Here's a example of this function:
<?php
  $img = new Imagick();
  $img->readImage($image_file_name);
  $img->whiteThresholdImage('grey');
  $img->writeImage($thumb_file_name);
  $img->clear();
  $img->destroy();
?>(PECL imagick 2, PECL imagick 3)
Imagick::whiteThresholdImage — Force all pixels above the threshold into white
Is like Imagick::ThresholdImage() but force all pixels above the threshold into white while leaving all pixels below the threshold unchanged.
threshold
   Повертає true в разі успіху.
  
| Версія | Опис | 
|---|---|
| PECL imagick 2.1.0 | Now allows a string representing the color as a parameter. Previous versions allow only an ImagickPixel object. | 
Приклад #1 Imagick::whiteThresholdImage()
<?php
function whiteThresholdImage($imagePath, $color) {
    $imagick = new \Imagick(realpath($imagePath));
    $imagick->whiteThresholdImage($color);
    header("Content-Type: image/jpg");
    echo $imagick->getImageBlob();
}
?>Here's a example of this function:
<?php
  $img = new Imagick();
  $img->readImage($image_file_name);
  $img->whiteThresholdImage('grey');
  $img->writeImage($thumb_file_name);
  $img->clear();
  $img->destroy();
?>