Unsharpen Mask, something for Glenn

farss wrote on 5/27/2007, 5:04 AM
A few days ago I read small but significan piece by Glenn about Vegas's Unsharpen Mask FX and how one can enter negative values to get a real unsharpening effect. Thinking about it some more I realised this could be a very valuable tool, if I'm understanding what Glenn is saying correctly. If this FX could be used to undo, to a limited extent, the impact of unwanted edge enhancement in a camera in a controlled way it could be be put to very good use. There's obvious limits, you can't undo the damage but maybe minimise the impact of it and it's effect on other visual problems.
If anyones got the inclination to delve into some serious analysis of video and the film look, here's a great article by the BBC:
http://www.bbc.co.uk/rd/pubs/whp/whp-pdf-files/WHP053.pdf
Seriously mathematical stuff, but there's quite a bit about how edge enhancement impacts the 'look' of video compared to film, some of it I doubt any amount of processing in post is going to help but other artifacts might well benefit from some Unsharpen Masking.

Bob.

Comments

GlennChan wrote on 5/27/2007, 12:41 PM
I don't believe that it will completely/perfectly undo the in-camera sharpening. Though in practice, it should be visually effective and get rid of those stupid halos around edges. Negative amounts in unsharp mask should be the same thing as a blur (if I remember the algorithm correctly)... so it's really just a blur. Might as well use the quick or gaussian blur, unless you need a bigger radius and a softfocus/diffusion effect at the same time.

farss wrote on 5/27/2007, 12:54 PM
Damn!
I thought you could set it up so it'd only affect say low or high frequency detail.
Bob.
GlennChan wrote on 5/27/2007, 1:31 PM
The reason why it won't reverse in camera sharpening (I think, if I got the unsharp mask algorithm correct) is that stuff will start bleeding towards the sides.

Try this in Vegas:
Add the convolution filter:
-0.5 2.0 -0.5 (in the middle part)

Add a second convolution filter
0.5 2.0 0.5 (make sure auto-normalize is checked)

At 100% zoom, things look roughly the same (unless you are looking at something evil, like a zone plate). Zoom in and you see that high contrast edges will have minor halos moved away from the center. You can see this if you have a text generator with say 50 for black and 200 for white.

I think that's why simple post processing won't perfectly get rid of the halos around edges when there's too much sharpening.

2- Potential other areas where you can get differences are:

-Processing on linear light versus gamma corrected values. The 'right' way to do things is to process on linear light values... though it's easier to work on gamma corrected values. Everything in Vegas works on gamma corrected values by default. I think most cameras sharpen gamma corrected values... but I could be wrong there.

In Photoshop, you can rough out how this looks. To get linear light processing, flip to 16-bit mode, adjust --> levels 2.2, apply sharpening of whatever effect, adjust --> levels 0.45. Dupe the original onto another layer and apply sharpening normally. You can see a subtle difference.
*Photoshop doesn't round numbers (and doesn't have enough bit depth), which is a noticeable error.

2b- A lot of cameras also oversample the signal (via pixel shifting or other means). If the sharpening or image processing is applied to the oversampled signal, then you can get results that you can't undo/reverse in post.

I think, anyways.

3- But the practical thing to do is...
A- Get a decent camera with wide exposure latitude and ability to tweak the sharpness setting. Use the BBC camera settings guidelines as guidelines (some of the settings are debatable).
B- If you camera has too much sharpening, some quick blur should be able to make it go away. If the camera isn't very sharp looking, then you may want to stick on a unsharp mask with a medium to large radius.
C- You probably get even better results with interesting lighting and good composition.
D- The 35mm adapters seem to do interesting things to the picture, and their natural softening is sort of like a blur... and this sort of cancels out the camera's edge enhancement.