gradient.php

<?php
/**
 * simpele gradient image adhv GET parameters
 *
 * @author Stefan Stubbe
 */

// default settings definiëren

$defaults = array(
	'height' => 100 ,
	'width'  => 220 ,
	'from'   => '8ae234' ,
	'to'     => '4e9a06' ,
);

// samenvoegen van default settings met parameters meegegeven in $_GET. 
// => GET parameters overschrijven dus de default settings

$options = array_merge( $defaults , $_GET );

// controleren of GD library beschikbaar is

if( ! function_exists( 'imagecreate' ) ) {
	die_with_error( 'GD library is missing' );
}

// valideren van de settings

$options = array_map( 'trim' , $options );

$height = (int) $options['height'];
if( $height <= 0 || $height > 200 ) {
	die_with_error( 'Invalid height given, must be between 1 and 200' );
}

$width = (int) $options['width'];
if( $width <= 0 || $width > 1000 ) {
	die_with_error( 'Invalid width given, must be between 1 and 1000' );
}

if( ! preg_match( '#^[a-z0-9]{6}$#i' , $options['from'] ) ) {
	die_with_error( 'Invalid start color, must be a hexadecimal RGB color' );
}

if( ! preg_match( '#^[a-z0-9]{6}$#i' , $options['to'] ) ) {
	die_with_error( 'Invalid end color, must be a hexadecimal RGB color' );
}

// afbeelding alloceren

if( false === ( $gd = imagecreate( $width , $height ) ) ) {
	die_with_error( 'Could not create image' );
}

// twee manieren om RGB op te splitsen in R, G en B
if( true ) {
	// een eerste manier om het hexadecimaal RGB kleur (RRGGBB) op te splitsen in rood, groen en blauw

	$redStart   = hexdec( substr( $options['from'] , 0 , 2 ) );
	$greenStart = hexdec( substr( $options['from'] , 2 , 2 ) );
	$blueStart  = hexdec( substr( $options['from'] , 4 , 2 ) );

	$redEnd     = hexdec( substr( $options['to'] , 0 , 2 ) );
	$greenEnd   = hexdec( substr( $options['to'] , 2 , 2 ) );
	$blueEnd    = hexdec( substr( $options['to'] , 4 , 2 ) );

} else {
	// de tweede manier: binair
	$start      = hexdec( $options['from'] );
	$end        = hexdec( $options['to'] );

	$redStart   = ( $start >> 16 ) & 0xFF;
	$greenStart = ( $start >> 8 ) & 0xFF;
	$blueStart  = $start & 0xFF;

	$redEnd     = ( $end >> 16 ) & 0xFF;
	$greenEnd   = ( $end >> 8 ) & 0xFF;
	$blueEnd    = $end & 0xFF;
}

// stappen tussen 'from' kleur en 'to' kleur bepalen

$redStep   = ( $redEnd   - $redStart   ) / $height;
$greenStep = ( $greenEnd - $greenStart ) / $height;
$blueStep  = ( $blueEnd  - $blueStart  ) / $height;

// gradient maken

$red   = $redStart;
$green = $greenStart;
$blue  = $blueStart; 

for( $y = 0 ; $y < $height ; $y++ ) {
	// kleur alloceren om te gebruiken in de afbeelding
	$color = imagecolorallocate( $gd , (int) $red , (int) $green , (int) $blue );

	// horizontale lijn tekenen in dit kleur
	imageline( $gd , 0 , $y  , $width , $y , $color );

	// kleuren aanpassen
	$red   += $redStep;
	$green += $greenStep;
	$blue  += $blueStep;
}


// output van afbeelding

header( 'Content-Type: image/png' );
imagepng( $gd );

exit;

// ==================================================================

function die_with_error( $msg )
{
	header( 'HTTP/1.0 500 Server error' );
	echo 'error: ' , $msg;
	exit;
}