Collaboration diagram for Font Handling:

As LaTeX also does, fonts are set by font classes. These are defined in JKQTMathTextEnvironmentFont:

A "roman" (MTEroman / MTEmathRoman) font used as the standard font ( JKQTMathText::setFontRoman() and for use in math mode JKQTMathText::setFontMathRoman() )
A "sans-serif" (MTEsans / MTEmathSans) font which may be activated with \sf ... ( JKQTMathText::setFontSans() and for use in math mode JKQTMathText::setFontMathSans() )
A "typewriter" (MTEtypewriter) font which may be activated with \tt ... ( JKQTMathText::setFontTypewriter() )
A "script" (MTEscript) font which may be activated with \script ... ( JKQTMathText::setFontScript() )
A "math-roman" (MTEmathRoman) font used as the standard font in math mode ( JKQTMathText::setFontMathRoman() )
A "math-sans-serif" (MTEmathSans) used as sans serif font in math mode ( JKQTMathText::setFontMathSans() )
A "blackboard" (MTEblackboard) font used to display double stroked characters ( JKQTMathText::setFontBlackboard() )
A "caligraphic" (MTEcaligraphic) font used to display caligraphic characters ( JKQTMathText::setFontCaligraphic() )
A "fraktur" (MTEfraktur) font used to display fraktur characters ( JKQTMathText::setFontFraktur() )
A fallback font MTEFallbackSymbols for (math) symbols, greek letters ... (if the symbols are not present in the currently used font). ( JKQTMathText::setFallbackFontSymbols() )

The default font class is "roman" (i.e. MTEroman in text-mode and MTEmathRoman in math-mode). You can switch to "sans-serif" by using LaTeX commands like \sf .

Especially when using the library in the context of a GUI, usually a sans-serif font is used to typeset text in the GUI as these are generally better readable on a screen than serif fonts. Therefore when using JKQTMathText for GUI rendering, it makes sense to redefine the class "roman" with the GUI font. You can e.g. write:

JKQTMathText::setFontRoman(QGuiApplication::font().family(), MTFEUnicode);
JKQTMathText::setFontMathRoman(QGuiApplication::font().family(), MTFEUnicode);
JKQTMathText::setFontSize(QGuiApplication::font().pointSize());

or the short hand:

JKQTMathText::setFontRomanAndMath(QGuiApplication::font().family(), MTFEUnicode);

JKQTMathText::setFontRomanAndMath

void setFontRomanAndMath(const QString &fontName, JKQTMathTextFontEncoding encoding=JKQTMathTextFontEncoding::MTFEStandard)

set the font fontName and it's encoding encoding to be used for text and math, i.e....

With JKQTMathTextFontSpecifier::fromFontSpec() there is a third way that allows to use a special string syntax (handy for INI files) to define fonts, see JKQTMathTextFontSpecifier . You can e.g. simply write:

JKQTMathText::setFontRomanAndMath("application+XITS", MTFEUnicode);

In all cases the default GUI font is used for rendering default output. For \sf to make a difference you can additionally set the "sans" class as a serif font, e.g.:

JKQTMathText::setFontSans("serif");

JKQTMathText::setFontMathSans("serif");

JKQTMathText::setFontMathSans

void setFontMathSans(const QString &fontName, JKQTMathTextFontEncoding encoding=JKQTMathTextFontEncoding::MTFEStandard)

set the font fontName and it's encoding encoding to be used for text in the logical font MTEmathSans

JKQTMathText::setFontSans

void setFontSans(const QString &fontName, JKQTMathTextFontEncoding encoding=JKQTMathTextFontEncoding::MTFEStandard)

set the font fontName and it's encoding encoding to be used for text in the logical font MTEsans

Using the font name "serif" will cause Qt to lookup a suitable serif font.

These fonts are generic font classes, which font is actually used can be configured in JKQTMathText class with the set...() functions mentioned above. Additionally some short ahnds exist:

JKQTMathText::useSTIX() use the STIX fonts from https://www.stixfonts.org/ in math-mode (or math- and text-mode, additionally also as MTEFallbackSymbols)
JKQTMathText::useXITS() use the XITS fonts from https://github.com/alif-type/xits in math-mode (or math- and text-mode, additionally also as MTEFallbackSymbols). These are included by default in this library and also activated by default.
JKQTMathText::useASANA() use the ASANA fonts from https://ctan.org/tex-archive/fonts/Asana-Math/ in math-mode (or math- and text-mode, additionally also as MTEFallbackSymbols)
JKQTMathText::useFiraMath() use the Fira Math fonts from https://github.com/firamath/firamath in math-mode (or math- and text-mode, additionally also as MTEFallbackSymbols)
JKQTMathText::useGuiFonts() use the default application font (e.g. Segoe UI on Win10/11) for the "roman" and "math-roman" and try to determine mathing fonts for "sans"/"math-sans" and MTEFallbackSymbols.
JKQTMathText::useAnyUnicode() set fonts for "roman" and "sans" in text- and math-mode with one call
JKQTMathText::useAnyUnicodeForMathOnly() set fonts for "roman" and "sans" in math-mode only with one call, leaves text-mode fonts as is
JKQTMathText::useAnyUnicodeForTextOnly() set fonts for "roman" and "sans" in text-mode only with one call, leaves math-mode fonts as is
JKQTMathText::setFontRomanAndMath() set "roman" font in text- and math-mode
JKQTMathText::setFontRomanOrSpecial() use an object of type JKQTMathTextFontSpecifier or a special string syntax (see struct documentation of JKQTMathTextFontSpecifier) to define the text- and math-mode "roman" fonts
JKQTMathText::setFontSpecial() seit "roman" and "sans" fonts from a parsed specifier, see function documentation for details

Here are some examples of using different, widely available, fonts:

using "Arial" JKQTMathText::setFontRomanAndMath("Arial"):
using "Courier New" JKQTMathText::setFontRomanAndMath("Courier New"):
using "Comic Sans MS" JKQTMathText::setFontRomanAndMath("Comic Sans MS"):
using "Old English Text" JKQTMathText::setFontRomanAndMath("Old English Text"):
using "Computer Modern" JKQTMathText::setFontRomanAndMath("CMU Serif"):
using "Fira" JKQTMathText::setFontRoman("Fira Sans"); setFontMathRoman("Fira Math"):
using "MS Segoe UI" (the default Win 10/11 GUI font) JKQTMathText::setFontRomanAndMath("MS Segoe UI"):
using "DejaVu Sans" JKQTMathText::setFontRomanAndMath("DejaVu Sans"):
using "DejaVu Serif" JKQTMathText::setFontRomanAndMath("DejaVu Serif"):
using "XITS" JKQTMathText::useXITS(false):
using "Noto Sans" JKQTMathText::setFontRomanAndMath("Noto Sans", "Noto Sans Math"):
using "Noto Serif" JKQTMathText::setFontRomanAndMath("Noto Serif", "Noto Serif"):

Math-mode is activated by enclosing your equation in $...$ or \[...\] . This mode is optimized for mathematical equations. Here is an example of the difference:

math-mode:
- use fonts MTEmathRoman and MTEmathSans
- whitespaces are mostly not drawn directly
- symbol spacing is different
- an example:

text-mode:
- use fonts MTEroman and MTEsans
- whitespaces are evaluated directly
- an example:

Font Lookup for symbols works as follows in JKQTMathTextSymbolNode:

if a character is found in the current (or to be used) font, it is taken from there
if the character is not found, it is looked for in the fallback fonts MTEFallbackSymbols
as a last resort, some symbols can be created otherwise, so if neither of the two options above contain the required symbol, the symbol might be synthesized otherwise, or a rectangle with the size of "X" is drawn instead

In "normal text", i.e. JKQTMathTextTextNode no font-lookup is perfomed, i.e. if a acharacter is not available in the selected font, it is not typeset (or typeset with a replacement, like a box). There are exceptions to this rule however: For some special fonts a lookup or synthetisation of the characters is performed. Currently this is implemented for:

"blackboard", the options are defined by setBlackboardFontMode(). See JKQTMathTextBlackboradDrawingMode