Exposure anchor - how to work with it?

[Abrimaal]

I have made thousands of panoramas and I still don't understand how the selector "anchor this image for exposure" works.
Especially I mean photo panoramas where partial photos differ a lot.
Which photo should I select for exposure anchor for the best results?

1. The one that looks the best?

2. The one with highest exposure?
3. The one with the most average exposure value?

When a photo is set as exposure anchor, it means that its exposure, colors and white balance should remain unchanged in the panorama.
All other photos are adjusted to the anchor.

I attach an example of a panorama, where the first photo from the left was locked as exposure anchor. Its exposure value was the highest.
At the edge the photo was not blended with any other photo,

so its exposure should be the same as the original photo,
but it doesn't look the same.

One more try: reset photometric values
The left edge of the panorama was darker, but still much brighter than the original

What should I do to make the panorama darker, lower all exposures proportionally at once?

If I don't select any image as exposure anchor, will the final exposure be calculated as average of all partial images?

[Paul Womack]

Selecting an image as anchor is stage 1 of the process, and your understanding is correct.

However, the anchoring is only APPLIED when you actually run a photometric optimisation, which checks corresponding pixels between the multiple images.

For the correspondance to be accurate, a well optimised alignment is a necessary precondition.

This optimisation is a button on the Exposure tab, which is only visible when using "Custom" optimisation parameters.

   BugBear

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[Bruno Postle]

On 13 February 2021 00:28:54 GMT, Abrimaal wrote:
>I have made thousands of panoramas and I still don't understand how the
>
>selector "anchor this image for exposure" works.

The anchor image retains the Ev exposure value during optimisation. A good anchor image has few 'blown-out' overexposed areas.

What you are noticing is that Hugin also has a project-wide Ev setting, which the assistant automatically sets to the average of all the photos in the project, so your anchor image may look different in the output because this average looks at all photos together.

You can change this global Ev in the panorama tab of the fast preview: set it to the same value as the anchor, and the anchor will have no exposure adjustment; there is also a reset to average button; and + - buttons that raise lower the global Ev by ⅓ of a stop to brighten/darken your final panorama.

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Bruno

[Abrimaal]

I think I understand it, although not everything.
Sometimes panoramas (especially taken in HDR - stacks of 3 photos with various exposures) look like this - there are lighter and darker areas in the sky at the stitching edges.
I think it may be a result of taking HDR photos from the hand, every photo is a little moved. (I use don't align stacks).
But the same panorama from the basic photos (the first exposure only, without the stacks) looks a lot better.

 

[Abrimaal]

"Edit message?" option unavailable.
I finally forgot to ask:
When making panoramas from stacks of HDR images
should I use exposure correction or reset all photometric parameters before stitching?
because after exposure correction all images look the same.

[Bruno Postle]

On Sat 27-Feb-2021 at 07:24 -0800, Abrimaal wrote:
>I think I understand it, although not everything.
>Sometimes panoramas (especially taken in HDR - stacks of 3 photos with
>various exposures) look like this - there are lighter and darker areas in
>the sky at the stitching edges.
>I think it may be a result of taking HDR photos from the hand, every photo
>is a little moved. (I use don't align stacks).
>But the same panorama from the basic photos (the first exposure only,
>without the stacks) looks a lot better.

It is difficult to say, this could be vignetting, I would normally
deal with this sort of low frequency pattern by increasing the
number of enblend levels.

>"Edit message?" option unavailable.

Hugin-ptx is a mailing list with a web-interface, you can't edit
your message because it has already been emailed to the subscribers.

>I finally forgot to ask:
>When making panoramas from stacks of HDR images
>should I use exposure correction or reset all photometric parameters before
>stitching?
>because after exposure correction all images look the same.

The Hugin Stitcher tab should do the right thing here: when using
the Exposure Fusion (with enfuse) option, Hugin resets exposure for
all images, so you don't have to; when you are merging HDR stacks
you do want the photos to be aligned for exposure, so Hugin applies
exposure correction.

So in general, you should optimise exposure for photos.

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Bruno