I have a bunch of objects, and I want to apply the same filter to them all. But when I create the filter and apply it, it is scaled differently on some objects.

In the attached image, all the objects have the filter "filter0", with the settings displayed. As you can see, the orange shape to the left has a very large filter effect, while the black object above it has a less strong one.

All the objects were draw/copied/arranged before applying the filter, so I don't think it's anything to do with scaling filters with objects.

Can anyone help?

I´d say this seems to happen when objects are scaled after the filter is applied - that´s how I can recreate this at least:

That's the thing - I though that might be the case, and I guess it is. But to make sure, I removed the filters completely, deleted all trace of the filters, and reapplied new filters. The problem was still there.

What I think is happening is that some of the objects have transforms applied - I didn't specifically apply the transforms, but somehow with the copying and moving they ended up with the transforms.

To test this, I created a new layer and drew new shapes from scratch over some of the problem objects. When I applied the filter to them, there was no problem.

I searched around for ways to remove transforms, and set the object size and position absolutely. But it looks like no-one knows. You can of course search in the XML and remove the transforms, but then the object goes back to it's original size and position.

So the question becomes, how do you remove a transform from an object, but leave the object where it was? Convert transforms to absolutes? Doing that in bulk would be great.

That was it! I found the extension "Apply transforms" which removes transforms but leaves the object where it was.

https://github.com/Klowner/inkscape-applytransforms

I installed that, used it on the problem objects, checking in the XML showed that the transforms were gone but the objects were still in place. Then I applied the filter to everything, and it was scaled the same for all of them. Problem solved.

That would have been my next guess and recommendation. Glad you get it sorted already. ;-)