based of the example in the docs (https://freetype.org/freetype2/docs/tutorial/step1.html)

font_rendering.c

#include <stdio.h>
#include <string.h>
#include <math.h>
#include <assert.h>
#include <Windows.h>

#include <ft2build.h>
#include FT_FREETYPE_H

#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "stb_image_write.h"



void show_image(unsigned char* image, int width, int height) {
	
	unsigned char* img = calloc(width*height*4, 1);
	// memset(img, 0xff, width*height*4);
	int  i, j;


	for ( i = 0; i < height; i++ )
	{
		for ( j = 0; j < width; j++ ){
			img[width*i*4 + j*4 + 0] = 0;//image[i * width + j];
			img[width*i*4 + j*4 + 1] = 0;//image[i * width + j];
			img[width*i*4 + j*4 + 2] = 0;//image[i * width + j];
			img[width*i*4 + j*4 + 3] = image[i * width + j];
		}
	}

	stbi_write_png("test.png", width, height, 4, img, width*4);
	free(img);
}

// FT_Error      error;

// unsigned char*         text;

// double        angle;
// int           n, num_chars;



typedef struct{
	int width;
	int height;
	int baseline;
} TextSize;

TextSize render_text(FT_Face face, unsigned char* text, unsigned char* image, TextSize size){
	FT_GlyphSlot slot = face->glyph;

	/* set up matrix */
	FT_Matrix matrix;
	matrix.xx = 0x10000L;
	matrix.xy = 0;
	matrix.yx = 0;
	matrix.yy = 0x10000L;

	FT_Vector pen = {0};
	if (image){
		pen.x = 0;
		pen.y = size.baseline;
	}

	int min_pos = 0;
	int max_pos = 0;

	for (int n = 0; n < strlen(text); n++) {
		/* set transformation */
		FT_Set_Transform( face, &matrix, &pen );

		FT_ULong utf8char;
		if (text[n] >> 3 == 0b11110){
			FT_ULong b1 = text[n];
			n++;
			FT_ULong b2 = text[n];
			n++;
			FT_ULong b3 = text[n];
			n++;
			FT_ULong b4 = text[n];
			utf8char = (b1&0b111)<<18 | (b2&0b111111)<<12 | (b3&0b111111)<<6 | (b4&0b111111);
		}else if (text[n] >> 4 == 0b1110){
			FT_ULong b1 = text[n];
			n++;
			FT_ULong b2 = text[n];
			n++;
			FT_ULong b3 = text[n];
			utf8char = (b1&0b1111)<<12 | (b2&0b111111)<<6 | (b3&0b111111);
		}else if (text[n] >> 5 == 0b110){
			FT_ULong b1 = text[n];
			n++;
			FT_ULong b2 = text[n];
			utf8char = (b1&0b11111)<<6 | (b2&0b111111);
		}else{
			utf8char = text[n];
		}

		/* load glyph image into the slot (erase previous one) */
		FT_Error error = FT_Load_Char(face, utf8char, image?FT_LOAD_RENDER:FT_LOAD_NO_BITMAP);
		if ( error )
			continue;                 /* ignore errors */

		/* now, draw to our target surface (convert position) */
		if (image){
			FT_Bitmap* bitmap = &slot->bitmap;
			FT_Int x = slot->bitmap_left;
			FT_Int y = size.height - slot->bitmap_top;
			FT_Int i, j, p, q;
			FT_Int x_max = x + bitmap->width;
			FT_Int y_max = y + bitmap->rows;


			/* for simplicity, we assume that `bitmap->pixel_mode' */
			/* is `FT_PIXEL_MODE_GRAY' (i.e., not a bitmap font)   */

			assert(bitmap->pixel_mode == FT_PIXEL_MODE_GRAY);

			for ( i = x, p = 0; i < x_max; i++, p++ )
			{
				for ( j = y, q = 0; j < y_max; j++, q++ )
				{
					if (i < 0 || j < 0 || i >= size.width || j >= size.height){ continue; }

					image[size.width * j + i] = (
						image[size.width * j + i] + bitmap->buffer[q * bitmap->width + p]
					) - (
						image[size.width * j + i] * bitmap->buffer[q * bitmap->width + p]
					);
					// image[j][i] |= bitmap->buffer[q * bitmap->width + p];
					// image[WIDTH*j*4 + i*4 + 0] |=bitmap->buffer[q * bitmap->width + p];
					// image[WIDTH*j*4 + i*4 + 1] |=bitmap->buffer[q * bitmap->width + p];
					// image[WIDTH*j*4 + i*4 + 2] |=bitmap->buffer[q * bitmap->width + p];
					// image[WIDTH*j*4 + i*4 + 3] = 0xFF;
				}
			}
		}

		if (slot->metrics.horiBearingY > max_pos){max_pos = slot->metrics.horiBearingY;}
		if (slot->metrics.horiBearingY-slot->metrics.height < min_pos){min_pos = slot->metrics.horiBearingY-slot->metrics.height;}
		/* increment pen position */
		pen.x += slot->advance.x;
		pen.y += slot->advance.y;
	}

	return (TextSize){
		pen.x/64,
		(max_pos - min_pos)/64,
		-min_pos
	};
}


int main(int _argc, unsigned char** _argv) {
	int argc;
	LPWSTR* argvW = CommandLineToArgvW(GetCommandLineW(), &argc);
	unsigned char** argv = malloc(argc * sizeof(*argv));
	for (size_t i = 0; i < argc; i++) {
		int len = WideCharToMultiByte(CP_UTF8, 0, argvW[i], -1, NULL, 0, NULL, NULL);
		argv[i] = malloc((len+1) * sizeof(*argv[i]));
		argv[i][len] = 0; // assure its null terminated
		WideCharToMultiByte(CP_UTF8, 0, argvW[i], -1, argv[i], len, NULL, NULL);
	}
	// leaking memory above. dosent matter as short run time

	if (argc != 3) {
		fprintf ( stderr, "usage: %s font sample-text\n", argv[0] );
		exit( 1 );
	}

	unsigned char* filename = argv[1];
	unsigned char* text = argv[2];
	
	FT_Library library;
	FT_Error error = FT_Init_FreeType( &library ); /* initialize library */
	/* error handling omitted */

	FT_Face face;
	error = FT_New_Face( library, filename, 0, &face ); /* create face object */
	/* error handling omitted */

	/* use 50pt at 100dpi */
	// error = FT_Set_Char_Size( face, 50 * 64, 0, 100, 0 ); /* set character size */
	error = FT_Set_Pixel_Sizes(face, 51, 51);
	/* error handling omitted */

	/* cmap selection omitted; */
	/* for simplicity we assume that the font contains a Unicode cmap */

	TextSize size = render_text(face, text, NULL, (TextSize){});
	unsigned char* image = calloc(size.width*size.height, 1);
	render_text(face, text, image, size);

	show_image(image, size.width, size.height);

	FT_Done_Face(face);
	FT_Done_FreeType(library);

	return 0;
}