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(PECL imagick 2.0.0)
Imagick::thresholdImage — Changes the value of individual pixels based on a threshold
$threshold 
   [,  int $channel  = Imagick::CHANNEL_ALL 
  ] )Changes the value of individual pixels based on the intensity of each pixel compared to threshold. The result is a high-contrast, two color image.
threshold 
channel 
   成功时返回  TRUE 。
  
Example #1 Imagick::thresholdImage()
  <?php
 function  thresholdimage ( $imagePath ,  $threshold ,  $channel ) {
     $imagick  = new \ Imagick ( realpath ( $imagePath ));
     $imagick -> thresholdimage ( $threshold  * \ Imagick :: getQuantum (),  $channel );
     header ( "Content-Type: image/jpg" );
    echo  $imagick -> getImageBlob ();
}
 ?>   [#1] php t traction dot de [2012-06-29 07:25:11]
getQuantumRange() can be handy here:
<?php
$i = new Imagick($imageFile);
$max = $i->getQuantumRange();
$max = $max["quantumRangeLong"]; 
$i->thresholdImage(0.77 * $max);
?>