Change the contrast of input values.

contrast_test.ino

// Check: https://plus.google.com/+KasperKamperman/posts/NmCDNXUVHYs

// #include "FastLED.h"

uint8_t inputValues[7] = {0, 30, 64, 128, 192, 221, 255};

int contrastValue = 8;

void setup() {  
  Serial.begin(57600);
}  

void loop() { 
  Serial.print("contrast: ");
  Serial.println(contrastValue);
  
  // display input
  Serial.print("input  :");
  
  for(int i = 0; i < 7; i++) {
    
    printWhiteSpace(inputValues[i]); // aligning
    Serial.print(inputValues[i]);
    Serial.print(", "); 
  }
  
  Serial.println();
  
  // display contrast
  Serial.print("output :");
  
  for(int i = 0; i < 7; i++) {
    
    int outputValue = applyContrast(inputValues[i], contrastValue);
    
    printWhiteSpace(outputValue);
    
    Serial.print(outputValue);
    Serial.print(", "); 
  }
  
  Serial.println();
  Serial.println("=============================================");
  
  delay(500);
  
  contrastValue = contrastValue + 10;
  
  if(contrastValue>254) contrastValue = 8;
  
}

void printWhiteSpace(int val) {  
    
    if(val>0)   
    { if(val<10)        Serial.print("   ");
      else if(val<100)  Serial.print("  ");
      else              Serial.print(" ");
    }
    else
    { if(val==0)        Serial.print("  ");
      else if(val>-10)  Serial.print(" ");
      if(val>-100)      Serial.print(" ");
    }
}

// constrast factor is from 1.0 - 2.0 or 0 - 255
int applyContrast(uint8_t input, uint8_t contrast_factor) {   
    // around 0
    int inputShift = (int) input - 128;  // -128 - 128
    
    // 1 + (i * scale) / 256;
    inputShift = inputShift + (inputShift * contrast_factor) / 256;
    
    return (int) inputShift + 128;
}

// below two functions 16bit and 8bit (didn't test it with this code, but I use them in other projects)

// constrast factor is from 1.0 - 2.0 or 0 - 255
uint8_t applyContrast8(uint8_t input, uint8_t contrast_factor) {   
  // around 0
  int inputShift = (int) input - 128;  // -128 - 128
    
  // 1 + (i * scale) / 256; // +128 shift back
  inputShift = 128 + inputShift + (inputShift * contrast_factor) / 256;
    
  return (uint8_t) constrain(inputShift,0,255);
}

// constrast factor is from 1.0 - 2.0 or 0 - 255
uint16_t applyContrast16by8(uint16_t input, uint8_t contrast_factor) {   
  
  // scaling around point / dilation middle grey = 32768
    
  // typecast to long otherwise we don't have enough space
  int32_t inputTest = input + (int32_t(input - 32768) * contrast_factor) / 256;
  return (uint16_t) constrain(inputTest,0,65535);
}