How to apply a gamma correction to a Graphics Draw (GD) image in PHP?

The imagegammacorrect() function is an inbuilt PHP function that applies gamma correction to a Graphics Draw (GD) image. Gamma correction adjusts the brightness and contrast of an image by modifying the relationship between input and output pixel values.

Syntax

bool imagegammacorrect(resource $image, float $inputgamma, float $outputgamma)

Parameters

The imagegammacorrect() function takes three parameters ?

• $image − The image resource to apply gamma correction to

• $inputgamma − The input gamma value (typically between 0.1 and 10.0)

• $outputgamma − The output gamma value (typically between 0.1 and 10.0)

Return Values

Returns TRUE on success or FALSE on failure.

Example

Here's how to apply gamma correction to brighten an image ?

<?php
    // Create a sample image
    $img = imagecreate(200, 200);
    
    // Allocate colors
    $white = imagecolorallocate($img, 255, 255, 255);
    $gray = imagecolorallocate($img, 128, 128, 128);
    $dark = imagecolorallocate($img, 64, 64, 64);
    
    // Fill background
    imagefill($img, 0, 0, $white);
    
    // Draw some shapes with different shades
    imagefilledellipse($img, 50, 50, 80, 80, $gray);
    imagefilledellipse($img, 150, 150, 80, 80, $dark);
    
    // Apply gamma correction to brighten the image
    // Input gamma: 1.0 (no input correction)
    // Output gamma: 0.5 (brightens the image)
    $result = imagegammacorrect($img, 1.0, 0.5);
    
    if ($result) {
        echo "Gamma correction applied successfully!";
        
        // Output image to browser
        header('Content-Type: image/png');
        imagepng($img);
    } else {
        echo "Gamma correction failed!";
    }
    
    // Clean up memory
    imagedestroy($img);
?>

How Gamma Correction Works

Gamma correction uses the formula: output = input^(inputgamma/outputgamma). Values less than 1.0 brighten the image, while values greater than 1.0 darken it ?

• Output gamma < 1.0 − Brightens the image

• Output gamma > 1.0 − Darkens the image

• Output gamma = 1.0 − No change in brightness

Conclusion

The imagegammacorrect() function provides an effective way to adjust image brightness and contrast in PHP. Use values below 1.0 for the output gamma to brighten images, or above 1.0 to darken them.