Color-Altering Filters
All the filters in this group work by altering colors. Each filter applies a specific operation to all or a subset of the image pixels, resulting in an entirely new and often quite different image from the original.
Grayscale Filter
By removing all the color values and replacing them with shades of gray, you can create nice-looking black and white images (see
Figure 5).
 | |
| Figure 5. Grayscale Filter: The grayscale filter removes color from images. |
// Listing grayscale_filter.php
<?php
$im = imagecreatefromjpeg("tree.jpg");
$size = getimagesize("tree.jpg");
$L=$size[0];
$H=$size[1];
for($j=0;$j<$H;$j++){
for($i=0;$i<$L;$i++){
// get the pixel color at i,j
$rgb = imagecolorat($im, $i, $j);
// get the individual color values
$r = $rgb&0x00FF0000;
$r = $r >> 16;
$g = $rgb&0x0000FF00;
$g = $g >> 8;
$b = $rgb&0x0000FF;
// calculate the grayscale
$bw=($r+$g+$b)/3;
$result = (0x000000FF<<24)|($bw<<16)|($bw<<8)|$bw;
// create the new color values
$new_r = ($result >> 16) & 0xFF;
$new_g = ($result >> 8) & 0xFF;
$new_b = $result & 0xFF;
// assign the grayscale color
$new_color = imagecolorallocate($im, $new_r, $new_g, $new_b);
imagesetpixel($im, $i, $j, $new_color);
}
}
header("Content-type: image/jpeg");
imagejpeg($im);
?>
Negate Filter
 | |
| Figure 6. Negate Filter: This filter reverses, or negates the colors in an image. |
This application creates a "negate" filter (see
Figure 6) by applying an exclusive OR operation to the red, green, and blue portions of the color while leaving the alpha channel intact:
// Listing negate_filter.php
<?php
$im = imagecreatefromjpeg("tree.jpg");
$size = getimagesize("tree.jpg");
$L=$size[0];
$H=$size[1];
for($j=0;$j<$H;$j++){
for($i=0;$i<$L;$i++){
$rgb = imagecolorat($im, $i, $j);
$result = ($rgb & 0xff000000)+($rgb & 0xffffff)^0xffffff;
$new_r = ($result >> 16) & 0xFF;
$new_g = ($result >> 8) & 0xFF;
$new_b = $result & 0xFF;
$new_color = imagecolorallocate($im, $new_r, $new_g, $new_b);
imagesetpixel($im, $i, $j, $new_color);
}
}
header("Content-type: image/jpeg");
imagejpeg($im);
?>
Flea Filter
 | |
| Figure 7. Flea Filter: An interesting colorization effect, this filter works by OR'ing the existing colors in an image with a random color. |
A flea filter is a random color value OR'd with the existing color values in an image. It produces a colorful and interesting effect (see
Figure 7):
// Listing flea_filter.php
<?php
$im = imagecreatefromjpeg("tree.jpg");
$size = getimagesize("tree.jpg");
$L=$size[0];
$H=$size[1];
for($j=0;$j<$H;$j++){
for($i=0;$i<$L;$i++){
$rgb = imagecolorat($im, $i, $j);
$new_r = rand(0,255);
$new_g = rand(0,255);
$new_b = rand(0,255);
$new_color = imagecolorallocate($im, $new_r, $new_g, $new_b);
$new_rgb = $rgb|$new_color;
imagesetpixel($im, $i, $j, $new_rgb);
}
}
header("Content-type: image/jpeg");
imagejpeg($im);
?>
Fog Filter
 | |
| Figure 8. Fog Filter: By using the alpha channel value (which requires the PNG format instead of JPG), this filter applies a fog-like film to an image. |
As the name implies, a fog filter applies a hazy film over an image (see
Figure 8). Note that in this case you have to covert the
tree.jpg file to a Portable Network Graphics (PNG) file (
tree.png) before beginning the operation, because images in PNG format can use alpha channel values while images in JPEG format do not.
// Listing fog_filter.php
<?php
$im = imagecreatefrompng("tree.png");
$size = getimagesize("tree.png");
$L=$size[0];
$H=$size[1];
for($j=0;$j<$H;$j++){
for($i=0;$i<$L;$i++){
$rgb = imagecolorat($im, $i, $j);
$r = ($rgb >> 16) & 0xFF;
$g = ($rgb >> 8) & 0xFF;
$b = $rgb & 0xFF;
//Turn off transparency blending
imagealphablending($im, false);
//Create a new transparent color for image
$color = imagecolorallocatealpha($im, $r, $g, $b, 100);
//Set the new color
imagesetpixel($im, $i, $j, $color);
//Restore transparency blending
imagesavealpha($im, true);
}
}
header('content-type: image/png');
imagepng($im);
?>
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