Prashant Ingale

The logic of batch processing thumbnails

Your server is running PHP, gd support is enabled, let’s start thinking about the code.

To generate thumbnails we do the following:

• Scan a folder for JPG and PNG files (gd does not support GIF anymore, because
  the packing algorithm in GIF is copyrighted).
• Take each of these images, and load it.
• Resize the image.
• Save the image as a thumbnail.

The resizing idea is the following:

• We assume the thumbnail should be 100 pixels, either wide or high.
• We load the original image, and check its dimensions.
• If the picture is higher than wide, we set the height of the thumb
  to 100 pixels.
• The width of the thumbnail is the original width multiplied with 100 pixels
  divided by its height.
• Thumbnail height = original width * (100 / original height)
• This way we preserve the original aspect ratio.
• If the original picture is wider than high, we do the same to the height of
  the thumbnail.
• If they are the same, we simply create a 100×100 pixels image.

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The PHP functions to use

PHP has a whole lot of functions

to help us with generating graphics with gd.

The ones we need to use here are:

• imageCreateFromJPEG() to create a copy to work on of a .jpg image.
• imageCreateFromPNG() to create a copy to work on of a .png image.
• imageSX() to get the width of the original image.
• imageSY() to get the height of the original image.
• ImageCreateTrueColor() to create a new truecolour image object.
• imageCopyResampled() to resample the image.

GD supports truecolour images from version 2.0 onwards, older versions of GD
will create JPGs and PNGs with 256 colours and need other functions. I deliberately
dropped the support here, as gd2 comes bundled with every newer install of PHP anyways.

• imageJPEG() to create a new JPEG image.
• imagePNG() to create a new PNG image.
• imagedestroy() to delete the old image objects in the memory.

Furthermore I created some functions I keep using:

• directory() reads a folder and returns all the files that apply to a certain filter.
• ditchtn() clears the resulting array from all files starting with a filter you define, like tn_".
• createthumb() creates a thumbnail and saves it to the server.

directory() and ditchtn() are filesystem and array functions, I will not explain them
here, they should be pretty self-explanatory.

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The main thumbnail generation function

createthumb() is the main thumbnail generation function, so let’s take a closer
look:

function createthumb($name,$filename,$new_w,$new_h){
	$system=explode('.',$name);
	if (preg_match('/jpg|jpeg/',$system[1])){
		$src_img=imagecreatefromjpeg($name);
	}
	if (preg_match(’/png/’,$system[1])){
		$src_img=imagecreatefrompng($name);
	}

createthumb() is called with the following parameters: The name of the original
image (if needed with folder name), the name of the thumbnail picture, and the
dimensions.

These lines get the information if gd is at least version 2.0 and check if the
original image is a JPEG or PNG.

Accordingly, a new image object is created called src_image.

$old_x=imageSX($src_img);
$old_y=imageSY($src_img);
if ($old_x > $old_y) {
	$thumb_w=$new_w;
	$thumb_h=$old_y*($new_h/$old_x);
}
if ($old_x < $old_y) {
	$thumb_w=$old_x*($new_w/$old_y);
	$thumb_h=$new_h;
}
if ($old_x == $old_y) {
	$thumb_w=$new_w;
	$thumb_h=$new_h;
}

These lines get the dimensions of the original image by using imageSX() and imageSY(),
and calculate the dimensions of the thumbnail accordingly, keeping the correct
aspect ratio. The desired dimensions are stored in thumb_w and thumb_h.

	$dst_img=ImageCreateTrueColor($thumb_w,$thumb_h);
	imagecopyresampled($dst_img,$src_img,0,0,0,0,$thumb_w,$thumb_h,$old_x,$old_y);

These lines create the image as a true colour version using
ImageCreateTrueColor() and resize and copy the original image into
the new thumbnail image, on the top left position.

if (preg_match("/png/",$system[1]))
{
	imagepng($dst_img,$filename);
} else {
	imagejpeg($dst_img,$filename);
}
imagedestroy($dst_img);
imagedestroy($src_img);
}

These lines check the file system extension of the original image and create
the thumbnail accordingly. The thumbnail gets saved onto the server (by adding
a filename to imagejpeg() or imagepng() function)
and the two image objects get destroyed to free the memory.

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How to use createthumb()

Let’s say you have an original picture called apple.jpg in a folder pics and you
want to generate the thumbnail in the folder thumbs as tn_apple.jpg 100×100 pixels big.

You can do this by calling this function with the following parameters:

createthumb('pics/apple.jpg','thumbs/tn_apple.jpg',100,100);

To batch process all the pictures in the folder pics and generate their
thumbnails in the folder thumbs you do the following:

$pics=directory('pics','jpg,JPG,JPEG,jpeg,png,PNG');
$pics=ditchtn($pics,'tn_');
if ($pics[0]!=”)
{
	foreach ($pics as $p)
	{
		createthumb(’pics/’.$p,’thumbs/tn_’.$p,100,100);
	}
}

You grab all the pictures from the pics folder via directory() and a filter for images.

Then you delete all the thumbnails by using ditchtn() and make sure that
the resulting array is not empty.

The second parameter in ditchtn() must match the name you give your
thumbnails, this is done to prevent thumbnails being generated from thumbnails.

You then loop over all images and generate the thumbnail with the desired
dimensions and names.

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