Fonts

In LittlevGL fonts are collections of bitmaps and other information required to render the images of the letters (glyph). A font is stored in a lv_font_t variable and can be set in style’s text.font field. For example:

my_style.text.font = &lv_font_roboto_28;  /*Set a larger font*/

The fonts have a bpp (bits per pixel) property. It shows how many bits are used to describe a pixel in the font. The value stored for a pixel determines the pixel’s opacity. This way, with higher bpp, the edges of the letter can be smoother. The possible bpp values are 1, 2, 4 and 8 (higher value means better quality).

The bpp also affects the required memory size to store the font. For example, bpp = 4 makes the font nearly 4 times greater compared to bpp = 1.

Unicode support

LittlevGL supports UTF-8 encoded Unicode characters. You need to configure your editor to save your code/text as UTF-8 (usually this the default) and be sure that, LV_TXT_ENC is set to LV_TXT_ENC_UTF8 in lv_conf.h. (This is the default value)

To test it try

lv_obj_t * label1 = lv_label_create(lv_scr_act(), NULL);
lv_label_set_text(label1, LV_SYMBOL_OK);

If all works well, a ✓ character should be displayed.

Built-in fonts

There are several built-in fonts in different sizes, which can be enabled in lv_conf.h by LV_FONT_… defines:

  • LV_FONT_ROBOTO_12 12 px

  • LV_FONT_ROBOTO_16 16 px

  • LV_FONT_ROBOTO_22 22 px

  • LV_FONT_ROBOTO_28 28 px

The built-in fonts are global variables with names like lv_font_roboto_16 for 16 px hight font. To use them in a style, just add a pointer to a font variable like shown above.

The built-in fonts have bpp = 4, contains the ASCII characters and uses the Roboto font.

In addition to the ASCII range, the following symbols are also added to the built-in fonts from the FontAwesome font.

Built-in Symbols in LittlevGL

The symbols can be used as:

lv_label_set_text(my_label, LV_SYMBOL_OK);

Or with together with strings:

lv_label_set_text(my_label, LV_SYMBOL_OK "Apply");

Or more symbols together:

lv_label_set_text(my_label, LV_SYMBOL_OK LV_SYMBOL_WIFI LV_SYMBOL_PLAY);

Special features

Bidirectional support

Most of the languages use Left-to-Right (LTR for short) writing direction, however some languages (such as Hebrew) uses Right-to-Left (RTL for short) direction.

LittlevGL not only supports RTL texts but supports mixed (a.k.a. bidirectional, BiDi) text rendering too. Some examples:

Bidirectional text examples

The BiDi support can be enabled by LV_USE_BIDI in lv_conf.h

All texts have a base direction (LTR or RTL) which determines some rendering rules and the default alignment of the text (Left or Right). However, in LittlevGL, base direction is not only for labels. It’s a general property which can be set for every object. If unset then it will be inherited from the parent. So it’s enough to set the base direction of the screen and every object will inherit it.

The default base direction of screen can be set by LV_BIDI_BASE_DIR_DEF in lv_conf.h.

To set an object’s base direction use lv_obj_set_base_dir(obj, base_dir). The possible base direction are:

  • LV_BIDI_DIR_LTR: Left to Right base direction

  • LV_BIDI_DIR_RTL: Right to Left base direction

  • LV_BIDI_DIR_AUTO: Auto detect base direction

  • LV_BIDI_DIR_INHERIT: Inherit the base direction from the parent (default for non-screen objects)

This list summarizes the effect of RTL base direction on objects:

  • Create objects by default on the right

  • lv_tabview: displays tabs from right to left

  • lv_cb: Show the box on the right

  • lv_btnm: Show buttons from right to left

  • lv_list: Show the icon on the right

  • lv_ddlist: Align the options to the right

  • The texts in lv_table, lv_btnm, lv_kb, lv_tabview, lv_ddlist, lv_roller are processed to display correctly with RTL parts too

Subpixel rendering

Subpixel rendering means to increase the horizontal resolution by rendering on Red, Green and Blue channel instead of pixel level. It results in higher quality letter anti-alaising.

Subpixel rendering requires to generate the fonts with special settings:

  • In the online converter tick the Subpixel box

  • In the command line tool use --lcd flag. Note that the generated font needs about 3 times more memory.

Subpixel rendering works only if the color channels of the pixels have a horizontal layout. That is the R, G, B channels are next eachother and not above eachother. The order of color channels also needs to match with the library settings. By default the LittlevGL assumes RGB order, however it can ba swapped by setting LV_SUBPX_BGR  1 in lv_conf.h.

Compress fonts

The bitmaps of the fonts can be compressed by

  • ticking the Compressed check box in the online converter

  • not passing --no-compressflag to the offline converter (applies compression by default)

The compression is more effective with larger fonts and higher bpp. However, it’s about 30% slower to render the compressed fonts. Therefore it’s recommended to compress only the largest fonts of user interface, because

  • they need the most memory

  • they can be compressed better

  • and probably they are used less frequently then the medium sized fonts. (so performance cost is smaller)

Add new font

There are several ways to add a new font to your project:

  1. The simplest method is to use the Online font converter. Just set the parameters, click the Convert button, copy the font to your project and use it. Be sure to carefully read the steps provided on that site or you will get an error while converting.

  2. Use the Offline font converter. (Requires Node.js to be installed)

  3. If you want to create something like the built-in fonts (Roboto font and symbols) but in different size and/or ranges, you can use the built_in_font_gen.py script in lvgl/scripts/built_in_font folder. (It requires Python and lv_font_conv to be installed)

To declare the font in a file, use LV_FONT_DECLARE(my_font_name).

To make the fonts globally available (like the builtin fonts), add them to LV_FONT_CUSTOM_DECLARE in lv_conf.h.

Add new symbols

The built-in symbols are created from FontAwesome font. To add new symbols from the FontAwesome font do the following steps:

  1. Search symbol on https://fontawesome.com. For example the USB symbol

  2. Open the Online font converter add FontAwesome.ttf and add the Unicode ID of the symbol to the range field. E.g. 0xf287 for the USB symbol. More symbols can be enumerated with ,.

  3. Convert the font and copy it to your project.

  4. Convert the Unicode value to UTF8. You can do it e.g.on this site. For 0xf287 the Hex UTF-8 bytes are EF 8A 87.

  5. Create a define from the UTF8 values: #define MY_USB_SYMBOL "\xEF\x8A\x87"

  6. Use the symbol as the built-in symbols. lv_label_set_text(label, MY_USB_SYMBOL)

Add a new font engine

LittlevGL’s font interface is designed to be very flexible. You don’t need to use LittlevGL’s internal font engine but, you can add your own. For example, use FreeType to real-time render glyphs from TTF fonts or use an external flash to store the font’s bitmap and read them when the library needs them.

To do this a custom lv_font_t variable needs to be created:

/*Describe the properties of a font*/
lv_font_t my_font;
my_font.get_glyph_dsc = my_get_glyph_dsc_cb;        /*Set a callback to get info about gylphs*/
my_font.get_glyph_bitmap = my_get_glyph_bitmap_cb;  /*Set a callback to get bitmap of a glyp*/
my_font.line_height = height;                       /*The real line height where any text fits*/
my_font.base_line = base_line;                      /*Base line measured from the top of line_height*/
my_font.dsc = something_required;                   /*Store any implementation specific data here*/
my_font.user_data = user_data;                      /*Optionally some extra user data*/

...

/* Get info about glyph of `unicode_letter` in `font` font.
 * Store the result in `dsc_out`.
 * The next letter (`unicode_letter_next`) might be used to calculate the width required by this glyph (kerning)
 */
bool my_get_glyph_dsc_cb(const lv_font_t * font, lv_font_glyph_dsc_t * dsc_out, uint32_t unicode_letter, uint32_t unicode_letter_next)
{
    /*Your code here*/

    /* Store the result.
     * For example ...
     */
    dsc_out->adv_w = 12;        /*Horizontal space required by the glyph in [px]*/
    dsc_out->box_h = 8;         /*Height of the bitmap in [px]*/
    dsc_out->box_w = 6;         /*Width of the bitmap in [px]*/
    dsc_out->ofs_x = 0;         /*X offset of the bitmap in [pf]*/
    dsc_out->ofs_y = 3;         /*Y offset of the bitmap measured from the as line*/
    dsc_out->bpp   = 2;         /*Bits per pixel: 1/2/4/8*/

    return true;                /*true: glyph found; false: glyph was not found*/
}


/* Get the bitmap of `unicode_letter` from `font`. */
const uint8_t * my_get_glyph_bitmap_cb(const lv_font_t * font, uint32_t unicode_letter)
{
    /* Your code here */

    /* The bitmap should be a continuous bitstream where
     * each pixel is represented by `bpp` bits */

    return bitmap;    /*Or NULL if not found*/
}