"halos" around bright objects

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bolonick
Posts: 14
Joined: February 18th, 2019, 4:23 pm

"halos" around bright objects

Post by bolonick »

On a scanned negative I have a very bright object next to a much darker background. I notice that while manipulating the image I am developing a "boundary effect" which consists of a slightly blurred halo around the bright object which does not seem to be there in the original scan. I notice I can minimize, but not completely eliminate, this effect by setting the "blur radius" to 0. I wonder if this is an unavoidable artifact of certain transformations or could have something to do with the way the scan, which I did not do, was done. Part of this "halo" seems to be a thin line of complementary color surrounding the bright object. The image is a high resolution .tiff file scanned in 48 bit mode.
MarkT
Posts: 366
Joined: April 24th, 2009, 2:07 pm

Re: "halos" around bright objects

Post by MarkT »

Any chance you can post small JPG versions of the original scan and the edited version? And also the transformation(s) you are employing?
jsachs
Posts: 4203
Joined: January 22nd, 2009, 11:03 pm

Re: "halos" around bright objects

Post by jsachs »

Halos are usually caused by sharpening. Which version of Picture Window are you using?
Jonathan Sachs
Digital Light & Color
bolonick
Posts: 14
Joined: February 18th, 2019, 4:23 pm

Re: "halos" around bright objects

Post by bolonick »

Yes, I think sharpening is the main issue.
The first scan I got of this image was, I believe, "over-sharpened."
I then got a higher resolution scan which was supposedly not sharpened at all and the "halo" was no longer visible in the original .tiff file, although I found it could get reintroduced unless I reset any sharpening to zero when resizing the image or doing other operations.
The slight reddish outline, complementary to the greenish background, still remains and I assume is an artifact of the scanning hardware or software, of which I have no control.
jsachs
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Joined: January 22nd, 2009, 11:03 pm

Re: "halos" around bright objects

Post by jsachs »

So the halos were from the scanning software and not Picture Window?

Newer versions of Picture Window have halo suppression sliders for the sharpening transformations.

Which version of Picture Window are you using and which sharpening transformation are you using?

The red outline, if it is consistently on the same side for all edges may be caused by a slight misregistration of the red, green and blue channels in the scanner. There is a Picture Window transformation to correct for this.

On the other hand, if the red outline is on one side on the left of the image and on the other side on the right of the image and there is none in the center, it may be chromatic aberration which there is a different transformation to correct.

Finally, if it is purple and only appears next to areas where the image is very bright, it could be purple fringing and there is yet another transformation to fix that.
Jonathan Sachs
Digital Light & Color
bolonick
Posts: 14
Joined: February 18th, 2019, 4:23 pm

Re: "halos" around bright objects

Post by bolonick »

Yes, the halos in the original image were from an original scanned image which apparently was sharpened but they also could be introduced into a higher resolution unsharpened image when resizing, for example, unless I set the sharpening slider to zero. (I suppose the solution to this is to do any sharpening explicitly with the sharpening transformation so that I could use the halo suppression.)

I will have to play with the image some more but from your description it sounds like the reddish outline (only a few pixels in width) is chromatic aberration.

Thanks for the advice.
bolonick
Posts: 14
Joined: February 18th, 2019, 4:23 pm

Re: "halos" around bright objects

Post by bolonick »

Yes, it was chromatic aberration.
Because it appeared almost identically in two different scanners, I assume the problem must be in the camera lens and not in the scanner lens.
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