(PECL imagick 2.0.0)
Imagick::sigmoidalContrastImage — Adjusts the contrast of an image
$sharpen
   , float $alpha
   , float $beta
   [, int $channel = Imagick::CHANNEL_ALL
  ] )
   Adjusts the contrast of an image with a non-linear sigmoidal contrast
   algorithm. Increase the contrast of the image using a sigmoidal transfer
   function without saturating highlights or shadows. Contrast indicates
   how much to increase the contrast (0 is none; 3 is typical; 20 is
   pushing it); mid-point indicates where midtones fall in the resultant
   image (0 is white; 50 is middle-gray; 100 is black). Set sharpen to
   TRUE to increase the image contrast otherwise the contrast is reduced.
  
See also » ImageMagick v6 Examples - Image Transformations — Sigmoidal Non-linearity Contrast
sharpenIf true increase the contrast, if false decrease the contrast.
alphaThe amount of contrast to apply. 1 is very little, 5 is a significant amount, 20 is extreme.
betaWhere the midpoint of the gradient will be. This value should be in the range 0 to 1 - mutliplied by the quantum value for ImageMagick.
channelWhich color channels the contrast will be applied to.
   Returns TRUE on success.
  
Example #1 Create a gradient image using Imagick::sigmoidalContrastImage() suitable for blending two images together smoothly, with the blending defined by $contrast and $the midpoint
<?php
function generateBlendImage($width, $height, $contrast = 10, $midpoint = 0.5) {
    $imagick = new Imagick();
    $imagick->newPseudoImage($width, $height, 'gradient:black-white');
    $quanta = $imagick->getQuantumRange();
    $imagick->sigmoidalContrastImage(true, $contrast, $midpoint * $quanta["quantumRangeLong"]);
    return $imagick; 
}
?>
Throws ImagickException on error.