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About the brightness of the final image in Zerene

 
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soldevilla



Joined: 16 Dec 2010
Posts: 550
Location: Barcelona, more or less

PostPosted: Mon Nov 14, 2016 2:54 pm    Post subject: About the brightness of the final image in Zerene Reply with quote

I have a question with Zerene, and I have not found an answer looking in the forum. Although I am very inept looking for things ...
I see that my stacked images are about 20% brighter than individual images. That produces that although I control the histogram in my individual images, the final image has areas burned or very close to the absolute white. I also check that when I am retouching the final image, covering the dark areas that the brightness has invaded, the contrast I get is much lower than the individual image.

I attach an image of the individual image and the stack showing the increase in brightness. I do not know how to put the link to the image in Google's albums (before, PICASA). I am sorry...



Last edited by soldevilla on Mon Nov 14, 2016 3:53 pm; edited 1 time in total
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Chris S.
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PostPosted: Mon Nov 14, 2016 3:33 pm    Post subject: Reply with quote

Of course Rik is the best person to answer this question, but I'll take a shot.

If you go into Zerene Stacker>Options>Preferences>Image Saving, is there a check in the box labeled "Retain extended dynamic range"? If so, uncheck it and run the stack again. Does this make your problem go away?

--Chris
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Lou Jost



Joined: 04 Sep 2015
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PostPosted: Mon Nov 14, 2016 4:27 pm    Post subject: Reply with quote

Is this PMax or DMap? PMax is known to generally increase contrast.
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rjlittlefield
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PostPosted: Mon Nov 14, 2016 8:28 pm    Post subject: Reply with quote

There is a FAQ on the Zerene Stacker website that addresses the general issue.

http://www.zerenesystems.com/cms/stacker/docs/faqlist#my_colors_changed_a_little_why_is_that
Quote:
My colors changed a little. Why is that?

There are three reasons that output images can have different colors from the input: 1) brightness adjustment, 2) PMax, and 3) “Retain extended dynamic range” when saving. “Brightness adjustment” refers to Zerene Stacker's attempt to correct for uneven exposure between various input images. That feature is turned on by default, but you can turn it off by un-checking Brightness at Options > Preferences > Alignment. “PMax” refers to the PMax stacking method, which often makes slight changes in brightness, contrast, and saturation as a side effect of doing its focus stacking. This behavior is an unavoidable side effect of PMax and should be considered as one of the tradeoffs of PMax versus DMap. “Retain extended dynamic range” when saving causes the range of internal pixel values to be compressed if necessary to fit within the 0-255 range of image files. Internally the range can exceed 0-255 as a result of PMax, brightness adjustment, or even just pixel interpolation during alignment.

Color/brightness/contrast changes can be completely avoided by using the DMap stacking method, with Brightness adjustment turned off at Options > Preferences > Alignment, and “Retain extended dynamic range” turned off at Options > Preferences > Image Saving or in the file save dialog.

From the crops that are shown, my guess is that both PMax and brightness adjustment are in play here. Often those two have opposite effects: brightness adjustment tends to reduce contrast because the starting end is mostly out of focus and all the other frames get adjusted to match, but PMax tends to increase contrast because that's just the way the math works.

Since soldevilla is seeing contrast increase, not decrease, I doubt that "Retain extended dynamic range" is involved. That never increases contrast, only reduces it.

To maintain the original histogram, there's really no substitute for turning off brightness adjustment and using DMap.

Of course that introduces other tradeoffs because of depth map haloing issues.

I don't know a perfect solution.

--Rik
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soldevilla



Joined: 16 Dec 2010
Posts: 550
Location: Barcelona, more or less

PostPosted: Wed Nov 16, 2016 3:14 pm    Post subject: Reply with quote

Thank you, Rik.

Yes, I stack with Pmax, I do not get good results with Dmax. The other parameters I have as you suggest me. All right. It's time to start learning again, using Dmax ... maybe also stacking with Pmax to retouch with the Pmax image the zones with banding in the Dmax image. When I get a new site for my photos I will put my tests here.

Thanks again.
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Lou Jost



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PostPosted: Wed Nov 16, 2016 3:21 pm    Post subject: Reply with quote

Quote:
...stacking with Pmax to retouch with the Pmax image the zones with banding in the Dmax image


That's what many of us do.
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Ultima_Gaina



Joined: 28 Jan 2017
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PostPosted: Thu May 18, 2017 10:56 am    Post subject: Re: About the brightness of the final image in Zerene Reply with quote

soldevilla wrote:
I have a question with Zerene, and I have not found an answer looking in the forum. Although I am very inept looking for things ...
I see that my stacked images are about 20% brighter than individual images. That produces that although I control the histogram in my individual images, the final image has areas burned or very close to the absolute white. I also check that when I am retouching the final image, covering the dark areas that the brightness has invaded, the contrast I get is much lower than the individual image.

I attach an image of the individual image and the stack showing the increase in brightness. I do not know how to put the link to the image in Google's albums (before, PICASA). I am sorry...



Here is a late reply. Even if it is late, it might help future "searchers".

What you experience is normal, in my opinion. The sharp, tiny in focus areas (e.g. bristles) might be close to pure white, with burnt details. However, the surrounded blurred areas will get those whites attenuated making your histogram look OK, in average.

However, the stacked result will have all of those very white details combined together and revealed, in a much brighter image.

This is why, when checking my histogram for individual shots, I make sure I underexpose, somewhere at 80% max. Otherwise, I'll end up burning the highlights beyond recovery.
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rjlittlefield
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PostPosted: Thu May 18, 2017 11:44 am    Post subject: Reply with quote

If your main problem is that highlights are getting blown out, then you should consider using "Retain extended dynamic range". The option is provided specifically to address that issue.

As written on the Zerene Stacker FAQs page:

Quote:
What does "Retain extended dynamic range" mean?

As background, you need to know that the PMax stacking method often causes contrast to increase, pushing darks darker and brights brighter. If your source images are already high contrast, then the increase can internally push pixel values to “darker than black” or “brighter than white”. Such values cannot be saved in ordinary image files. By default, Zerene Stacker clips these pixels to exactly black or white when the file is saved, thus throwing away some information you might like to keep. Placing a checkmark on “Retain extended dynamic range” essentially does a “levels adjustment” that reduces contrast and possibly brightens the image, exactly enough to occupy the full range of allowed pixel values, 0-255 in an 8-bit image. This preserves all the computed pixel values so that you can apply your own levels or curves adjustment in Photoshop or any similar tool to get whatever appearance you like best. When using “Retain extended dynamic range”, it's also a good idea to use 16-bit TIFF output, so as to preserve good gradation that might be lost if the extra dynamic range were compressed into 8 bits.


You might also want to consider selecting Options > Preferences > PMax Settings > Retain UDR Image. That one is explained as:

Quote:
What is the PMax setting "Retain UDR Image"?

UDR stands for Unrestricted Dynamic Range. As discussed in the previous FAQ, the PMax stacking method often causes contrast to increase, pushing darks darker and brights brighter. To compensate for this effect, the last step of the PMax method is a type of HDR (High Dynamic Range) local adjustment of contrast & brightness that preserves detail in overly light regions while pushing those regions back into standard bounds. By selecting the option to “Retain UDR Image”, you can preserve the image as it stood just before the HDR step. This allows you to run your own HDR methods outside Zerene Stacker, say in Photoshop. When you select “Retain UDR Image”, each run of PMax produces two Output Images, one of them with “UDR” in its name. To finish processing that image outside Zerene Stacker, you should save it as 16-bit TIFF using “Retain extended dynamic range” on the save.

--Rik
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mawyatt



Joined: 22 Aug 2013
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Location: Clearwater

PostPosted: Thu May 18, 2017 12:25 pm    Post subject: Reply with quote

Rik,

If I understand correctly, using 16 bit TIFF input images and selecting Retain Extended DR, this will place the final TIFF image pixel value within 16 bit scaling, thus whites at max of 65535 and blacks min at 0? While selecting Retain UDR will produce and image pixel possibly beyond 65535 and below 0 (negative value), unless Retain EDR is selected?

Mike
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rjlittlefield
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PostPosted: Thu May 18, 2017 2:48 pm    Post subject: Reply with quote

mawyatt wrote:
If I understand correctly...?

Yes, your description sounds correct to me.

--Rik
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Ultima_Gaina



Joined: 28 Jan 2017
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PostPosted: Fri May 19, 2017 7:48 am    Post subject: Reply with quote

The UDR option looks interesting, since my goal is to do most of processing needed on the RAW file and only the minimum required on TIFF files. Thanks, Rik!

Will the UDR option hopefully reduce some of the noise accumulated on the PMax file? I will test it this weekend.

I just want to clarify the point I initially tried to make. I am referring at burnt highlights on the original RAW file, that might go unnoticed when checking a live-view histogram, because of negligible contribution the in-focus band has on the overall image brightness.

In other words, in-camera histogram is heavily biased towards the not so representative, out-of-focus areas.

The information in these burnt highlights is lost forever and no post-processing trick will bring it back.
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rjlittlefield
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PostPosted: Fri May 19, 2017 11:33 am    Post subject: Reply with quote

Ultima_Gaina wrote:
Will the UDR option hopefully reduce some of the noise accumulated on the PMax file?

I do not expect you will see much difference in noise.

For pixels that are in about the bottom 90% of the histogram (that is, brightness below about 230 on a 0-255 scale), the UDR and regular PMax images will be identical. For pixels that are in the upper 10% of the histogram (above 230) there may be a slight reduction in noise, but typically these are not the regions where noise is most obvious.

--Rik
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