PHP GD Functions

GAMMA CORRECTION

is a nonlinear operation, applied to encode or decode LUMINANCE parameters or TRISTIMULUS values in images, to make their appearance close to that perceived by humans.

LUNINANCE

is the measure of the density of the intensity of a light reflected in a particular direction.

TRISTIMULUS

is a description of how the three primary colors are blended to obtain a particular secondary color.

Gamma is defined by V_out = V_in^gamma,

where:

V_out is the OUTPUT luminance value
and
V_in is the INPUT/actual luminance value.

This formula causes the graphic line to curve.

When gamma < 1, the line arches upward.

gamma_chart3c.png apr

When gamma > 1, the line arches down.

gamma_chart4d.png apr

* Images from CAMBRIDGE in COLOR

This function returns TRUE on success or FALSE on failure.


<?php

bool imagegammacorrect ( GdImage $image, 
                                                  float $input_gamma, 
                                                  float $output_gamma )


where,

$image = The image identifier

$input_gamma = The input gamma
              
$output_gamma = The output gamma

?>

$image

The image identifier.

$input_gamma

The input gamma.

$output_gamma

The output gamma.

EXERCISE


<?php

echo 'PHP VERSION: ' . PHP_VERSION . '<br><br>';

$lang = 'en';

$org_file = "jpeg/JPEG 033 01.jpg";

$des_file = "jpeg/JPEG 033 01 imagegammacorrect.jpg";

$gamma_in = 2.0;

$gamma_out = 0.9;

echo basename($org_file); ?><br><br>
<img src="<?php echo $org_file; ?>"
alt="<?php echo $org_file; ?>" 
title="<?php echo $org_file; ?>"><br><br>

<?php
$id_HS01 = imagecreatefromjpeg( $org_file);

$gamacorrectd = 
imagegammacorrect($id_HS01, $gamma_in, $gamma_out);

if($gamacorrectd == true)
{
imagejpeg($id_HS01, $des_file);

echo '<br><br>' . basename($des_file); ?><br><br>
<img src="<?php echo $des_file; ?>"
alt="<?php echo $des_file; ?>" 
title="<?php echo $des_file; ?>">
<?php
imagedestroy($id_HS01);
}
else
{
$en = 'GAMMA was not corrected!';
echo '' . $$lang . '';
}

?>