Validate image size in django admin

Question

I see a lot of people with Django apps that have image uploads are automatically resizing the images after they are uploaded. That is well and good for some cases, but I don't want to do this. Instead, I simply want to force the user to upload a file that is already the proper size.

I want to have an ImageField where I force the user to upload an image that is 100x200. If the image they upload is not exactly that size, I want the admin form to return as invalid. I would also like to be able to do the same thing for aspect ratios. I want to force the user to upload an image that is 16:9 and reject any upload that does not conform.

I already know how to get the width and height of the image, but I can't do that server-side until after the image is already uploaded, and the form is submitted successfully. How can I check this earlier, if possible?

Answer 1

The right place to do this is during form validation.
A quick example (will edit/integrate with more info later):

from django.core.files.images import get_image_dimensions
class myForm(forms.ModelForm):
   class Meta:
       model = myModel
   def clean_picture(self):
       picture = self.cleaned_data.get("picture")
       if not picture:
           raise forms.ValidationError("No image!")
       else:
           w, h = get_image_dimensions(picture)
           if w != 100:
               raise forms.ValidationError("The image is %i pixel wide. It's supposed to be 100px" % w)
           if h != 200:
               raise forms.ValidationError("The image is %i pixel high. It's supposed to be 200px" % h)
       return picture

Answer 2

This may be a dupe of/very similar to this question:

Using jQuery, Restricting File Size Before Uploading

I don't think there is a way to do it the way you want. Agos solution is the way to go...

Edit: One of the answers in the linked SO question talks about using flash to do it. So while it may be possible to do with some other technology, I don't think you can do it with straight javascript.