GFX Lib & Adafruit 3.5 display getPixel color?

Projects Programming
1

The code below draws a sine wave on the Adafruit 3.5" TFT 32x480 display using the adafruit GFX lib. I need to determine the pixel color at x, y as shown at the bottom of the script (Slide function). The line "Serial.println(tft.getPixel(100, 100);" generates an error because the GFX library apparently doesn't contain a getPixel function the user can use.

So how does one get the color of a pixel on the screen?

// for teensy 4.0
/***************************************************************************************************************/
#include <arduino.h>
#include "Adafruit_HX8357.h"                        // the 480x320 display
#include <Adafruit_GFX.h>                           // Core graphics library

// The display uses hardware SPI, plus pins 9 & 10
#define TFT_DC_PIN            9
#define TFT_CS_PIN           10
#define TFT_RST_PIN          15                     // tie this to arduino RST if you like

Adafruit_HX8357 tft = Adafruit_HX8357(TFT_CS_PIN, TFT_DC_PIN, TFT_RST_PIN);

// Colors
#define BACKGROUND  0xA514
#define FILL        0xAD75
#define BLACK       0x0000
#define BLUE        0x0019
#define YELLOW      0xFFE0
#define RED         0xA800
#define GREEN       0x0360
#define WHITE       0xFFFF
#define GRAY        0x9CD3

const float rad_per_deg = 0.01745329251;          // value of a radian per degree
float theta = 0;
int pulse_width_sin = 0;
int pulse_width_cos = 0;
int sign = 1;                                     // to control the increments or decrements of angle parameter
int h, v;

/***************************************************************************************************************/
void setup(void)
{
  Serial.begin(115200);
  delay(250);

  tft.begin();
  delay(200);
  tft.setRotation(1);
  tft.fillScreen(BLACK);
}

/***************************************************************************************************************/
void loop(void)
{
  Sine();
}

void Sine()
{
  if (theta == 3.14159265359) sign = -1;        // keep increasing the value of theta till pi and then decrease till zero
  else if (theta == 0) sign = 1;

  theta = theta + (rad_per_deg * sign);
  pulse_width_sin = 255 * sin(theta);
  pulse_width_cos = 255 * cos(theta);

  v = 160 + pulse_width_sin / 2;
  h += 1;
  if (h > 478)
    h = 0;
  tft.drawLine(h, v, h, v + 1, 0x07e0);
  pulse_width_sin = abs(pulse_width_sin);
  pulse_width_cos = abs(pulse_width_cos);

  delay(10);  // control the frequency here
}

void Slide()
{
  //  Serial.println(tft.getPixel(100, 100);
}

`
2

It's the colour you last wrote there.

3

That doesn't seem very efficient when there are 480x320 pixels which works out to 153,600 individual locations to keep track of. Drawing using formulas would require you to keep track of all that data if you want to detect and change or move a pixel at any given location. Many other graphic programs have a get pixel or get pixel color because there are very good reasons for using them. The example shown in the Slide function is not what the end program will use to detect the color. The location will be computer generated and it will not be known by the user.

4

Try digging into the examples. It looks to me as if the getPixel function is associated with a canvas which underlies the display class.