Mapping values from analog input giving unexpected values?

Question

Mega 2560

I am reading in values from 3 pots then mapping them to values between 0 - 255

#define REDPIN   7
#define GREENPIN 8
#define BLUEPIN  9

int red_pot   = A0;  // analog pin used to connect the potentiometer
int green_pot = A1;
int blue_pot  = A3;


void setup() {

  pinMode(REDPIN,   OUTPUT);
  pinMode(GREENPIN, OUTPUT);
  pinMode(BLUEPIN,  OUTPUT);

  Serial.begin(38400); //Debug port
  Serial.setTimeout(50);
}


void loop() {

  int red, green, blue; 

  red   = analogRead(red_pot);
  green = analogRead(green_pot);
  blue  = analogRead(blue_pot);

  Serial.print(" r = ");
  Serial.print(red);

  Serial.print(" b = ");
  Serial.print(blue);

  Serial.print(" g = ");
  Serial.print(green);
  Serial.print("\n\n\n");


  // map values
  red   = map(red,   20, 650, 0, 255);     
  green = map(green, 20, 570, 0, 255);
  blue  = map(blue,  20, 570, 0, 255);


  Serial.print("red map : ");
  Serial.print(red);

  Serial.print(" green map : ");
  Serial.print(green);

  Serial.print(" blue map : ");
  Serial.print(blue);
  Serial.print('\n\n\n');

  analogWrite(REDPIN,   red); 
  analogWrite(GREENPIN, green);
  analogWrite(BLUEPIN,  blue);    
}

red & green behave as expected but blue is not, from the serial monitor...

When blue is at its lowest value,

When blue is at its highest value,

I have changed blue's analog pin assignment but the same issue.

Any clues what's happening?

EDIT #1

I thought it might be some int sign error so I changed the map operation to,

blue  = map(abs(blue), 20, 670, 0, 255);

It made no difference.

Answer 1

Note that the Arduino function "map" is really this:

long map(long x, long in_min, long in_max, long out_min, long out_max) {
  return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min;
}

...so, if you pass a "minimum from" value which is less than the actual value you are mapping from you will get a negative number given your "minimum out" is zero.

In several of your examples you have set the minimum to 20 but pass a value of 19. If your program is intolerant of negative numbers (for instance, how would a negative number effect the PWM hardware), consider testing for lower than minimum and higher than maximum values before using the map function.