reducing memory required for KeDei TFT library used with 3.5" TFT display with Arduino

Question

I need to reduce the memory required by the KeDei TFT library used with the Osoyoo 3.5" TFT touch screen display shield for Arduino Uno and ATmega 2560. When I try writing a simple Arduino application that uses the TFT display with the KeDei library, most of the available memory on the Arduino is taken up by the library itself.

Unfortunately I have discovered that while an ATmega 2560 does have the necessary amount of memory, the KeDei TFT library does not provide correct touch coordinates when the TFT display is used with that device so the ATmega 2560 is not feasible unless Osoyoo customer support comes though with a solution.

Investigating the library source code, I found in the file KeDei_font.cpp a bitmap font table being used to generate the characters displayed. This bitmap font table is an array, unsigned char font16_B[96][16] and appears to be the main memory hog. This array contains bitmap fonts for the ASCII characters from the space character, 0x20, to the tilde character, 0x7e.

One thing that I have done is to reduce the number of characters by eliminating the lower case letters and transforming lower case letters to upper case. This results in a table const unsigned char font16_B[59][16] which is a bit more than half the size of the original table.

With this approach I also eliminate a few other punctuation type characters but as long as I'm displaying only alphanumeric characters and spaces, this will work.

#if defined(SMALLER_FONT_TABLE)
    static const unsigned char font16_B[59][16] = {
#else
    static const unsigned char font16_B[95][16] = {
#endif
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00},/*" ",0*/
            {0x00,0x00,0x00,0x18,0x18,0x18,0x18,0x18,0x18,0x18,0x00,0x00,0x38,0x38,0x00,0x00},/*"!",1*/
            {0x00,0xD8,0xFC,0x6C,0x36,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00},/*""",2*/
            {0x00,0x00,0x00,0x6C,0x6C,0x6C,0xFF,0x36,0x36,0x36,0xFF,0x36,0x36,0x36,0x00,0x00},/*"#",3*/
            {0x00,0x00,0x18,0x3C,0x7E,0x7E,0x1E,0x1C,0x38,0x78,0x78,0x7E,0x7E,0x3C,0x18,0x18},/*"$",4*/
            {0x00,0x00,0x00,0x66,0x6F,0x3F,0x3F,0x3F,0x7E,0xF8,0xFC,0xFC,0xFC,0x66,0x00,0x00},/*"%",5*/
            {0x00,0x00,0x00,0x1C,0x36,0x36,0x36,0x1E,0xFE,0x6F,0x7B,0x33,0xB3,0xFE,0x00,0x00},/*"&",6*/
            {0x00,0x0E,0x0E,0x0C,0x07,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00},/*"'",7*/
            {0x00,0xC0,0x60,0x30,0x30,0x18,0x18,0x18,0x18,0x18,0x18,0x30,0x30,0x60,0xC0,0x00},/*"(",8*/
            {0x00,0x06,0x0C,0x18,0x18,0x30,0x30,0x30,0x30,0x30,0x30,0x18,0x18,0x0C,0x06,0x00},/*")",9*/
            {0x00,0x00,0x00,0x00,0x18,0x18,0xFF,0x3C,0x3C,0xFF,0x18,0x18,0x00,0x00,0x00,0x00},/*"*",10*/
            {0x00,0x00,0x00,0x00,0x18,0x18,0x18,0x18,0xFF,0x18,0x18,0x18,0x18,0x00,0x00,0x00},/*"+",11*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x0E,0x0E,0x0C,0x07},/*",",12*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0xFE,0x00,0x00,0x00,0x00,0x00,0x00,0x00},/*"-",13*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x0E,0x0E,0x00,0x00},/*".",14*/
            {0x00,0x00,0x80,0xC0,0xC0,0x60,0x60,0x30,0x30,0x18,0x18,0x0C,0x0C,0x06,0x06,0x00},/*"/",15*/
            {0x00,0x00,0x00,0x38,0x6C,0xC6,0xC6,0xC6,0xC6,0xC6,0xC6,0xC6,0x6C,0x38,0x00,0x00},/*"0",16*/
            {0x00,0x00,0x00,0x18,0x1E,0x18,0x18,0x18,0x18,0x18,0x18,0x18,0x18,0x7E,0x00,0x00},/*"1",17*/
            {0x00,0x00,0x00,0x7C,0xC6,0xC6,0xC6,0x60,0x60,0x30,0x18,0x0C,0xC6,0xFE,0x00,0x00},/*"2",18*/
            {0x00,0x00,0x00,0x7C,0xC6,0xC6,0x60,0x38,0x60,0xC0,0xC0,0xC6,0x66,0x3C,0x00,0x00},/*"3",19*/
            {0x00,0x00,0x00,0x60,0x70,0x78,0x6C,0x6C,0x66,0x66,0xFE,0x60,0x60,0xF8,0x00,0x00},/*"4",20*/
            {0x00,0x00,0x00,0xFE,0x06,0x06,0x06,0x3E,0x6E,0xC0,0xC0,0xC6,0x66,0x3C,0x00,0x00},/*"5",21*/
            {0x00,0x00,0x00,0x78,0x6C,0x06,0x06,0x3E,0x6E,0xC6,0xC6,0xC6,0x6C,0x38,0x00,0x00},/*"6",22*/
            {0x00,0x00,0x00,0xFE,0x66,0x66,0x30,0x30,0x18,0x18,0x18,0x18,0x18,0x18,0x00,0x00},/*"7",23*/
            {0x00,0x00,0x00,0x7C,0xC6,0xC6,0xC6,0x6C,0x38,0x6C,0xC6,0xC6,0xC6,0x7C,0x00,0x00},/*"8",24*/
            {0x00,0x00,0x00,0x38,0x6C,0xC6,0xC6,0xC6,0xEC,0xF8,0xC0,0xC0,0x6C,0x3C,0x00,0x00},/*"9",25*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x38,0x38,0x00,0x00,0x00,0x00,0x38,0x38,0x00,0x00},/*":",26*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x18,0x00,0x00,0x00,0x00,0x00,0x18,0x18,0x0C},/*";",27*/
            {0x00,0x00,0x00,0xC0,0x60,0x30,0x18,0x0C,0x06,0x0C,0x18,0x30,0x60,0xC0,0x00,0x00},/*"<",28*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0xFF,0x00,0x00,0x00,0xFF,0x00,0x00,0x00,0x00,0x00},/*"=",29*/
            {0x00,0x00,0x00,0x06,0x0C,0x18,0x30,0x60,0xC0,0x60,0x30,0x18,0x0C,0x06,0x00,0x00},/*">",30*/
            {0x00,0x00,0x00,0x7C,0xC6,0xC6,0xCE,0xC0,0x60,0x30,0x30,0x00,0x38,0x38,0x00,0x00},/*"?",31*/
            {0x00,0x00,0x00,0x3C,0x66,0xFE,0xFF,0xFF,0xFF,0xFF,0x7F,0xC6,0x66,0x3C,0x00,0x00},/*"@",32*/
            {0x00,0x00,0x00,0x18,0x18,0x38,0x3C,0x3C,0x6C,0x7C,0x66,0xC6,0xC6,0xEF,0x00,0x00},/*"A",33*/
            {0x00,0x00,0x00,0x3F,0x66,0x66,0x66,0x3E,0x66,0xC6,0xC6,0xC6,0x66,0x3F,0x00,0x00},/*"B",34*/
            {0x00,0x00,0x00,0xFC,0xC6,0xC6,0x03,0x03,0x03,0x03,0x03,0xC6,0x66,0x3C,0x00,0x00},/*"C",35*/
            {0x00,0x00,0x00,0x3F,0x66,0xC6,0xC6,0xC6,0xC6,0xC6,0xC6,0xC6,0x66,0x3F,0x00,0x00},/*"D",36*/
            {0x00,0x00,0x00,0x7F,0xC6,0x36,0x36,0x3E,0x36,0x36,0x06,0xC6,0xC6,0x7F,0x00,0x00},/*"E",37*/
            {0x00,0x00,0x00,0x7F,0xC6,0x36,0x36,0x3E,0x36,0x36,0x06,0x06,0x06,0x0F,0x00,0x00},/*"F",38*/
            {0x00,0x00,0x00,0x7C,0x66,0x66,0x03,0x03,0x03,0xF3,0x63,0x66,0x66,0x3C,0x00,0x00},/*"G",39*/
            {0x00,0x00,0x00,0xEF,0xC6,0xC6,0xC6,0xC6,0xFE,0xC6,0xC6,0xC6,0xC6,0xEF,0x00,0x00},/*"H",40*/
            {0x00,0x00,0x00,0x7E,0x18,0x18,0x18,0x18,0x18,0x18,0x18,0x18,0x18,0x7E,0x00,0x00},/*"I",41*/
            {0x00,0x00,0x00,0xFC,0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x33,0x1F},/*"J",42*/
            {0x00,0x00,0x00,0xFF,0x66,0x36,0x1E,0x1E,0x1E,0x36,0x36,0x66,0x66,0xFF,0x00,0x00},/*"K",43*/
            {0x00,0x00,0x00,0x0F,0x06,0x06,0x06,0x06,0x06,0x06,0x06,0x06,0xC6,0xFF,0x00,0x00},/*"L",44*/
            {0x00,0x00,0x00,0xFF,0x7E,0x7E,0x7E,0x7E,0x7E,0x7E,0x7E,0x7E,0x7E,0xFF,0x00,0x00},/*"M",45*/
            {0x00,0x00,0x00,0xE7,0xCE,0xCE,0xDE,0xDE,0xF6,0xF6,0xF6,0xE6,0xE6,0xCF,0x00,0x00},/*"N",46*/
            {0x00,0x00,0x00,0x3C,0x66,0xC3,0xC3,0xC3,0xC3,0xC3,0xC3,0xC3,0x66,0x3C,0x00,0x00},/*"O",47*/
            {0x00,0x00,0x00,0x7F,0xC6,0xC6,0xC6,0xC6,0x7E,0x06,0x06,0x06,0x06,0x0F,0x00,0x00},/*"P",48*/
            {0x00,0x00,0x00,0x3C,0x66,0xC3,0xC3,0xC3,0xC3,0xC3,0xDF,0xF7,0x76,0x3C,0xE0,0x00},/*"Q",49*/
            {0x00,0x00,0x00,0x7F,0xC6,0xC6,0xC6,0x7E,0x36,0x36,0x66,0x66,0xC6,0xCF,0x00,0x00},/*"R",50*/
            {0x00,0x00,0x00,0xFC,0xC6,0xC6,0x06,0x0C,0x38,0x60,0xC0,0xC6,0xC6,0x7E,0x00,0x00},/*"S",51*/
            {0x00,0x00,0x00,0xFF,0xDB,0x18,0x18,0x18,0x18,0x18,0x18,0x18,0x18,0x3C,0x00,0x00},/*"T",52*/
            {0x00,0x00,0x00,0xEF,0xC6,0xC6,0xC6,0xC6,0xC6,0xC6,0xC6,0xC6,0xC6,0x7C,0x00,0x00},/*"U",53*/
            {0x00,0x00,0x00,0xEF,0xC6,0xC6,0x66,0x6C,0x6C,0x3C,0x3C,0x38,0x18,0x18,0x00,0x00},/*"V",54*/
            {0x00,0x00,0x00,0xFF,0xDB,0xDB,0xDB,0xDB,0xFF,0xFF,0x7E,0x66,0x66,0x66,0x00,0x00},/*"W",55*/
            {0x00,0x00,0x00,0xEF,0xC6,0x6C,0x6C,0x38,0x38,0x38,0x6C,0x6C,0xC6,0xEF,0x00,0x00},/*"X",56*/
            {0x00,0x00,0x00,0xFF,0x66,0x66,0x3C,0x3C,0x18,0x18,0x18,0x18,0x18,0x3C,0x00,0x00},/*"Y",57*/
            {0x00,0x00,0x00,0xFE,0x63,0x60,0x30,0x30,0x18,0x0C,0x0C,0xC6,0xC6,0x7F,0x00,0x00},/*"Z",58*/
#if !defined(SMALLER_FONT_TABLE)
            // In the interest of reducing the memory requirements for the TFT library, we
            // allow for the use of a smaller font table which eliminates the lower case letters
            // as well as a few symbols. See below conversion of lower case letters to upper case.
            //    Richard Chambers, 04-25-2021
            {0x00,0xF8,0x18,0x18,0x18,0x18,0x18,0x18,0x18,0x18,0x18,0x18,0x18,0x18,0xF8,0x00},/*"[",59*/
            {0x00,0x00,0x06,0x06,0x0C,0x0C,0x18,0x18,0x18,0x30,0x30,0x60,0x60,0x60,0xC0,0xC0},/*"\",60*/
            {0x00,0x3E,0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x3E,0x00},/*"]",61*/
            {0x00,0x78,0xCC,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00},/*"^",62*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0xFF},/*"_",63*/
            {0x00,0x0E,0x18,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00},/*"`",64*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x7C,0xC6,0xF8,0xCC,0xC6,0xC6,0xFC,0x00,0x00},/*"a",65*/
            {0x00,0x00,0x00,0x07,0x06,0x06,0x06,0x3E,0x6E,0xC6,0xC6,0xC6,0x6E,0x3E,0x00,0x00},/*"b",66*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x78,0xCC,0x06,0x06,0x06,0xCC,0x78,0x00,0x00},/*"c",67*/
            {0x00,0x00,0x00,0xE0,0xC0,0xC0,0xC0,0xF8,0xCC,0xC6,0xC6,0xC6,0xEC,0xF8,0x00,0x00},/*"d",68*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x7C,0xC6,0xFE,0x06,0x06,0xC6,0x7C,0x00,0x00},/*"e",69*/
            {0x00,0x00,0x00,0xF0,0x98,0x18,0x18,0xFE,0x18,0x18,0x18,0x18,0x18,0x7E,0x00,0x00},/*"f",70*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0xFC,0x66,0x66,0x3C,0x06,0x7C,0xC6,0xC6,0x7C},/*"g",71*/
            {0x00,0x00,0x00,0x07,0x06,0x06,0x06,0x7E,0xCE,0xC6,0xC6,0xC6,0xC6,0xEF,0x00,0x00},/*"h",72*/
            {0x00,0x00,0x00,0x1C,0x1C,0x00,0x00,0x1E,0x18,0x18,0x18,0x18,0x18,0x7E,0x00,0x00},/*"i",73*/
            {0x00,0x00,0x00,0x70,0x70,0x00,0x00,0x78,0x60,0x60,0x60,0x60,0x60,0x60,0x66,0x3E},/*"j",74*/
            {0x00,0x00,0x00,0x07,0x06,0x06,0x06,0xF6,0x36,0x1E,0x3E,0x36,0x66,0xFF,0x00,0x00},/*"k",75*/
            {0x00,0x00,0x00,0x1E,0x18,0x18,0x18,0x18,0x18,0x18,0x18,0x18,0x18,0x7E,0x00,0x00},/*"l",76*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0xFF,0xB6,0xB6,0xB6,0xB6,0xB6,0xFF,0x00,0x00},/*"m",77*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x7F,0xCE,0xC6,0xC6,0xC6,0xC6,0xEF,0x00,0x00},/*"n",78*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x7C,0xC6,0xC6,0xC6,0xC6,0xC6,0x7C,0x00,0x00},/*"o",79*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x3F,0x6E,0xC6,0xC6,0xC6,0x66,0x3E,0x06,0x0F},/*"p",80*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0xF8,0xCC,0xC6,0xC6,0xC6,0xCC,0xF8,0xC0,0xE0},/*"q",81*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0xFF,0xDC,0x0C,0x0C,0x0C,0x0C,0x3F,0x00,0x00},/*"r",82*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0xFC,0xC6,0x06,0x7C,0xC0,0xC6,0x7E,0x00,0x00},/*"s",83*/
            {0x00,0x00,0x00,0x00,0x00,0x18,0x18,0x7E,0x18,0x18,0x18,0x18,0x18,0x70,0x00,0x00},/*"t",84*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0xE7,0xC6,0xC6,0xC6,0xC6,0xE6,0xFC,0x00,0x00},/*"u",85*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0xEF,0xC6,0x6C,0x6C,0x3C,0x18,0x18,0x00,0x00},/*"v",86*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0xFF,0xDB,0xDB,0xFF,0xFF,0x66,0x66,0x00,0x00},/*"w",87*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0xFE,0x6C,0x38,0x38,0x38,0x6C,0xFE,0x00,0x00},/*"x",88*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0xEF,0xC6,0x6C,0x6C,0x3C,0x38,0x18,0x18,0x0F},/*"y",89*/
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0xFE,0x66,0x30,0x18,0x18,0xCC,0xFE,0x00,0x00},/*"z",90*/
            {0x00,0xC0,0x60,0x60,0x60,0x60,0x60,0x30,0x60,0x60,0x60,0x60,0x60,0x60,0xC0,0x00},/*"{",91*/
            {0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x30,0x30},/*"|",92*/
            {0x00,0x0E,0x18,0x18,0x18,0x18,0x18,0x30,0x18,0x18,0x18,0x18,0x18,0x18,0x0E,0x00},/*"}",93*/
            {0x1C,0x76,0xC6,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00},/*"~",94*/
#endif
        };

Then in the function that is generating the characters and displaying them pixel by pixel, I have the following bit of code that transforms lower case characters into upper case characters should I have the smaller table enabled.

#if defined(SMALLER_FONT_TABLE)
    // Lets translate the character to one of the printable
    // characters in our font table. What we are doing is
    // to translate lower case letters to upper case letters
    // since in order to have a smaller font table taking up
    // less memory we remove the lower case letters.
    //    Richard Chambers, 04-25-2021
    if (_data < ' ' || _data > 'z') return;
    if (_data >= 'a') _data = _data - 'a' + 'A';
#endif
    char_i=(int)_data - ' ';

With this approach, when I compile my Arduino sketch with the SMALLER_FONT_TABLE defined, the compile completes with the following warning:

Sketch uses 12844 bytes (39%) of program storage space. Maximum is 32256 bytes.
Global variables use 1556 bytes (75%) of dynamic memory, leaving 492 bytes for local variables. Maximum is 2048 bytes.
Low memory available, stability problems may occur.

What can I do to further reduce the memory required for the display font? Is there a different kind of bitmap font that uses less memory?

Strictly "alphanumeric" should be 36 characters A-Z, 0-9, + 1 for space = 37. Try that. Or rewrite the library, which is probably bloated and wasteful, as almost every Arduino library is. Other thing to consider is PROGMEM. Or a smaller font of your own design. — TomServo, May 02 '21 at 15:40
@TomServo I am in the process of rewriting the library. Currently I allow the choices of either the original 16x16 bitmap font or an 8x8 bitmap font I found in GitHub. I've also added the ability to use a bitmap font double high, double wide, or combine the two which provides a nice mix of styles. I'm now working on the bitmap display portion of the library. — Richard Chambers, May 02 '21 at 18:12
That is good. I've never seen a "popular" Arduino library that wasn't a slow, bloated mess, designed for simplicity for the tinkerer and hobbyist above all else. I've book marked your question because I'm interested in this type of display also. — TomServo, May 02 '21 at 20:32
@TomServo a fork of the KeDei TFT library with my changes is in my github repository at https://github.com/RichardChambers/driver/tree/master/KeDeiTFT — Richard Chambers, May 03 '21 at 03:43
@TomServo I've done further work and have posted a new question regarding standards and specifications for bitmap fonts. I've been looking for additional varieties of bitmap fonts with the idea of finding a smaller one and have met with some success. However what I am also finding is bitmap font tables that when I try to use them result in squiggles and doodles rather than characters. if you can shed any light on this, take a look at https://stackoverflow.com/questions/67465098/differences-in-bitmap-or-rasterized-font-bitmaps-and-text-display-on-3-5-tft-lc — Richard Chambers, May 10 '21 at 17:44
See as well the answers to this posted question which contains some font examples. https://stackoverflow.com/questions/2156572/c-header-file-with-bitmapped-fonts — Richard Chambers, May 17 '21 at 01:28

Richard Chambers · Accepted Answer · 2021-05-05T20:16:32.940

The first approach I looked into was to use some kind of compression on the bitmap font table such as run length encoding as so many entries were binary zero. I tested this approach and it did reduce the amount of memory while adding a bit of complexity. However the amount of memory saved was around 200 bytes with the simple approach I tested.

The second approach I looked at was reducing the size of the array by first eliminating the lower case letters and then by changing the bitmap font as well. Changing the bitmap font from a 16x16 size font to an 8x8 size font makes a significant difference in memory usage.

However changing the size of the table from const unsigned char font16_B[96][16] to const unsigned char font16_B[96][8] means that the characters displayed on the TFT screen will be smaller.

So there is a tradeoff between the amount of memory used and the character display size. Larger displayed characters requires more memory for the description of the glyphs.

A quick search for "8 bitmap font" finds this GitHub repository of Daniel Hepper, https://github.com/dhepper/font8x8, with an 8x8 size font and the license is Public Domain.

Using Preprocessor directives to select the font table to use and selecting a subsection of the file font8x8_basic.h from Hepper's GitHub repository, I added the following to the KeDei TFT library.

#if defined(USE_FONT_8_B)
#define FONT_ROW_LEN  8
#if defined(SMALLER_FONT_TABLE)
    static const unsigned char font8_B[59][8] = {
#else
    static const unsigned char font8_B[96][8] = {
#endif
        { 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 },   // U+0020 (space)
        { 0x18, 0x3C, 0x3C, 0x18, 0x18, 0x00, 0x18, 0x00 },   // U+0021 (!)
        { 0x36, 0x36, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 },   // U+0022 (")
        { 0x36, 0x36, 0x7F, 0x36, 0x7F, 0x36, 0x36, 0x00 },   // U+0023 (#)
        { 0x0C, 0x3E, 0x03, 0x1E, 0x30, 0x1F, 0x0C, 0x00 },   // U+0024 ($)
        { 0x00, 0x63, 0x33, 0x18, 0x0C, 0x66, 0x63, 0x00 },   // U+0025 (%)
        { 0x1C, 0x36, 0x1C, 0x6E, 0x3B, 0x33, 0x6E, 0x00 },   // U+0026 (&)
        { 0x06, 0x06, 0x03, 0x00, 0x00, 0x00, 0x00, 0x00 },   // U+0027 (')
        { 0x18, 0x0C, 0x06, 0x06, 0x06, 0x0C, 0x18, 0x00 },   // U+0028 (()
        { 0x06, 0x0C, 0x18, 0x18, 0x18, 0x0C, 0x06, 0x00 },   // U+0029 ())
        { 0x00, 0x66, 0x3C, 0xFF, 0x3C, 0x66, 0x00, 0x00 },   // U+002A (*)
        { 0x00, 0x0C, 0x0C, 0x3F, 0x0C, 0x0C, 0x00, 0x00 },   // U+002B (+)
        { 0x00, 0x00, 0x00, 0x00, 0x00, 0x0C, 0x0C, 0x06 },   // U+002C (,)
        { 0x00, 0x00, 0x00, 0x3F, 0x00, 0x00, 0x00, 0x00 },   // U+002D (-)
        { 0x00, 0x00, 0x00, 0x00, 0x00, 0x0C, 0x0C, 0x00 },   // U+002E (.)
        { 0x60, 0x30, 0x18, 0x0C, 0x06, 0x03, 0x01, 0x00 },   // U+002F (/)
        { 0x3E, 0x63, 0x73, 0x7B, 0x6F, 0x67, 0x3E, 0x00 },   // U+0030 (0)
        { 0x0C, 0x0E, 0x0C, 0x0C, 0x0C, 0x0C, 0x3F, 0x00 },   // U+0031 (1)
        { 0x1E, 0x33, 0x30, 0x1C, 0x06, 0x33, 0x3F, 0x00 },   // U+0032 (2)
        { 0x1E, 0x33, 0x30, 0x1C, 0x30, 0x33, 0x1E, 0x00 },   // U+0033 (3)
        { 0x38, 0x3C, 0x36, 0x33, 0x7F, 0x30, 0x78, 0x00 },   // U+0034 (4)
        { 0x3F, 0x03, 0x1F, 0x30, 0x30, 0x33, 0x1E, 0x00 },   // U+0035 (5)
        { 0x1C, 0x06, 0x03, 0x1F, 0x33, 0x33, 0x1E, 0x00 },   // U+0036 (6)
        { 0x3F, 0x33, 0x30, 0x18, 0x0C, 0x0C, 0x0C, 0x00 },   // U+0037 (7)
        { 0x1E, 0x33, 0x33, 0x1E, 0x33, 0x33, 0x1E, 0x00 },   // U+0038 (8)
        { 0x1E, 0x33, 0x33, 0x3E, 0x30, 0x18, 0x0E, 0x00 },   // U+0039 (9)
        { 0x00, 0x0C, 0x0C, 0x00, 0x00, 0x0C, 0x0C, 0x00 },   // U+003A (:)
        { 0x00, 0x0C, 0x0C, 0x00, 0x00, 0x0C, 0x0C, 0x06 },   // U+003B (;)
        { 0x18, 0x0C, 0x06, 0x03, 0x06, 0x0C, 0x18, 0x00 },   // U+003C (<)
        { 0x00, 0x00, 0x3F, 0x00, 0x00, 0x3F, 0x00, 0x00 },   // U+003D (=)
        { 0x06, 0x0C, 0x18, 0x30, 0x18, 0x0C, 0x06, 0x00 },   // U+003E (>)
        { 0x1E, 0x33, 0x30, 0x18, 0x0C, 0x00, 0x0C, 0x00 },   // U+003F (?)
        { 0x3E, 0x63, 0x7B, 0x7B, 0x7B, 0x03, 0x1E, 0x00 },   // U+0040 (@)
        { 0x0C, 0x1E, 0x33, 0x33, 0x3F, 0x33, 0x33, 0x00 },   // U+0041 (A)
        { 0x3F, 0x66, 0x66, 0x3E, 0x66, 0x66, 0x3F, 0x00 },   // U+0042 (B)
        { 0x3C, 0x66, 0x03, 0x03, 0x03, 0x66, 0x3C, 0x00 },   // U+0043 (C)
        { 0x1F, 0x36, 0x66, 0x66, 0x66, 0x36, 0x1F, 0x00 },   // U+0044 (D)
        { 0x7F, 0x46, 0x16, 0x1E, 0x16, 0x46, 0x7F, 0x00 },   // U+0045 (E)
        { 0x7F, 0x46, 0x16, 0x1E, 0x16, 0x06, 0x0F, 0x00 },   // U+0046 (F)
        { 0x3C, 0x66, 0x03, 0x03, 0x73, 0x66, 0x7C, 0x00 },   // U+0047 (G)
        { 0x33, 0x33, 0x33, 0x3F, 0x33, 0x33, 0x33, 0x00 },   // U+0048 (H)
        { 0x1E, 0x0C, 0x0C, 0x0C, 0x0C, 0x0C, 0x1E, 0x00 },   // U+0049 (I)
        { 0x78, 0x30, 0x30, 0x30, 0x33, 0x33, 0x1E, 0x00 },   // U+004A (J)
        { 0x67, 0x66, 0x36, 0x1E, 0x36, 0x66, 0x67, 0x00 },   // U+004B (K)
        { 0x0F, 0x06, 0x06, 0x06, 0x46, 0x66, 0x7F, 0x00 },   // U+004C (L)
        { 0x63, 0x77, 0x7F, 0x7F, 0x6B, 0x63, 0x63, 0x00 },   // U+004D (M)
        { 0x63, 0x67, 0x6F, 0x7B, 0x73, 0x63, 0x63, 0x00 },   // U+004E (N)
        { 0x1C, 0x36, 0x63, 0x63, 0x63, 0x36, 0x1C, 0x00 },   // U+004F (O)
        { 0x3F, 0x66, 0x66, 0x3E, 0x06, 0x06, 0x0F, 0x00 },   // U+0050 (P)
        { 0x1E, 0x33, 0x33, 0x33, 0x3B, 0x1E, 0x38, 0x00 },   // U+0051 (Q)
        { 0x3F, 0x66, 0x66, 0x3E, 0x36, 0x66, 0x67, 0x00 },   // U+0052 (R)
        { 0x1E, 0x33, 0x07, 0x0E, 0x38, 0x33, 0x1E, 0x00 },   // U+0053 (S)
        { 0x3F, 0x2D, 0x0C, 0x0C, 0x0C, 0x0C, 0x1E, 0x00 },   // U+0054 (T)
        { 0x33, 0x33, 0x33, 0x33, 0x33, 0x33, 0x3F, 0x00 },   // U+0055 (U)
        { 0x33, 0x33, 0x33, 0x33, 0x33, 0x1E, 0x0C, 0x00 },   // U+0056 (V)
        { 0x63, 0x63, 0x63, 0x6B, 0x7F, 0x77, 0x63, 0x00 },   // U+0057 (W)
        { 0x63, 0x63, 0x36, 0x1C, 0x1C, 0x36, 0x63, 0x00 },   // U+0058 (X)
        { 0x33, 0x33, 0x33, 0x1E, 0x0C, 0x0C, 0x1E, 0x00 },   // U+0059 (Y)
        { 0x7F, 0x63, 0x31, 0x18, 0x4C, 0x66, 0x7F, 0x00 },   // U+005A (Z)
#if !defined(SMALLER_FONT_TABLE)
        // In the interest of reducing the memory requirements for the TFT library, we
        // allow for the use of a smaller font table which eliminates the lower case letters
        // as well as a few symbols. See below conversion of lower case letters to upper case.
        //    Richard Chambers, 04-25-2021
        { 0x1E, 0x06, 0x06, 0x06, 0x06, 0x06, 0x1E, 0x00 },   // U+005B ([)
        { 0x03, 0x06, 0x0C, 0x18, 0x30, 0x60, 0x40, 0x00 },   // U+005C (\)
        { 0x1E, 0x18, 0x18, 0x18, 0x18, 0x18, 0x1E, 0x00 },   // U+005D (])
        { 0x08, 0x1C, 0x36, 0x63, 0x00, 0x00, 0x00, 0x00 },   // U+005E (^)
        { 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xFF },   // U+005F (_)
        { 0x0C, 0x0C, 0x18, 0x00, 0x00, 0x00, 0x00, 0x00 },   // U+0060 (`)
        { 0x00, 0x00, 0x1E, 0x30, 0x3E, 0x33, 0x6E, 0x00 },   // U+0061 (a)
        { 0x07, 0x06, 0x06, 0x3E, 0x66, 0x66, 0x3B, 0x00 },   // U+0062 (b)
        { 0x00, 0x00, 0x1E, 0x33, 0x03, 0x33, 0x1E, 0x00 },   // U+0063 (c)
        { 0x38, 0x30, 0x30, 0x3e, 0x33, 0x33, 0x6E, 0x00 },   // U+0064 (d)
        { 0x00, 0x00, 0x1E, 0x33, 0x3f, 0x03, 0x1E, 0x00 },   // U+0065 (e)
        { 0x1C, 0x36, 0x06, 0x0f, 0x06, 0x06, 0x0F, 0x00 },   // U+0066 (f)
        { 0x00, 0x00, 0x6E, 0x33, 0x33, 0x3E, 0x30, 0x1F },   // U+0067 (g)
        { 0x07, 0x06, 0x36, 0x6E, 0x66, 0x66, 0x67, 0x00 },   // U+0068 (h)
        { 0x0C, 0x00, 0x0E, 0x0C, 0x0C, 0x0C, 0x1E, 0x00 },   // U+0069 (i)
        { 0x30, 0x00, 0x30, 0x30, 0x30, 0x33, 0x33, 0x1E },   // U+006A (j)
        { 0x07, 0x06, 0x66, 0x36, 0x1E, 0x36, 0x67, 0x00 },   // U+006B (k)
        { 0x0E, 0x0C, 0x0C, 0x0C, 0x0C, 0x0C, 0x1E, 0x00 },   // U+006C (l)
        { 0x00, 0x00, 0x33, 0x7F, 0x7F, 0x6B, 0x63, 0x00 },   // U+006D (m)
        { 0x00, 0x00, 0x1F, 0x33, 0x33, 0x33, 0x33, 0x00 },   // U+006E (n)
        { 0x00, 0x00, 0x1E, 0x33, 0x33, 0x33, 0x1E, 0x00 },   // U+006F (o)
        { 0x00, 0x00, 0x3B, 0x66, 0x66, 0x3E, 0x06, 0x0F },   // U+0070 (p)
        { 0x00, 0x00, 0x6E, 0x33, 0x33, 0x3E, 0x30, 0x78 },   // U+0071 (q)
        { 0x00, 0x00, 0x3B, 0x6E, 0x66, 0x06, 0x0F, 0x00 },   // U+0072 (r)
        { 0x00, 0x00, 0x3E, 0x03, 0x1E, 0x30, 0x1F, 0x00 },   // U+0073 (s)
        { 0x08, 0x0C, 0x3E, 0x0C, 0x0C, 0x2C, 0x18, 0x00 },   // U+0074 (t)
        { 0x00, 0x00, 0x33, 0x33, 0x33, 0x33, 0x6E, 0x00 },   // U+0075 (u)
        { 0x00, 0x00, 0x33, 0x33, 0x33, 0x1E, 0x0C, 0x00 },   // U+0076 (v)
        { 0x00, 0x00, 0x63, 0x6B, 0x7F, 0x7F, 0x36, 0x00 },   // U+0077 (w)
        { 0x00, 0x00, 0x63, 0x36, 0x1C, 0x36, 0x63, 0x00 },   // U+0078 (x)
        { 0x00, 0x00, 0x33, 0x33, 0x33, 0x3E, 0x30, 0x1F },   // U+0079 (y)
        { 0x00, 0x00, 0x3F, 0x19, 0x0C, 0x26, 0x3F, 0x00 },   // U+007A (z)
        { 0x38, 0x0C, 0x0C, 0x07, 0x0C, 0x0C, 0x38, 0x00 },   // U+007B ({)
        { 0x18, 0x18, 0x18, 0x00, 0x18, 0x18, 0x18, 0x00 },   // U+007C (|)
        { 0x07, 0x0C, 0x0C, 0x38, 0x0C, 0x0C, 0x07, 0x00 },   // U+007D (})
        { 0x6E, 0x3B, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 },   // U+007E (~)
#endif
    };
#else
#define FONT_ROW_LEN  16
#if defined(SMALLER_FONT_TABLE)
    static const unsigned char font16_B[59][16] = {
#else
    static const unsigned char font16_B[95][16] = {
#endif
            {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00},/*" ",0*/

I also had to modify the character drawing loop so that rather than using 16 columns, it used 8. The number of columns was a hardcoded constant but is now specified by the class variable font_size which is set to either 16 or 8 depending on whether the defined constant USE_FONT_8_B is defined or not.

#if defined(USE_FONT_8_B)
        font_size = 8;
#else
        font_size = 16;
#endif

The loop looks like:

    for(char_m = 0; char_m < font_size; char_m++)
    {
        for(char_n = 0; char_n < 8; char_n++)
        {

#if defined(USE_FONT_8_B)
            if (font8_B[char_i][char_m] & 1 << char_n)
#else
            if(font16_B[char_i][char_m] & 1 << char_n)
#endif
            {
               TFT.set_area(now_x, now_y, ++now_x, now_y);
               TFT.w_data(font_color >> 8);
               TFT.w_data(font_color);
            }
            else
            {
              now_x++;
  
            }
         }
         now_y++;
         now_x -= 8;
    }
    now_y -= font_size;
    now_x += 8 + font_interval;

If I turn on using both the 8x8 font and the SMALLER_FONT_TABLE define then when I compile my sketch I get the following compiler output about memory usage:

Sketch uses 12390 bytes (38%) of program storage space. Maximum is 32256 bytes.
Global variables use 1084 bytes (52%) of dynamic memory, leaving 964 bytes for local variables. Maximum is 2048 bytes.

Here is a comparison of the two different sizes on the 3.5" TFT display attached to an Arduino Uno. The button is 64 by 40 pixels.

Note

I have forked the KeDei TFT library source code from Osoyoo's GitHub and have begun modifications to the source. The fork is located at https://github.com/RichardChambers/driver/tree/master/KeDeiTFT

One of the changes made was to allow text to be double high and/or double wide. This image is of the current version of a scanner/scale simulator showing the difference in style between text that is double high and double wide as well as text that is double high.

This updated image show what is expected to be the final GUI for now. It allows for changing the weight and it also allows for setting various status bits to indicate scale error conditions such as under capacity.

In order to support the number of buttons, I rewrote the Button class so that I could have buttons which share some data thus saving about 11 bytes per button. So this GUI with eight buttons that are using the data sharing feature saves some 77 bytes of memory, a significant saving for an Arduino.

The Button class is now derived from a ButtonShared class. The ButtonShared class is used to declare groups of related buttons which will have the same style. The ButtonShared class contains all the button mechanics and a Button object is a ButtonShared object with its own ButtonData member rather than a shared ButtonData member.

struct ButtonData {
public:
    ButtonData(unsigned short sizeWidth = 64, unsigned short sizeHigh = 40)
    {
        //the  button size
        x_size = sizeWidth;
        y_size = sizeHigh;
        //the button  abort color
        edge_up_color = 0xffff;    // RGB565 value for white
        edge_down_color = 0x6b4d;
        button_color = 0x4898;
        font_color = 0xffff;    // RGB565 value for white
                                       //the  botton moder 0--cube_button  1--circle_button
        botton_moder = 0;
    }

    //the  button size
    unsigned char   x_size;
    unsigned char   y_size;
    //the  button  abort color
    TFTLCD::TftColor    edge_up_color;
    TFTLCD::TftColor    edge_down_color;
    TFTLCD::TftColor    button_color;
    TFTLCD::TftColor    font_color;
    //the  botton moder 0--cube_button , 1--circle_button
    bool            botton_moder;
};


class  ButtonShared
{
public:
    ButtonShared(ButtonData &buttonData) : myData(&buttonData)
    {
    }

    ButtonShared() : myData(nullptr)
    {
    }

    bool setSharedData(ButtonData &buttonData) { myData = &buttonData; return 1; }
    bool drawButton(unsigned short _x, unsigned short _y, bool _botton_moder, const char *str, Font & myFont);
    bool drawButton(TFTLCD::TftPos &p, bool _botton_moder, const char *str, Font & myFont);
    bool istouch(unsigned short _x, unsigned short _y);
    bool istouch(void);
    bool isTouchState(void);
    bool pendown(void);
    bool penup(void);
    /*if you want  to change the butom size or the color,you can use  the  follow function to achieve your purpose,
    but you  must do it before use the drawbutton() function;also  if you  always change the auto value ,you can change the button() function*/
    void resetsize(unsigned char _x_size, unsigned char _y_size);
    void resetcolor(TFTLCD::TftColor _edge_up_color, TFTLCD::TftColor _edge_down_color, TFTLCD::TftColor _button_color, TFTLCD::TftColor _font_color);

private:
    ButtonData *myData;

    // following data is unique to each button and can not be shared
    // between buttons.
    //the  position(x,y);
    unsigned short  x;
    unsigned short  y;
    unsigned char   penDownFlag;      // set by successful istouch() call or when the pendown() function is called. cleared when the penup() function is called.
};

class  Button : public ButtonShared
{
public:
    Button(unsigned short sizeWidth = 64, unsigned short sizeHigh = 40) : localData(sizeWidth, sizeHigh)
    {
        setSharedData(localData);
    }
    
private:
    ButtonData localData;
};

Just one more note -- if the font size is a problem, and you're not opposed to "blocky" characters, perhaps you could run your 8x8 characters through a "2x magnifier" on the way to the screen... You'd still have square characters, and they'd be blocky, but they would be twice as wide and twice as high. I've done similar things in the past with low-res graphics where "larger and pixelated" was just fine. — TomServo, May 03 '21 at 17:40
@TomServo I added a photo of using the current library for a UI on the 3.5" TFT LCD display using the modified KeDei TFT library. — Richard Chambers, May 03 '21 at 21:28

reducing memory required for KeDei TFT library used with 3.5" TFT display with Arduino

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