(PECL imagick 2.0.0)
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
   Returns TRUE on success.
  
| Version | Description | 
|---|---|
| 2.1.0 | Now allows a string representing the color as a parameter. Previous versions allow only an ImagickPixel object. | 
Example #1 Imagick::whiteThresholdImage()
<?php
function whiteThresholdImage($imagePath, $color) {
    $imagick = new \Imagick(realpath($imagePath));
    $imagick->whiteThresholdImage($color);
    header("Content-Type: image/jpg");
    echo $imagick->getImageBlob();
}
?>