Code:

// Draw a rectangle. Note: damages text color register
void RA8876_t3::drawRect(int16_t x, int16_t y, int16_t w, int16_t h,uint16_t color) {
drawSquare(x, y, x+w-1, y+h-1, color);
}
// Draw a filled rectangle. Note: damages text color register
void RA8876_t3::fillRect(int16_t x, int16_t y, int16_t w, int16_t h,uint16_t color) {
drawSquareFill(x, y, x+w-1, y+h-1, color);
}
// Draw a round rectangle.
void RA8876_t3::drawRoundRect(uint16_t x, uint16_t y, uint16_t w, uint16_t h, uint16_t xr, uint16_t yr, uint16_t color) {
drawCircleSquare(x, y, x+w-1, y+h-1, xr, yr, color);
}
// Draw a filed round rectangle.
void RA8876_t3::fillRoundRect(uint16_t x, uint16_t y, uint16_t w, uint16_t h, uint16_t xr, uint16_t yr, uint16_t color) {
drawCircleSquareFill(x, y, x+w-1, y+h-1, xr, yr, color);
}

To this:
Code:

// Draw a rectangle. Note: damages text color register
void RA8876_t3::drawRect(int16_t x, int16_t y, int16_t w, int16_t h,uint16_t color) {
drawSquare(x, y, w-1, h-1, color);
}
// Draw a filled rectangle. Note: damages text color register
void RA8876_t3::fillRect(int16_t x, int16_t y, int16_t w, int16_t h,uint16_t color) {
drawSquareFill(x, y, w-1, h-1, color);
}
// Draw a round rectangle.
void RA8876_t3::drawRoundRect(uint16_t x, uint16_t y, uint16_t w, uint16_t h, uint16_t xr, uint16_t yr, uint16_t color) {
drawCircleSquare(x, y, w-1, h-1, xr, yr, color);
}
// Draw a filed round rectangle.
void RA8876_t3::fillRoundRect(uint16_t x, uint16_t y, uint16_t w, uint16_t h, uint16_t xr, uint16_t yr, uint16_t color) {
drawCircleSquareFill(x, y, w-1, h-1, xr, yr, color);
}

This eliminated the addition of x+w and y+h which if x = 1023 and w = 1023 would equal 2048 and is obviously beyond the screen width of 1023. Same with y+h.
Code:

/***************************************************************
* graphics.ino
*
* Basic graphics test for RA8876 based display
***************************************************************/
#include "Arduino.h"
//#include "Ra8876_Lite.h"
#include "RA8876_t3.h"
#define RA8876_CS 10
#define RA8876_RESET 8
#define BACKLITE 7 //External backlight control connected to this Arduino pin
RA8876_t3 tft = RA8876_t3(RA8876_CS, RA8876_RESET); //Using standard SPI pins
// Array of Simple RA8876 Basic Colors
PROGMEM uint16_t myColors[] = {
0x0000,
0xffff,
0xf800,
0xfc10,
0x8000,
0x07e0,
0x87f0,
0x0400,
0x001f,
0x051f,
0x841f,
0x0010,
0xffe0,
0xfff0,
0x8400,
0x07ff,
0x87ff,
0x0410,
0xf81f,
0xfc1f,
0x8010,
0xA145
};
int interations = 0;
// Draw random unfilled rectangle boxes
void rectangles(uint16_t thickness) {
uint16_t x0, y0, x1, y1, c;
uint16_t j;
for(int i=0; i < interations; i++) {
x0 = (uint16_t)random(1,512);
y0 = (uint16_t)random(1,288);
x1 = (uint16_t)random(512,1023);
y1 = (uint16_t)random(288,575);
c = (uint16_t)random(21);
if(x0 > tft.width()) x0 = tft.width();
if(y0 > tft.height()) y0 = tft.height();
if(x1 > tft.width()) x1 = tft.width();
if(y1 > tft.height()) y1 = tft.height();
if(thickness > 0) {
for(j = 1; j <= thickness; j++) {
tft.drawRect(x0,y0,x1,y1,myColors[c]);
if(x0 <= tft.width())
x0++;
if(y0 <= tft.height())
y0++;
if(x1 > 0)
x1--;
if(y1 > 0)
y1--;
}
} else {
tft.drawRect(x0,y0,x1,y1,myColors[c]);
}
}
tft.fillStatusLine(myColors[11]);
}
// Draw random filled rectangle boxes
void filledRectangles(void) {
uint16_t x0, y0, x1, y1, c;
for(int i=0; i< interations; i++) {
x0 = (uint16_t)random(1023);
y0 = (uint16_t)random(575);
x1 = (uint16_t)random(1023);
y1 = (uint16_t)random(575);
c = (uint16_t)random(21);
tft.fillRect(x0,y0,x1,y1,myColors[c+1]);
}
tft.fillStatusLine(myColors[11]);
}
// Draw random round rectangle boxes
void rRectangles(uint16_t thickness) {
uint16_t x0, y0, x1, y1, xr, yr, c;
uint16_t j;
for(int i=0; i < interations; i++) {
x0 = (uint16_t)random(1,512);
y0 = (uint16_t)random(1,288);
x1 = (uint16_t)random(512,1023);
y1 = (uint16_t)random(288,575);
xr = 20; // Major Radius - Line Thickness must be less than
yr = 20; // Minor Radius - Major and Minor radiuses by at
// - least half of xr and yr.
c = (uint16_t)random(21);
if(x0 > tft.width()) x0 = tft.width();
if(y0 > tft.height()) y0 = tft.height();
if(x1 > tft.width()) x1 = tft.width();
if(y1 > tft.height()) y1 = tft.height();
// Make sure major radius (xr) is less than x1 - x0
// Must be xr * 2 + 1 less than x1 - x0
// RA8876.pdf section 12.6 page 62
if((xr * 2 + 1) >= (x1 - x0))
xr = (x1 - x0) / 2 - 1;
// Same for minor radius (yr)
if((yr * 2 + 1) >= (y1 - y0))
yr = (y1 - y0) / 2 - 1;
if(thickness > 0) {
for(j = 1; j <= thickness; j++) {
tft.drawRoundRect(x0,y0,x1,y1,xr,yr,myColors[c]);
if(x0 <= tft.width())
x0++;
if(y0 <= tft.height())
y0++;
if(x1 > 0)
x1--;
if(y1 > 0 )
y1--;
if(xr > 0)
xr--;
if(yr > 0)
yr--;
}
} else {
tft.drawRoundRect(x0,y0,x1,y1,xr,yr,myColors[c]);
}
}
tft.fillStatusLine(myColors[11]);
}
// Draw random filled round rectangle boxes
void filledRRectangles(void) {
uint16_t x0, y0, x1, y1, xr, yr, c;
for(int i=0; i < interations; i++) {
x0 = (uint16_t)random(1,512);
y0 = (uint16_t)random(1,288);
x1 = (uint16_t)random(512,1023);
y1 = (uint16_t)random(288,575);
xr = 20; // Major Radius
yr = 20; // Minor Radius
c = (uint16_t)random(21);
// Keep x,y within 1024x576 boundries
if(x0 > tft.width()) x0 = tft.width();
if(y0 > tft.height()) y0 = tft.height();
if(x1 > tft.width()) x1 = tft.width();
if(y1 > tft.height()) y1 = tft.height();
// Make sure major radius (xr) is less than x1 - x0
// Must be xr * 2 + 1 less than x1 - x0
// RA8876.pdf section 12.6 page 62
if((xr * 2 + 1) >= (x1 - x0))
xr = (x1 - x0) / 2 - 1;
// Same for minor radius (yr)
if((yr * 2 + 1) >= (y1 - y0))
yr = (y1 - y0) / 2 - 1;
tft.fillRoundRect(x0, y0, x1, y1, xr, yr, myColors[c]);
}
tft.fillStatusLine(myColors[11]);
}
// Draw random circles
void drawcircles(uint16_t thickness) {
uint16_t x0, y0, r, c;
int j;
for(int i=0; i < interations; i++) {
x0 = (uint16_t)random(1023);
y0 = (uint16_t)random(575);
r = (uint16_t)random(239);
c = (uint16_t)random(21);
if(x0-r <= 0)
x0 += (uint16_t)r;
if(y0-r <= 0)
y0 += (uint16_t)r;
if(x0+r >= tft.width())
x0 = (uint16_t)(tft.width() - r);
if(y0+r >= tft.height())
y0 = (uint16_t)(tft.height() - r);
if(thickness > 0) {
for(j = 1; j <= thickness; j++) {
tft.drawCircle(x0, y0, r, myColors[c]);
if(r > 0)
r--;
}
} else {
tft.drawCircle(x0, y0, r, myColors[c]);
}
tft.drawCircle(x0, y0, r, myColors[c]);
}
tft.fillStatusLine(myColors[11]);
}
// Draw random filled circles
void fillcircles(void) {
uint16_t x0, y0, r, c;
for(int i=0; i< interations; i++) {
x0 = (uint16_t)random(1023);
y0 = (uint16_t)random(575);
r = (uint16_t)random(239);
c = (uint16_t)random(21);
if(x0-r <= 0)
x0 += (uint16_t)r;
if(y0-r <= 0)
y0 += (uint16_t)r;
if(x0+r >= tft.width())
x0 = (uint16_t)(tft.width() - r);
if(y0+r >= tft.height())
y0 = (uint16_t)(tft.height() - r);
tft.fillCircle(x0, y0, r, myColors[c]);
}
tft.fillStatusLine(myColors[11]);
}
// Draw random unfilled tritangles
void drawTriangles(void) {
uint16_t x0, y0, x1, y1, x2, y2, c;
for(int i=0; i < interations; i++) {
x0 = (uint16_t)random(1023);
y0 = (uint16_t)random(575);
x1 = (uint16_t)random(1023);
y1 = (uint16_t)random(575);
x2 = (uint16_t)random(1023);
y2 = (uint16_t)random(575);
c = (uint16_t)random(21);
tft.drawTriangle(x0,y0,x1,y1,x2,y2,myColors[c+1]);
}
tft.fillStatusLine(myColors[11]);
}
// Draw random filled triangles
void fillTriangles(void) {
uint16_t x0, y0, x1, y1, x2, y2, c;
for(int i=0; i < interations; i++) {
x0 = (uint16_t)random(1023);
y0 = (uint16_t)random(575);
x1 = (uint16_t)random(1023);
y1 = (uint16_t)random(575);
x2 = (uint16_t)random(1023);
y2 = (uint16_t)random(575);
c = (uint16_t)random(21);
tft.fillTriangle(x0,y0,x1,y1,x2,y2,myColors[c+1]);
}
tft.fillStatusLine(myColors[11]);
}
// Draw random unfilled ellipses
void drawEllipses(void)
{
int16_t x0, y0, xr, yr;
uint16_t color;
for(int i=0; i < interations; i++) {
x0 = (uint16_t)random(1023);
y0 = (uint16_t)random(575);
xr = (uint16_t)random(239);
yr = (uint16_t)random(239);
color = (uint16_t)random(21);
if(x0-xr <= 0)
x0 += (uint16_t)xr;
if(y0-yr <= 0)
y0 += (uint16_t)yr;
if(x0+xr >= tft.width())
x0 = (uint16_t)(tft.width() - xr);
if(y0+yr >= tft.height())
y0 = (uint16_t)(tft.height() - yr);
tft.drawEllipse(x0, y0, xr, yr, myColors[color]);
}
tft.fillStatusLine(myColors[11]);
}
// Draw random filled ellipses
void fillEllipses(void)
{
int16_t x0, y0, xr, yr;
uint16_t color;
for(int i=0; i < interations; i++) {
x0 = (uint16_t)random(1023);
y0 = (uint16_t)random(575);
xr = (uint16_t)random(239);
yr = (uint16_t)random(239);
color = (uint16_t)random(21);
if(x0-xr <= 0)
x0 += (uint16_t)xr;
if(y0-yr <= 0)
y0 += (uint16_t)yr;
if(x0+xr >= tft.width())
x0 = (uint16_t)(tft.width() - xr);
if(y0+yr >= tft.height())
y0 = (uint16_t)(tft.height() - yr);
tft.fillEllipse(x0, y0, xr, yr, myColors[color]);
}
tft.fillStatusLine(myColors[11]);
//timeOn(); // Turn on time/date display on status line
}
// Draw random Lines
void drawlines(void) {
uint16_t x0, y0, x1, y1, c;
for(int i=0; i < interations; i++) {
x0 = (uint16_t)random(1,1023);
y0 = (uint16_t)random(1,575);
x1 = (uint16_t)random(1,1023);
y1 = (uint16_t)random(1,575);
c = (uint16_t)random(21);
if(x0 > tft.width()) x0 = tft.width();
if(y0 > tft.height()) y0 = tft.height();
if(x1 > tft.width()) x1 = tft.width();
if(y1 > tft.height()) y1 = tft.height();
tft.drawLine(x0,y0,x1,y1,myColors[c]);
}
tft.fillStatusLine(myColors[11]);
}
int i = 0;
void setup() {
//I'm guessing most copies of this display are using external PWM
//backlight control instead of the internal RA8876 PWM.
//Connect a Teensy pin to pin 14 on the display.
//Can use analogWrite() but I suggest you increase the PWM frequency first so it doesn't sing.
pinMode(BACKLITE, OUTPUT);
digitalWrite(BACKLITE, HIGH);
tft.begin();
//initVT100();
tft.setTextCursor(0,0);
tft.fillScreen(myColors[11]);
tft.setFontSize(1,false);
}
void loop() {
tft.printStatusLine(0,myColors[1],myColors[11],"Rectangles");
interations = 30000;
rectangles(0);
tft.fillScreen(myColors[11]);
delay(10);
tft.printStatusLine(0,myColors[1],myColors[11],"Rectangles 10 pixel line thickness");
interations = 4000;
rectangles(10);
tft.fillScreen(myColors[11]);
delay(10);
tft.printStatusLine(0,myColors[1],myColors[11],"Filled Rectangles");
interations = 4000;
filledRectangles();
tft.fillScreen(myColors[11]);
delay(10);
tft.printStatusLine(0,myColors[1],myColors[11],"Round Rectangles");
interations = 30000;
rRectangles(0);
tft.fillScreen(myColors[11]);
delay(10);
tft.printStatusLine(0,myColors[1],myColors[11],"Round Rectangles 10 pixel line thickness");
interations = 4000;
rRectangles(10);
tft.fillScreen(myColors[11]);
delay(10);
tft.printStatusLine(0,myColors[1],myColors[11],"Filled Round Rectangles");
interations = 4000;
filledRRectangles();
tft.fillScreen(myColors[11]);
delay(10);
tft.printStatusLine(0,myColors[1],myColors[11],"Circles");
interations = 30000;
drawcircles(0);
tft.fillScreen(myColors[11]);
delay(10);
tft.printStatusLine(0,myColors[1],myColors[11],"Circles 10 pixel circle thickness");
interations = 4000;
drawcircles(10);
tft.fillScreen(myColors[11]);
delay(10);
tft.printStatusLine(0,myColors[1],myColors[11],"Filled Circles");
interations = 4000;
fillcircles();
tft.fillScreen(myColors[11]);
delay(10);
tft.printStatusLine(0,myColors[1],myColors[11],"Triangles");
interations = 30000;
drawTriangles();
tft.fillScreen(myColors[11]);
delay(10);
tft.printStatusLine(0,myColors[1],myColors[11],"Filled Triangles");
interations = 4000;
fillTriangles();
tft.fillScreen(myColors[11]);
delay(10);
interations = 30000;
tft.printStatusLine(0,myColors[1],myColors[11],"Ellipses");
drawEllipses();
tft.fillScreen(myColors[11]);
delay(10);
tft.printStatusLine(0,myColors[1],myColors[11],"Filled Ellipses");
interations = 4000;
fillEllipses();
tft.fillScreen(myColors[11]);
delay(10);
tft.printStatusLine(0,myColors[1],myColors[11],"Lines");
interations = 100000;
drawlines();
tft.fillScreen(myColors[11]);
delay(10);
}

Hopefully I did not miss something