[wp-hackers] hacking image resizing

[Otto]

wp_generate_attachment_metadata is what creates the various sizes, at
upload time. That's where the "sizes" metadata is created.

It does this by calling image_make_intermediate_size for each separate
size. That function calls image_resize to do the actual work of
resizing the image, which uses a few gd functions to do the bulk of
the work (imagecopyresampled, imagetruecolortopalette, imagegif,
imagepng, imagejpg, etc).

This process can fail for a few reasons. wp_load_image loads the image
from disk by doing imagecreatefromstring( file_get_contents( $file )
). So if file_get_contents is unavailable, the whole shebang fails.

Basically, if the calls to the subfunctions fail to return valid
images for whatever reason, no sizes meta gets added.

-Otto



On Mon, Dec 7, 2009 at 10:53 PM, Sergiy Kuzmenko <s.kuzmenko at gmail.com> wrote:
> I had a bit of a problem with image resizing. Images sized stay greyed out
> no matter what. GD library is enabled. In fact I tried it on two machines.
> It worked fine on my Fedora desktop but failed on on the Ubuntu (hardy)
> server. There is a thread in support forum about this (apparently I am not
> alone) so I suspected it might be a bug. Looking at the code and trying to
> find the root cause I discovered that on the  machine where I have this
> problem the assoc. array containing image data has no "sizes" element
> required to trigger image resizing (wp-includes/media.php, function
> image_get_intermediate_size).
>
> Apparently, this assoc. array gets loaded from $wp_object_cache where it is
> supposed to be initialized somehow at an earlier stage.  And this is where I
> get stuck. It is not very obvious which code is loading that info into
> $wp_object_cache. Doing "grep -r sizes | grep -v .js" does not reveal
> anything worth attention. I wonder if someone could elucidate me on how
> image sizes get loaded into cache.
>
> Many thanks.
> Sergiy
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