[pmwiki-users] Best Place to Resize an Image . . .

[H. Fox]

On 4/11/06, Joachim Durchholz <jo at durchholz.org> wrote:
> H. Fox schrieb:
> > On 4/10/06, Ben Wilson <dausha at gmail.com> wrote:
> >> I want to automatically create thumbnails of uploaded images when they
> >> exceed a certain size. I know "how" to do this, since I used to do the
> >> same thing to images up to 4.5GB in size.
> >
> > Using a web server?  If so, I'm very curious how you are doing that.
>
> Use gdlib. It's part of the standard install of PHP (see
> http://www.php.net/manual/en/ref.image.php ).
> The usual sequence is:
> * getimagesize (also provides the image type)
> * imagecreatefromgif/imagecreatefromjpeg/imagecreatefrompng
> * imagecreatetruecolor (with the intended target size)
> * imagecopyresampled (fallback: imagecopyresized)
> * imagegif/imagejpeg/imagepng

I'm familiar with GD because I use it in Qdig, my GPL image gallery
script.  I contributed a function to determine the GD version to PHP
documentation about a year ago.

http://us2.php.net/manual/en/function.gd-info.php#52481

One essential step you left out, for large images at least, is getting
the source image out of memory using imagedestroy() before the last
step of converting the resized one to a compressed format.  I learned
about that by not doing it Qdig's original GD conversion code and
getting support requests from people experiencing "Allowed memory size
exhausted" errors.  :-)

I don't think you can convert a 4.5GB file with a web server,
especially using GD.  AMOF I suspect almost no typical webserver could
convert a 4.5MB compressed JPEG image using GD because PHP's
memory_limit will become exhausted, probably a few times over.  A
4.5MB true-color image will expand to something like 40 or 50 MB in
memory, and there still needs to be room for the resampled version. 
The default PHP memory_limit is 8MB.

More info:

    http://qdig.sourceforge.net/Support/AllowedMemorySize

Hagan