M Sokolewice got it nearly correct. However, the situation is a little
more complex than he has discussed.
The % compression figure for jpeg is translated into the amount of
information stored in the reverse cosine matrix. The size of the
compressed file is not proportional to the % you set in the compressor.
Thus 100% actually means store all the information in the reverse
cosine matrix. This is like storing the image in a 24 bit png, but with
the compressor turned off. So at 100% jpeg is quite inefficient.
The other issue is the amount of high frequency information in your
images. If you have a 2000x2000 image with most of the image dynamics
at a 10 pixel frequency, and you reduce this to 200x200 then the JPEG
compression algorithm will 'see' approximately the same amount of
information in the image :-( The reality is not quite as simple as this
because of the way JPEG uses blocks etc, but it is an easy way of
thinking about it.
What all this means is that as you reduce the size of an image, if you
want it to retain some of the detail of the original but at a smaller
size, there will be a point at which 8 or 24 bit PNG will become a
better bet.
Clear as mud?
AJ
M. Sokolewicz wrote:
I'm not quite sure, but consider the following:
Considering the fact that most JPEG images are stored with some form of
compression usually ~75% that would mean the original image, in actual
size, is about 1.33x bigger than it appears in filesize. When you make a
thumbnail, you limit the amount of pixels, but you are setting
compression to 100% (besides that, you also use a truecolor pallete
which adds to its size). So, for images which are scaled down less than
25% (actually this will prob. be more around 30-ish, due to palette
differences) you'll actually see the thumbnail being bigger in
*filesize* than the original (though smaller in memory-size)
- tul
P.S. isn't error_reporting( FATAL | ERROR | WARNING ); supposed to be
error_reporting( E_FATAL | E_ERROR | E_WARNING ); ??
tedd wrote:
Hi gang:
I have a thumbnail script, which does what it is supposed to do.
However, the thumbnail image generated is larger than the original
image, how can that be?
Here's the script working:
http://xn--ovg.com/thickbox
And, here's the script:
<?php /* thumb from file */
/* some settings */
ignore_user_abort();
set_time_limit( 0 );
error_reporting( FATAL | ERROR | WARNING );
/* security check */
ini_set( 'register_globals', '0' );
/* start buffered output */
ob_start();
/* some checks */
if ( ! isset( $_GET['s'] ) ) die( 'Source image not specified' );
$filename = $_GET['s'];
// Set a maximum height and width
$width = 200;
$height = 200;
// Get new dimensions
list($width_orig, $height_orig) = getimagesize($filename);
if ($width && ($width_orig < $height_orig))
{
$width = ($height / $height_orig) * $width_orig;
}
else
{
$height = ($width / $width_orig) * $height_orig;
}
// Resample
$image_p = imagecreatetruecolor($width, $height);
$image = imagecreatefromjpeg($filename);
imagecopyresampled($image_p, $image, 0, 0, 0, 0, $width, $height,
$width_orig, $height_orig);
// Output & Content type
header('Content-type: image/jpeg');
imagejpeg($image_p, null, 100);
/* end buffered output */
ob_end_flush();
?>
---
Thanks in advance for any comments, suggestions or answers.
tedd
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