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15 years ago
This is based on the Skew function from designerkamal at gmail dot com.

This is a function for skewing images in PHP with anti-aliasing. It works with alpha PNG images.

Warning: the bigger the image you skew, the longer it will take to process. It's about 3 times longer than without anti-aliasing.

<?php
// $img: handle of an image
// $skew_val: level of skew to apply (0 being none, 1 being 45°)
function imageskewantialiased($img, $skew_val)
{
    $width = imagesx($img);
    $height = imagesy($img);
    $height2 = $height + ($width * $skew_val);
    
    // See below for definition of imagecreatealpha
    $imgdest = imagecreatealpha($width, $height2);
    
    // Process the image
    for($x = 0, $level = 0; $x < $width - 1; $x++)
    {
        $floor = floor($level);
        
        // To go faster, some lines are being copied at once
        if ($level == $floor)
            imagecopy($imgdest, $img, $x, $level, $x, 0, 1, $height - 1);
        else
        {
            $temp = $level - $floor;
            
            // The first pixel of the line
            // We get the color then apply a fade on it depending on the level
            $color1 = imagecolorsforindex($img, imagecolorat($img, $x, 0));
            $alpha = $color1['alpha'] + ($temp * 127);
            if ($alpha < 127)
            {
                $color = imagecolorallocatealpha($imgdest, $color1['red'], $color1['green'], $color1['blue'], $alpha);
                imagesetpixel($imgdest, $x, $floor, $color);
            }
            
            // The rest of the line
            for($y = 1; $y < $height - 1; $y++)
            {
                // Merge this pixel and the upper one
                $color2 = imagecolorsforindex($img, imagecolorat($img, $x, $y));
                $alpha = ($color1['alpha'] * $temp) + ($color2['alpha'] * (1 - $temp));
                if ($alpha < 127)
                {
                    $red   = ($color1['red']   * $temp) + ($color2['red']   * (1 - $temp));
                    $green = ($color1['green'] * $temp) + ($color2['green'] * (1 - $temp));
                    $blue  = ($color1['blue']  * $temp) + ($color2['blue']  * (1 - $temp));
                    $color = imagecolorallocatealpha($imgdest, $red, $green, $blue, $alpha);
                    imagesetpixel($imgdest, $x, $floor + $y, $color);
                }
                
                $color1 = $color2;
            }
            
            // The last pixel of the line
            $color1 = imagecolorsforindex($img, imagecolorat($img, $x, $height - 1));
            $alpha = $color1['alpha'] + ((1 - $temp) * 127);
            if ($alpha < 127)
            {
                $color = imagecolorallocatealpha($imgdest, $color1['red'], $color1['green'], $color1['blue'], $alpha);
                imagesetpixel($imgdest, $x, $floor + $height - 1, $color);
            }
        }
        
        // The line is finished, the next line will be lower
        $level += $skew_val;
    }
    
    // Finished processing, return the skewed image
    return $imgdest;
}

// Creates a new image of the size specified with a blank background (transparent)
function imagecreatealpha($width, $height)
{
    // Create a normal image and apply required settings
    $img = imagecreatetruecolor($width, $height);
    imagealphablending($img, false);
    imagesavealpha($img, true);
    
    // Apply the transparent background
    $trans = imagecolorallocatealpha($img, 0, 0, 0, 127);
    for ($x = 0; $x < $width; $x++)
    {
        for ($y = 0; $y < $height; $y++)
        {
            imagesetpixel($img, $x, $y, $trans);
        }
    }
    
    return $img;
}

// Here's an example of how to use it
imagepng(imageskewantialiased(imagecreatefrompng('test.png'), 0.15), 'skew.png');
?>
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