(PECL imagick 2.0.0)
Imagick::linearStretchImage — Stretches with saturation the image intensity
$blackPoint
   , float $whitePoint
   )Stretches with saturation the image intensity.
blackPointThe image black point
whitePointThe image white point
   Returns TRUE on success.
  
Example #1 Imagick::linearStretchImage()
<?php
function linearStretchImage($imagePath, $blackThreshold, $whiteThreshold) {
    $imagick = new \Imagick(realpath($imagePath));
    $pixels = $imagick->getImageWidth() * $imagick->getImageHeight();
    $imagick->linearStretchImage($blackThreshold * $pixels, $whiteThreshold * $pixels);
    header("Content-Type: image/jpg");
    echo $imagick->getImageBlob();
}
?>