unicode_first = 0,
  .unicode_last = 9999999,
  .h_px = 20,
  .glyph_bitmap = NULL,
  .glyph_dsc = NULL,
  .unicode_list = NULL,
  .get_bitmap = get_ttf_bitmap,
  .get_width = get_ttf_width,
  .bpp = 8,
  .next_page = NULL
};

#endif // CUSTOM_FONT

main.c

#define USE_SDL 1

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

#if USE_SDL
#include <SDL2/SDL.h>
#include "lv_drivers/display/monitor.h"
#else
#include "lv_drivers/display/fbdev.h"
#endif

#include "custom_font.h"

static char * label_text = "The quick brown fox jumps over the lazy";

static void hal_init(void);
void disp_init(void);
void build_container(lv_obj_t * parent);
LV_FONT_DECLARE(custom_font);

int main (int argc, char ** argv)
{
    load_custom_font();
    
    lv_init();
    hal_init();
    disp_init();

    while (1) {
        lv_tick_inc(5);
        lv_task_handler();
        usleep(5 * 1000);
    }
}

static void hal_init(void)
{
    #if USE_SDL
        monitor_init();
    #else
        fbdev_init();
    #endif

    lv_disp_drv_t disp_drv;
    lv_disp_drv_init(&disp_drv);

    #if USE_SDL
        disp_drv.disp_flush = monitor_flush;
        disp_drv.disp_fill = monitor_fill;
        disp_drv.disp_map = monitor_map;
    #else
        disp_drv.disp_flush = fbdev_flush;
        disp_drv.disp_fill = fbdev_fill;
        disp_drv.disp_map = fbdev_map;
    #endif

    lv_disp_drv_register(&disp_drv);
}

void disp_init(void)
{
    lv_obj_t *scr = lv_obj_create(NULL, NULL);
    lv_scr_load(scr);

    build_container(scr);
}

lv_style_t label_style;

void build_container(lv_obj_t *parent)
{
    lv_obj_t *main_container = lv_obj_create(parent, NULL);
    lv_obj_set_size(main_container, 800, 480);    

    lv_obj_t *label = lv_label_create(main_container, NULL);

    lv_style_copy(&label_style, &lv_style_plain);
    label_style.text.font = &custom_font;
    label_style.text.color = LV_COLOR_RED;
    lv_obj_set_style(label, &label_style);

    lv_label_set_text(label, label_text);
    lv_obj_align(label, main_container, LV_ALIGN_CENTER, 0, 0);
}