I figured out it.

if you pre generate the thumbnail or othersize (for example websize) in advance and upload to _data\i\galleries, you should make sure those files have a newer date than the original image it self, otherwise piwigo will regenerate it again.

ok
I'm looking at the code and i see only one thing :
      if ($mtime===false or $mtime < $params->last_mod_time)
      {
        $is_cached = false;
I notify the developer of that part

I didn't code that part but normally the script check if the file exists, and if not generate : so you should be able to generate locally in _data/i/...-suffix.ext and upload it. You might have not used the correct suffix, or path

deserteagle wrote:

I had 2 weeks vacation in NZ with my family, so there are more than 2000 photos in one album, so there will be near 20k photos (9 multiple size) in the cache folder.

I do agree with that. I have some users (on Piwigo.com) who add sometimes 4000 photos on the same day. That may mean 36000 files in the same folder. Not good for filesystem performances.

If you add photos with the web form or with a remote tool (like iPhoto or Lightroom), the thumbnail is automatically generated at upload time, to avoid this slow loading "first time". Read more on topic:18755

I tried 2.4RC3, the view experience is not good because I have to wait the thumbnail to be generated, when I change the picture size by click the multiple size button on the top left of page, I also have to wait seconds for the generate process.

I strong believe "generate on the fly" sound cool but not practicable, especially for huge number of photos.

Create the thumbnail and web size in advance will not use too many disk quote (for my case, 50G photos with web size(800x600) use about 300M), but make the page load and show photos to visitor quickly!

I strong suggest that make the thumbnail/ HD folders be able to customize are more useful compare to "generate on the fly".

deserteagle369

Put the data and pic directory outside the install directory is a very useful and reasonable feature.

1.
like rvelices said, it's easy to upgrade

2.
make system backup size easy, my pic directory over 50G.

3.
can view the websize pics by other image view software when the piwigo are not reachable.
In my case, I use piwigo built a family gallery, so every pc in my home can easy access piwigo gallery by web, but sometimes I go out with notebook, I prefer only keep a copy of gallery directory without pwg_high directory, then I can use image view software like faststone to play slideshow of whole gallery directory in websize (800x600) with background musics.

If the pwg_high and thumbnail directory can seperate out of gallery directory that will make batch management easy. for example I use faststone to generate the whole gallery directory from my original photo directory at one time(maybe several hours), then I use synchronize and thumbnail function in piwigo to generate thumbnail at one time too(this also can be done by faststone if the thumbnail directory are out of install directory.

The multiple size function can generate different size pics, that's great if the pics in gallery are contribute by different users, but if the pics are mostly upload by admin like me, I prefer batch generate the websize and thumbnail before upload it.

plg wrote:

So it can't work if Piwigo is installed at the root of your website, do you confirm?

I confirm unless the data directory is a symbolic link to a real directory outside the install.