As I promised, here is the writeup with examples:
http://nerds-central.blogspot.com/2006/08/choosing-file-format-for-small-web.html
Cheers
AJ
tedd wrote:
Alex:
Excuse for top posting:
You said: Clear as mud?
Well actually, it's simperer than I thought. After your reply, I did
some reading on jpeg and found it's simply a transform, not unlike FFT
where two-dimensional temporal data is transformed from the time domain
to the frequency domain -- very interesting reading.
The reverse cosine matrix you mention is probably the discrete cosine
transform (DCT) matrix where the x, y pixels of an image file have a z
component representing color. From that you can translate the data into
the frequency domain, which actually generates more data than the original.
However, the quality setting is where you make it back up in compression
ratio's by trimming off higher frequencies which don't add much to the
data. Unlike the FFT, the algorithm does not address phasing, which I
found interesting.
However, the answer to my question deals with the quality statement. In
the statement:
imagejpeg($image_p, null, 100);
I should have used something less than 100.
I've change the figure to 25 and don't see any noticeable difference in
quality of the thumbnail.
It seems to me there should be a table (or algorithm) somewhere that
would recommend what quality to use when reducing the size of an image
via this method. In this case, I reduced an image 62 percent (38% of the
original) with a quality setting of 25 and "see" no difference. I think
this (the quality factor) is programmable.
As for png images, I would probably agree (if I saw comparisons), but
not all browsers accept them. I belive that at least one IE has problems
with png's, right?
tedd
At 4:45 PM +0100 8/23/06, Alex Turner wrote:
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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