Fonts

In LVGL 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:

lv_style_set_text_font(&my_style, LV_STATE_DEFAULT, &lv_font_montserrat_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

LVGL supports UTF-8 encoded Unicode characters. Your editor needs to be configureed 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_MONTSERRAT_12 12 px ASCII + built-in symbol

  • LV_FONT_MONTSERRAT_14 14 px ASCII + built-in symbol

  • LV_FONT_MONTSERRAT_16 16 px ASCII + built-in symbol

  • LV_FONT_MONTSERRAT_18 18 px ASCII + built-in symbol

  • LV_FONT_MONTSERRAT_20 20 px ASCII + built-in symbol

  • LV_FONT_MONTSERRAT_22 22 px ASCII + built-in symbol

  • LV_FONT_MONTSERRAT_24 24 px ASCII + built-in symbol

  • LV_FONT_MONTSERRAT_26 26 px ASCII + built-in symbol

  • LV_FONT_MONTSERRAT_28 28 px ASCII + built-in symbol

  • LV_FONT_MONTSERRAT_30 30 px ASCII + built-in symbol

  • LV_FONT_MONTSERRAT_32 32 px ASCII + built-in symbol

  • LV_FONT_MONTSERRAT_34 34 px ASCII + built-in symbol

  • LV_FONT_MONTSERRAT_36 36 px ASCII + built-in symbol

  • LV_FONT_MONTSERRAT_38 38 px ASCII + built-in symbol

  • LV_FONT_MONTSERRAT_40 40 px ASCII + built-in symbol

  • LV_FONT_MONTSERRAT_42 42 px ASCII + built-in symbol

  • LV_FONT_MONTSERRAT_44 44 px ASCII + built-in symbol

  • LV_FONT_MONTSERRAT_46 46 px ASCII + built-in symbol

  • LV_FONT_MONTSERRAT_48 48 px ASCII + built-in symbol

  • LV_FONT_MONTSERRAT_12_SUBPX 12 px font with subpixel rendering

  • LV_FONT_MONTSERRAT_28_COMPRESSED 28 px compressed font with 3 bpp

  • LV_FONT_DEJAVU_16_PERSIAN_HEBREW 16 px Hebrew, Arabic, Perisan letters and all their forms

  • LV_FONT_SIMSUN_16_CJK 16 px 1000 most common CJK radicals

  • LV_FONT_UNSCII_8 8 px pixel perfect font

The built-in fonts are global variables with names like lv_font_montserrat_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 Montserrat font.

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

../_images/symbols.png

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, Persian or Arabic) uses Right-to-Left (RTL for short) direction.

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

../_images/bidi.png

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 LVGL, base direction is applied 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 and other objects inherit the base direction from their parent.

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_checkbox: Show the box on the right

  • lv_btnmatrix: Show buttons from right to left

  • lv_list: Show the icon on the right

  • lv_dropdown: Align the options to the right

  • The texts in lv_table, lv_btnmatrix, lv_keyboard, lv_tabview, lv_dropdown, lv_roller are "BiDi processed" to be displayed correctly

Arabic and Persian support

There are some special rules to display Arabic and Persian characters: the form of the character depends on their position in the text. A different form of the same letter needs to be used if it isolated, start, middle or end position. Besides these some conjunction rules also should be taken into account.

LVGL supports to apply these rules if LV_USE_ARABIC_PERSIAN_CHARS is enabled.

However, there some limitations:

  • Only displaying texts is supported (e.g. on labels), text inputs (e.g. text area) doesn't support this feature

  • Static text (i.e. const) are not processed. E.g. texts set by lv_label_set_text() will "Arabic processed" but lv_lable_set_text_static() won't.

  • Text get functions (e.g. lv_label_get_text()) will return the processed text.

Subpixel rendering

Subpixel rendering means to triple the horizontal resolution by rendering on Red, Green and Blue channel instead of pixel level. It takes advantage of the position of physical color channels of each pixel. It results in higher quality letter anti-aliasing. Lear more here.

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 each other and not above each other. The order of color channels also needs to match with the library settings. By default the LVGL assumes RGB order, however it can be 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-compress flag 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.

  1. Search symbol on https://fontawesome.com. For example the USB symbol. Copy it's Unicode ID which is 0xf287 in this case.

  2. Open the Online font converter. Add Add FontAwesome.woff. .

  3. Set the parameters such as Name, Size, BPP. You'll use this name to declare and use the font in your code.

  4. Add the Unicode ID of the symbol to the range field. E.g. 0xf287 for the USB symbol. More symbols can be enumerated with ,.

  5. Convert the font and copy it to your project. Make sure to compile the .c file of your font.

  6. Declare the font using extern lv_font_t my_font_name; or simply LV_FONT_DECLARE(my_font_name);.

Using the symbol

  1. 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.

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

  3. Create a label and set the text. Eg. lv_label_set_text(label, MY_USB_SYMBOL)

Note - lv_label_set_text(label, MY_USB_SYMBOL) searches for this symbol in the font defined in style.text.font properties. To use the symbol you may need to change it. Eg style.text.font = my_font_name

Load font in run-time

lv_font_load can be used to load a font from a file. The font to load needs to have a special binary format. (Not TTF or WOFF). Use lv_font_conv with --format bin option to generate an LVGL compatible font file.

Note that to load a font LVGL's filesystem needs to be enabled and a driver needs to be added.

Example

lv_font_t * my_font;
my_font = lv_font_load(X/path/to/my_font.bin);

/*Use the font*/

/*Free the font if not required anymore*/
lv_font_free(my_font);

Add a new font engine

LVGL's font interface is designed to be very flexible. You don't need to use LVGL'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.

A raedy to use FreeType can be found in lv_freetype repository.

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*/
}