Zoom Blur

[Marpel]

I have used the Zoom Blur tool on a couple images.

An issue I am seeing so far is for images which have the full blur (50%, slider moved farthest to the right). I haven't yet tried with a less than full blur, so can't comment on that. I am attaching a full image, and a crop of the area which shows the problem most clearly. It seems the tool is somehow including the edge of the image in it's calculations, which results in a visible rectangular "halo" in the image. Not easy to see at first glance, but once noticed it seems to stick out like a sore thumb.

Is there any way to mitigate this?

[Marpel]

By the way, I post with attached images to a couple sites and am forever confused on image size requirements. I have tried to find details in FAQ (even under attachments heading) and elsewhere on this site but am unable to find the info, so usually just wing it. Is there a location specific to attachment requirements?

Marv

[Marpel]

Any idea if a potential fix for this may be likely? I have been asked to do up a couple similar images but the halo makes this difficult in PWP.

I've tried using PS, but I've found that program is not a good option either, due to other issues.

Marv

[jsachs]

Max image size is 1200 x 1200 -- only the administrator (me) can see or change the setting.

I thought I had answered this post earlier, but I must have forgotten to click the Submit button.

The problem, I believe, is that when you get near the edges of the image, the transformation runs out of image data to use, so it blurs based on what it has and this in effect reduces the amount of blurring and causes a visible artifact.

What I recommend, and it should work for the image you are using, is to add a generous border around the edges and crudely clone image data into the border to make a plausible surround. Then do the zoom blur and finally crop out the border you added.

[Marpel]

Jonathan,

Thanks for the response.

The image I posted is actually a composite of two, one has the steeple (with a mask for that steeple) and the other is an image from a set of clouds I photographed. I comp'd the steeple over the clouds and ran the zoom blur to get what you see.

Because of the halo though, prior to my first post, I was making do with resizing the cloud image larger, compositing the steeple into that larger image, zoom blurring, then cropping the image to the size it would have been prior to resizing. Just a derivation of your suggestion.

However, and the reason for my post, the regular sized image from my camera (7360 x 4912) results in a long wait (some 6-ish minutes, if I recall), and making the image larger so it can be later cropped, added significantly to that time. The result is certainly satisfactory, but the added time is the issue.

But if that is the only way, it will have to do.

I should add, comparing PWP to PS for this, although I didn't see the same halo from PS, it has no easy way to position the centre point of the zoom and the result also has horrible graininess in the image. PWP is way better in many respects.

Marv

[jsachs]

I double checked and I'm pretty sure the halo comes from blurring with pixels as you get near the edges. I have no idea how Photoshop avoids this problem.

As I understand it, the Photoshop algorithm uses a clever trick to greatly speed up the computation, but it does not accurately emulate what happens when you zoom and it can introduce some artifacts.

The other thing you could try is to successively blur by say 10% for five times rather than once at 50%. This might produce a smoother result, but I'm not sure it would be the same effect.