wasm_transcript_browser/src/render/font.c

typedef struct rn_glyph_t rn_glyph_t;
struct rn_glyph_t {
    v2f32_t size;
    v2f32_t offset;
    f32     xadvance;
    f32     left_side_bearing;
    r2f32_t atlas_bounding_box;
};

typedef struct rn_font_t rn_font_t;
struct rn_font_t {
    rn_glyph_t *glyphs;
    i32         glyph_count;

    u32 first_char, last_char;
    u32 texture_id;

    f32 size;
    f32 descent;
    f32 ascent;
    f32 line_gap;

    r2f32_t white_texture_bounding_box;
};

typedef struct rn_atlas_t rn_atlas_t;
struct rn_atlas_t {
    u8 *bitmap;
    v2i32_t  size;
    v2f32_t  inverse_size;
    i32      xcursor;
    i32      ycursor;

    i32 biggest_height;
    r2f32_t white_texture_bounding_box;
    u32     texture_id;
};

rn_atlas_t *rn_create_atlas(ma_arena_t *arena, v2i32_t size) {
    rn_atlas_t *result     = ma_push_type(arena, rn_atlas_t);
    result->size           = size;
    result->inverse_size.x = 1.f / (f32)result->size.x;
    result->inverse_size.y = 1.f / (f32)result->size.y;
    result->bitmap         = ma_push_array(arena, u8, size.x * size.y);

    // Add a whitebox first for rectangle rendering
    for (i32 y = 0; y < 16; y++) {
        for (i32 x = 0; x < 16; x++) {
            u8 *dst = result->bitmap + x + y * result->size.x;
            *dst         = 0xff;
        }
    }

    result->white_texture_bounding_box = (r2f32_t){
        .min = { 2.f * result->inverse_size.x,  2.f / (f32)result->size.y},
        .max = {14.f * result->inverse_size.x, 14.f / (f32)result->size.y}
    };
    result->xcursor += 16;
    result->biggest_height += 16;
    return result;
}

r2f32_t rn_pack_bitmap(rn_atlas_t *atlas, u8 *bitmap, i32 width, i32 height) {
    // Packing into a texture atlas
    // @Inefficient The algorithm is a simplest thing I had in mind, first we advance
    // through the atlas in X packing consecutive glyphs. After we get to the end of the row
    // we advance to the next row by the Y size of the biggest packed glyph. If we get to the
    // end of atlas and fail to pack everything the app panics.

    i32 spacing = 4;
    if (atlas->xcursor + width > atlas->size.x) {
        if (atlas->ycursor + height < atlas->size.y) {
            atlas->xcursor = 0;
            atlas->ycursor += atlas->biggest_height + spacing;
        } else {
            fatalf("error while packing a font into atlas. rn_atlas_t size for this font scale is a bit too small");
        }
    }

    u8 *src = bitmap;
    for (i32 y = atlas->ycursor; y < atlas->ycursor + height; y += 1) {
        for (i32 x = atlas->xcursor; x < atlas->xcursor + width; x += 1) {
            u8 *dst = atlas->bitmap + x + y * atlas->size.x;
            *dst         = *src++;
        }
    }

    v2f32_t  size   = {(f32)width * atlas->inverse_size.x, (f32)height * atlas->inverse_size.y};
    v2f32_t  pos    = {(f32)atlas->xcursor * atlas->inverse_size.x, (f32)atlas->ycursor * atlas->inverse_size.y};
    r2f32_t result = {pos.x, pos.y, pos.x + size.x, pos.y + size.y};

    atlas->xcursor += width + spacing;
    if (height > atlas->biggest_height) {
        atlas->biggest_height = height;
    }

    return result;
}

rn_font_t rn_create_font(ma_arena_t *glyph_arena, s8_t font_data, rn_atlas_t *atlas, i32 size) {
    rn_font_t result             = {};
    stbtt_fontinfo stb_font;
    i32 success = stbtt_InitFont(&stb_font, (const unsigned char *)font_data.str, 0);
    if (!success) {
        return result;
    }

    f32 scale = stbtt_ScaleForPixelHeight(&stb_font, (f32)size);
    // f32 em_scale = stbtt_ScaleForMappingEmToPixels(&stb_font, (f32)size);

    i32 ascent, descent, line_gap;
    stbtt_GetFontVMetrics(&stb_font, &ascent, &descent, &line_gap);
    result.ascent                     = (f32)ascent * scale;
    result.descent                    = (f32)descent * scale;
    result.line_gap                   = (f32)line_gap * scale;
    result.size                       = (f32)size;
    result.first_char                 = ' ';
    result.last_char                  = '~';
    result.white_texture_bounding_box = atlas->white_texture_bounding_box;

    i32 glyph_count = result.last_char - result.first_char + 1; // + 1 because it's '<=' not '<'
    result.glyphs = ma_push_array(glyph_arena, rn_glyph_t, glyph_count);

    for (u32 ascii_symbol = result.first_char; ascii_symbol <= result.last_char; ascii_symbol++) {
        i32      width, height, xoff, yoff;
        u8 *bitmap = (u8 *)stbtt_GetCodepointBitmap(&stb_font, 0, scale, ascii_symbol, &width, &height, &xoff, &yoff);

        i32 xadvance, left_side_bearing;
        stbtt_GetCodepointHMetrics(&stb_font, ascii_symbol, &xadvance, &left_side_bearing);

        rn_glyph_t glyph              = {};
        glyph.atlas_bounding_box = rn_pack_bitmap(atlas, bitmap, width, height);
        glyph.size               = (v2f32_t){(f32)width, (f32)height};
        glyph.offset             = (v2f32_t){(f32)xoff, (f32)yoff};
        glyph.xadvance           = (f32)xadvance * scale;
        glyph.left_side_bearing  = (f32)left_side_bearing * scale;

        assert(result.glyph_count + 1 <= glyph_count);
        result.glyphs[result.glyph_count++] = glyph;

        stbtt_FreeBitmap(bitmap, 0);
    }

    return result;
}

rn_glyph_t *rn_get_glyph(rn_font_t *font, u32 codepoint) {
    b32 is_in_range = codepoint >= font->first_char && codepoint <= font->last_char;
    if (is_in_range) {
        u32 index = codepoint - font->first_char;
        return &font->glyphs[index];
    } else {
        u32 index = '?' - font->first_char;
        return &font->glyphs[index];
    }
}