Arc - multiple outputs from one upload?


I’m using Waffle (a fork of Arc) for image uploads in my Phoenix project. When an image is successfully uploaded, I would like to generate a bunch of smaller images from it, which will then be used in a tiled image pyramid viewer. Of course if the user deletes the image the tiles should be automatically deleted as well.

How can I accomplish this?
If I understand it correctly, Arc’s default “transformations” flow assumes each input file may map to at most one output file for each version, even if I want to run arbitrary commands. I already have the command to generate the images, I just need to understand how to integrate this with Arc.

Thanks in advance!

Hello and welcome,

It is possible to specify multiple versions in the uploader, something like…

  @versions [:original, :large, :thumb, :mini]

  def transform(:mini, _) do
    {:convert, "-resize 160x90^ -gravity center -extent 160x90 -format png", :png}

  def transform(:thumb, _) do

  def transform(:large, _) do

For delete, I did not solve it yet in the best way… In case of cascade delete for example…

But I have a cleaning task running periodically, deleting what needs to be.


I think you’re talking about images files associated with database records, right? Indeed I can’t see really a better solution.

But just in case the OP author is not aware of it, I would like to add this:

A Waffle uploader module has a delete function that should delete all the versions of an image (:mini, :large, :original…) all together when called.

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Yes, I was talking about associated files :slight_smile:

The problem is not to hook file deletion, but to know when the resource is deleted.

I have been thinking of postgres triggers, but it is postgres specific, and a super simple cron task…

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Hey @kokolegorille, thanks for your reply!

Sorry, I’m not sure I was clear enough. I know I can define these versions, but each version is assumed to only produce one file, right?

I’m implementing an image viewer that allows users to pan and zoom very large images. For this I need to generate an arbitrary number of files based on the dimensions of the input image. More specifically, from a single image I must produce many “slices” (cropped and resized squares) that correspond to x, y and z coordinates (z for zoom). This is common practice for “image pyramids”, “slippy maps”, etc. (if you want to look up those terms). So I don’t know how many images will be produced, and I don’t want to refer to each of them by name, but by “coordinate”. For instance, /uploads/myimg/0/2/3.jpg would be x=0, y=2, z=3. So I imagine it being a single version (e.g. :slices) with 3 parameters (x, y and z).

I hope I’ve explained it well enough!

I have also used transformation tools with System.cmd…

Just I did not use waffle transform, but a custom job. I used ffmpeg, to capture screen from video, but it was too heavy for waffle (taking too long). I did a custom background job in that case.

If You have a file location, You can apply anything on it… with imagemagick convert, or ffmpeg, or any other tools.

Would you mind explaining how you did this background job?

And do you think it would be possible to run the command and make transform return the path to a folder that contains all generated images for the input file, rather than return the path to a single file? Then I’d be able to do something like:
<img src="<%= Pyramid.url({"myimg.png", img}, :slices) %>/0/0/0.jpg">

There are some ways to do it.

Oban by @sorentwo.

or a simple task… like in this post by @alvises

BTW, You may find more articles on using mogrify, or doing raw file upload in the previous linked site.

After this, You can create your own helper to display the associated files. For example it could be


I’m still very much a newbie so I’m still pretty lost, but I’ll try to understand things a little better before asking more questions. Thank you very much!