Problem with drawing lines in TFT_eSPI

[seyedmm044]

Hi. I have a 3.5" 480x320 TFT display with SPI connection to an ESP32. I'm using TFT_eSPI library to communicate with display and Setup 21 but with DC connected to GPIO16. I am drawing a TFT_GOLD line using drawLine. After that, I'm drawing another line but TFT_TRANSPARENT to clear that line. My problem is that a green-ish trace is left behind as you can see in images. I tried using TFT_BLACK but it doesn't do anything. here is my code:

#include <TFT_eSPI.h>
#include <SPI.h>
#include "dial.h"

TFT_eSPI tft = TFT_eSPI();
TFT_eSprite needle = TFT_eSprite(&tft);
TFT_eSprite label = TFT_eSprite(&tft);

void setup() {
  Serial.begin(115200);

  tft.init();
  tft.setRotation(3);
  tft.fillScreen(TFT_BLACK);

  tft.setSwapBytes(true);
  tft.pushImage(0,120,480,80,dial);

  needle.setColorDepth(8);
  needle.createSprite(320,50);
}

int getTopX(int speed) {
  double tmp = speed * 290;
  int topX = (int)(tmp/60 + 0.5);
  return topX;
}

int getBottomX(int speed) {
  double tmp = speed * 160;
  int topX = (int)(tmp/60 + 65.5);
  return topX;
}

void drawNeedle(int last_speed, int speed) {
  // clearing last line:
  int ltopx = getTopX(last_speed);
  int lbtmx = getBottomX(last_speed);
  needle.drawLine(lbtmx,50,ltopx,0, TFT_TRANSPARENT);

  needle.setTextColor(TFT_TRANSPARENT);
  needle.drawString(String(last_speed), 40,60, 7);
  needle.setTextColor(TFT_WHITE);
  needle.drawString(String(speed),40,60,7);

  // drawing new line:
  int topx = getTopX(speed);
  int btmx = getBottomX(speed);
  needle.drawLine(btmx,50,topx,0,TFT_WHITE);

  needle.pushSprite(90,200, TFT_BLACK);
}

int lspeed=0;
void loop() {
  for (int speed= 0; speed < 61; speed++) {
    int start = millis();
    drawNeedle(lspeed, speed);
    lspeed = speed;
    Serial.println(String(millis() - start));
  }
}

the problem i have is that drawing each line/speed should not take mor than 80 ms. currently it takes around 35, so everything is good (speed number not showing is not a problem as it is drawn outside of sprite dimensions. I will fix that later). here are the images:

TFT_TRANSPARENT (Don't mind double lines. That's because i took this image while it was drawing a new line.):

TFT_BLACK (IRL all lines are same color IDK why it is like this in the image:

[xfpd]

@seyedmm044

I tried using TFT_BLACK

You must store the old line values, change the color to background (green?), and draw the old line values again... then draw the new line values (white?)

[seyedmm044]

line is rendered by it's speed (so the value you say is the speed) which is saved in the main loop (line 59) and took as a parameter in drawNeedle (line 35). I also fill screen with TFT_BLACK in setup function (line 14) so background should be black. When I try to draw the line with background color, second image appears.

[seyedmm044]

I found the solution. as you can see in line 45, I am preventing TFT_eSPI to update pixels with color black (third parameter in needle.pushSprite(90,200,TFT_BLACK);) and that's why @xfpd 's answer doesn't work.

[xfpd]

@seyedmm044

second image appears.

If you "draw a line with the background color, a second image appears" you are not drawing with the background color. Post #2 is the solution... where I noted green and white being used. You fixated on black as the background color. I did not.