Imagick::sigmoidalContrastImage

(PECL imagick 2, PECL imagick 3)

Imagick::sigmoidalContrastImage — Adjusts the contrast of an image

Description

public function Imagick::sigmoidalContrastImage(
    bool $sharpen,
    float $alpha,
    float $beta,
    int $channel = Imagick::CHANNEL_DEFAULT
): bool

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. alpha indicates how much to increase the contrast: 0.00 is none; 3.00 is typical; 20.00 is pushing it. beta indicates where midtones fall in the resultant image, as a fraction of the quantum range: 0.0 is white, 0.5 is middle-gray, 1.0 is black (multiply by getQuantumRange()['quantumRangeLong'] to get the value to pass).

Parameters

sharpen: If true increase the contrast, if false decrease the contrast.
alpha: The amount of contrast to apply. 1.00 is very little, 5.00 is a significant amount, 20.00 is extreme.
beta: Where the midpoint of the gradient will be. This value should be in the range 0.00 to 1.00 - multiplied by the quantum value for ImageMagick.
channel: Which color channels the contrast will be applied to.

Return Values

Returns true on success.

Errors/Exceptions

Throws ImagickException on error.

Examples

Example #1 Create a gradient image using Imagick::sigmoidalContrastImage() suitable for blending two images together smoothly, with the blending defined by alpha and beta

<?php

function generateBlendImage($width, $height, $alpha = 10, $beta = 0.5)
{
    $imagick = new Imagick();
    $imagick->newPseudoImage($width, $height, 'gradient:black-white');
    $quanta = $imagick->getQuantumRange();
    $imagick->sigmoidalContrastImage(true, $alpha, $beta * $quanta["quantumRangeLong"]);

    return $imagick;
}

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User Contributed Notes 2 notes

down

acameron at theatomgroup dot com ¶

12 years ago

The documentation for this function really isn't useful or helpful at all. "Sharpen", "Contrast" and "Midpoint" aren't the names of the params, which are referred to as "alpha" and "beta".

The Imagemagick manual entry:

For those interested, the corrected formula for the 'sigmoidal non-linearity contrast control' is...
( 1/(1+exp(β*(α-u))) - 1/(1+exp(β)) ) / ( 1/(1+exp(β*(α-1))) - 1/(1+exp(β*α)) )
Where α is the threshold level, and β the contrast factor to be applied.
The formula is actually very simple exponential curve, with the bulk of the above formula is designed to ensure that 0 remains zero and 1 remains one. That is, the graph always goes though the points 0,0 and 1,1. And the highest gradient of change is at the given threshold.

Src: http://www.imagemagick.org/Usage/color_mods/#sigmoidal

In the Imagick extension source, the function call is:

 status = MagickSigmoidalContrastImageChannel(intern->magick_wand, channel, sharpen, alpha, beta);

Src: https://github.com/mkoppanen/imagick/ 

Referring back to the binary API call gives:

foreign import ccall "MagickSigmoidalContrastImageChannel" magickSigmoidalContrastImageChannel
  :: Ptr MagickWand
  -> ChannelType       -- ^ identify which channel to level: `redChannel`, `greenChannel`
  -> MagickBooleanType -- ^ increase or decrease image contrast
  -> CDouble           -- ^ strength of the contrast, the larger the number the more 'threshold-like' it becomes
  -> CDouble           -- ^ midpoint of the function as a color value 0 to `quantumRange`
  -> IO MagickBooleanType

Src: http://hackage.haskell.org/package/imagemagick-0.0.2/docs/src/Graphics-ImageMagick-MagickWand-FFI-WandImage.html

So the parameters should apparently be interpreted:

Sharpen:     0/1 (increase/decrease contrast)
Alpha:       Strength of the contrast (typically 3-20)
Beta:        Midpoint of the contrast (typically 50)

down

SkepticaLee ¶

10 years ago

The formula for sigmoidal contrast given by Thyssen is missing a term. It should be:

( 1/(1+exp(β*(α-u))) - 1/(1+exp(β*α)) ) / ( 1/(1+exp(β*(α-1))) - 1/(1+exp(β*α)) )

where there was an α missing from the second term.

Note that a sigmoidal contrast with α = 6 and β = 0.46 is approximately equivalent to combining a "screen" overlay of the image on itself, followed by a "multiply" overlay.

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