Thumbnails in PHP - Explanation
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We'll now proceed through the script and explain each block in excruciating detail. I hope you have caffeine ready, you may need it.
<?php
# Constants
define(IMAGE_BASE, '/var/www/html/mbailey/images');
define(MAX_WIDTH, 150);
define(MAX_HEIGHT, 150);
?> |
First we should define a few constants. IMAGE_BASE points to the directory from which images are to be served. Only images in or under this directory are available. MAX_WIDTH and MAX_HEIGHT specify the maximum size to which images will be shrunk. If an image is already smaller than these sizes, it is returned without modifications. Proportions are also retained so that if the width is reduced by a factor of 2, the height will be reduced by that same factor.
<?php
# Get image location
$image_file = str_replace('..', '', $_SERVER['QUERY_STRING']);
$image_path = IMAGE_BASE . "/$image_file";
# Load image
$img = null;
$ext = strtolower(end(explode('.', $image_path)));
if ($ext == 'jpg' || $ext == 'jpeg') {
$img = @imagecreatefromjpeg($image_path);
} else if ($ext == 'png') {
$img = @imagecreatefrompng($image_path);
# Only if your version of GD includes GIF support
} else if ($ext == 'gif') {
$img = @imagecreatefrompng($image_path);
}
?> |
The image location is passed as the entire query string as explained above. To prevent people from accessing images outside of the specified path, the string replace function is used to remove any "..".
The extension is checked to find out what type of image has been requested. GD has a number of different load functions. Each is of the form imageloadfrom[type](). I have included only the three most common image formats, though others can easily be added.
If the extension is unrecognized or the specified file does not exist, $img is left empty. When this occurs, an error is generated (see below). The @ before the load functions suppresses errors so that the user won't be shown a page of garbage.
NOTE: gif support isn't available in the newer versions of GD. If your version of GD does not support gif images, you should probably remove the lines for handling gif images.
<?php
# If an image was successfully loaded, test the image for size
if ($img) {
# Get image size and scale ratio
$width = imagesx($img);
$height = imagesy($img);
$scale = min(MAX_WIDTH/$width, MAX_HEIGHT/$height);
?> |
imagesx() and imagesy() return the width and height of the image respectively. The maximum size of the image is then divided by the actual size of the image. This results in a scaling ratio. This scaling ratio is multiplied by the actual image size in order to give the reduced size. This value is calculated for both the width and height seperately; though the smaller of the two is used. By using the same value for both width and height, we preserve the correct proportions.
<?php
# If the image is larger than the max shrink it
if ($scale < 1) {
$new_width = floor($scale*$width);
$new_height = floor($scale*$height);
# Create a new temporary image
$tmp_img = imagecreatetruecolor($new_width, $new_height);
# Copy and resize old image into new image
imagecopyresized($tmp_img, $img, 0, 0, 0, 0,
$new_width, $new_height, $width, $height);
imagedestroy($img);
$img = $tmp_img;
}
}
?> |
If the scale ratio is greater than or equal to 1 the image does not need to be modified in any way. Otherwise, the image must be reduced in size by that value. The new image size is generated by multiplying the scale by the current size.
Using these new dimensions a buffer image is created using imagecreatetruecolor(). If you are using a GD library prior to 2.0, you will need to change this function to imagecreate(). The true color version of this function was added in 2.0 to preserve color integrity when parts of an image are copied, which we are about to do. The older function will still work fine for many images, but high color photos and the like may not maintain the correct colors using the older function.
The original image is then resized into the buffer image. Since we will now be using this new image, we are able to free the original image with imagedestroy(). The buffer image is then renamed to $img.
<?php
# Create error image if necessary
if (!$img) {
$img = imagecreate(MAX_WIDTH, MAX_HEIGHT);
imagecolorallocate($img,0,0,0);
$c = imagecolorallocate($img,70,70,70);
imageline($img,0,0,MAX_WIDTH,MAX_HEIGHT,$c2);
imageline($img,MAX_WIDTH,0,0,MAX_HEIGHT,$c2);
}
?> |
As was explained earlier, if the image's extension was unrecognizable, or the file did not exist all together, $img was left empty. If this is the case, we need to generate a place holder image to notify the user that the image they requested was unavailable. We do this by drawing a black box with a gray X through the middle. You may want to make this more explicit, by adding text or even redirecting the browser to a premade image.
<?php
# Display the image
header("Content-type: image/jpeg");
imagejpeg($img);
?> |
The image is finally displayed to the user in jpeg format.
Next: Conclusion >>
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