Basic texture atlas using freetype in odin

freetype.go

// Store the glyph index in the font for faster kerning lookup?
Glyph :: struct {
	bearingX, bearingY: int,
	width, height: int,
	advanceX: int,
}

Font :: struct {
	face: FT_Face,
	contex: stbrp_context,
	nodes: []stbrp_node,
	atlas_w, atlas_h: int,
	pixels: []u8,
	glyphs: map[rune]Glyph,
}

the_library: FT_Library;

load_font :: proc(path: string, pixel_size: int, atlas_width, atlas_height: int) -> ^Font {
	font := new(Font);
	font.atlas_w = atlas_width;
	font.atlas_h = atlas_height;
	using font;

	cpath := strings.new_c_string(path);
	defer free(cpath);
	if FT_New_Face(the_library, cpath, 0, &face) != 0 {
		return nil;
	}
	FT_Set_Pixel_Sizes(face, 0, auto_cast pixel_size);

	pixels = make([]u8, atlas_w*atlas_h);

	nodes = make([]stbrp_node, atlas_w);
	stbrp_init_target(&contex, i32(atlas_w-1), i32(atlas_h-1), &nodes[0], auto_cast len(nodes));

	pack_range(font, ' ', '~');

	return font;
}

pack_range :: proc(using font: ^Font, first, last: rune) -> (packed_all: bool = true) {
	glyph_count := int(last-first+1);
	reserve(&glyphs, glyph_count);
	rects := make([]stbrp_rect, glyph_count);
	defer free(rects);

	fmt.printf("glyph_count %v\n", glyph_count);
	
	// Get glyph data and setup packing
	for cp in first...last {
		if FT_Load_Char(face, auto_cast cp, FT_LOAD_RENDER) != 0 {
			assert(false, "Invalid font cp!");
		}
		glyph := face.glyph;
		g: Glyph;
		g.width = auto_cast glyph.metrics.width;
		g.height = auto_cast glyph.metrics.height;
		g.bearingX = auto_cast glyph.metrics.horiBearingX;
		g.bearingY = auto_cast glyph.metrics.horiBearingY;
		g.advanceX = auto_cast glyph.metrics.horiAdvance;
		glyphs[cp] = g;

		rects[cp-first].w = auto_cast (glyph.bitmap.width+1);
		rects[cp-first].h = auto_cast (glyph.bitmap.rows+1);
	}

	stbrp_pack_rects(&contex, &rects[0], i32(len(rects)));

	copy_to_atlas :: proc(using font: ^Font, r: ^stbrp_rect, bitmap: []u8) {
		for y in 0..r.h {
			yy := int(r.y+y);
			for x in 0..r.w {
				xx := int(r.x+x);
				font.pixels[xx+yy*font.atlas_w] = bitmap[x+y*r.w];
			}
		}
	}


	// Do packing and writing to atlas
	for cp in first...last {
		if FT_Load_Char(face, auto_cast cp, FT_LOAD_RENDER) != 0 {
			assert(false, "Invalid font cp!!??!?!");
		}
		r := &rects[cp-first];
		if r.was_packed == 0 {
			packed_all = false;
			continue;
		}

		r.x += 1;
		r.y += 1;
		r.w -= 1;
		r.h -= 1;

		pixels := mem.slice_ptr(face.glyph.bitmap.buffer, int(face.glyph.bitmap.width*face.glyph.bitmap.rows));
		copy_to_atlas(font, r, pixels);
	}

	return;
}

import stbiw "shared:odin-stb/stb_image_write.odin"

main :: proc() {
    FT_Init_FreeType(&the_library);
    defer FT_Done_FreeType(the_library);

    font := load_font("fonts/arial.ttf", 48, 1024, 1024);
    if font == nil {
    	assert(false, "failed to init font!");
    }
	pack_range(font, 0xA0, 0xFF);
    pack_range(font, 0x400, 0x4FF);
    stbiw.write_png("test.png\x00", font.atlas_w, font.atlas_h, 1, font.pixels, 0);
}