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(PECL imagick 2.0.0)
ImagickPixel::setColorValue — Sets the normalized value of one of the channels
$color 
   ,  float $value 
   )Sets the value of the specified channel of this object to the provided value, which should be between 0 and 1. This function can be used to provide an opacity channel to an ImagickPixel object.
color One of the Imagick color constants e.g. \Imagick::COLOR_GREEN or \Imagick::COLOR_ALPHA.
value The value to set this channel to, ranging from 0 to 1.
   成功时返回  TRUE 。
  
Example #1 Basic Imagick::setColorValue() usage
  <?php
$color   = new \ ImagickPixel ( 'firebrick' );
 $color -> setColorValue ( Imagick :: COLOR_ALPHA ,  0.5 );
 print_r ( $color -> getcolor ( true ));
 ?>   以上例程会输出:
Array
(
    [r] => 0.69803921568627
    [g] => 0.13333333333333
    [b] => 0.13333333333333
    [a] => 0.50000762951095
)