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Old Feb 15, 2010, 3:17 PM   #11
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What is the effect on noise when adding multiple images in HDR ? I would assume that noise is increased in dark areas ?
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Old Feb 15, 2010, 5:05 PM   #12
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This example might explain it. It's from v1.17 of AutoHDR. Input image on the left and processed on the right. The two histograms correspond to the two pictures.



In the original, the histogram has two quite distinct regions corresponding roughly to the dark building and the light sky. By stretching the building histogram upwards and the sky histogram downwards, the two regions eventually overlap so that both the building and the sky have values spanning the entire range.

You definitely can manipulate the histograms - that's how the simple tone mapping functions work as far as I understand. The clever thing that HDR programs do is to work out that it needs to tone-map different parts of the image differently.

I've also re-created the process at a simple level in Paintshop Pro here. This is exactly what HDR software does but at a much finer level than I've done here by hand:
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Old Feb 15, 2010, 5:08 PM   #13
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@amcam - I think noise is just caused by stretching the histogram too far. If your dark area only has 8 levels of dark grey in it, the stretched histogram will still only have 8 levels but they'll be further apart so you can see them as different (or noisy).
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Old Feb 15, 2010, 5:24 PM   #14
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@amcam - I think noise is just caused by stretching the histogram too far. If your dark area only has 8 levels of dark grey in it, the stretched histogram will still only have 8 levels but they'll be further apart so you can see them as different (or noisy).
I don't have the knowledge to do more than make cute noises - But I have noticed that chroma noise kills detail far more than luminense noise. I notice that HDR shots done by any method seem to be eliminating chroma noise, and saturate the image. This in turn seems to allow a greater amount of non-noise creating sharpening...

Dave
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Old Feb 15, 2010, 5:27 PM   #15
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Well, Bynx and Martin, you both give a lot of food for thought... It is all very interesting.

Thanks.

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Old Feb 15, 2010, 6:50 PM   #16
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This is an interesting conversation. Some of my thoughts:

I've never quite thought of Photomatix in quite the terms either Bynx or Martin have proposed, though I've tended to think of it more in the symbols that Bynx used than in histogram terms that Martin used. I'm not so sure the program is as interested in what's around each "spot" (to use his terms). I had always thought that the program chose the "spot" on each layer that was closest to mid-grey, and then because that would be really flat, it adds some extra saturation. That's why it will sometimes take white and make it greyer than it should be. As you change the light smoothing and strength, you change where it lightens and where it darkens, and I think it also adds or subtracts saturation. Perhaps it's a tone version of USM, where the program changes contrast at the edges - which would explain the halos. But I really have no idea how the program works - just that it does, but with some limitations.

As far as altering histograms, anyone who uses curves or levels in either Lightroom, ACR or Photoshop does it. I changed the histogram with the middle, underexposed shot - I lightened the darks far more than I lightened the lights (used Lightroom, but the controls I used are the same in ACR), changing the histogram rather than just moving the original one over to the right.

Up until a couple of days ago I was using an older version of Photomatix that tended to give me a flatter, less contrasty results. I actually liked that because I had the program automatically re-import the file into Lightroom and I added a curves adjustment to add extra contrast, as much or little as I wanted. I've found that the latest version of Photomatix goes about things differently and I'm having more trouble getting used to it - I don't think it's as much trying to even out tones as much as it used to.

Noise may or may not be increased. I've had more trouble with noise in some sunset pictures, but mainly in the lighter areas not in the darker areas. You get a lot more noise if your originals are noisy - it accentuates what's already there. I've done a number of night light pictures that were noiseless (they were shot using ISO 100, a tripod and long shutter speeds, then I used Lightroom to export to Photomatix, which means that LR dealt with any hot pixels before going through the HDR process as I don't think my camera does the black frame noise reduction (is that the right term?) when you use auto bracketing. I know that made a huge difference as I was playing with DPHDR at the same time I took them and ran the raw files through it - there were quite a few hot pixels that DPHDR picked up.
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Old Feb 15, 2010, 7:40 PM   #17
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Quote:
Originally Posted by mtngal View Post
This is an interesting conversation. Some of my thoughts:

...Noise may or may not be increased. I've had more trouble with noise in some sunset pictures, but mainly in the lighter areas not in the darker areas. You get a lot more noise if your originals are noisy - it accentuates what's already there. I've done a number of night light pictures that were noiseless (they were shot using ISO 100, a tripod and long shutter speeds, then I used Lightroom to export to Photomatix, which means that LR dealt with any hot pixels before going through the HDR process as I don't think my camera does the black frame noise reduction (is that the right term?) when you use auto bracketing. I know that made a huge difference as I was playing with DPHDR at the same time I took them and ran the raw files through it - there were quite a few hot pixels that DPHDR picked up.
I just checked on something. I notice that my Photomatix shots are a bit noiser than either my normal processing or pseudo Photoshop HDR's. I just zoomed in on one of them, and while there Is more luminense noise, I don't see Any chroma noise...

Dave
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Old Feb 15, 2010, 8:32 PM   #18
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I would have to dig up my manuals to say for certain, but, as I recall, my HDR software, when using multi-exposures, is creating a layer for each of the exposures, and divides the histogram into sections. The low tones from the overexposed frame, the light tones from the underexposed frame, and the mid tones from the middle exposure are combined to create the final result. The cutoffs for the histogram sections aren't discrete, but have some overlap.
From observation, it appears that a certain amount of unsharp mask is done to each portion individually prior to the combining.
The dark noise is amplified if only using a single exposure, along with the rest of the information. It is more noticeable if the psuedo HDR is done on a full resolution shot than one which has been downsized for web viewing.

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Old Feb 15, 2010, 10:20 PM   #19
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Noise may or may not be increased. I've had more trouble with noise in some sunset pictures, but mainly in the lighter areas not in the darker areas.
I thought that noise was always more evident in dark areas with digital cameras due to a lower signal/noise ratio ?

Where dark areas are not noisy, is there any benefit in using HDR ? The approach that VT describes would minimise noise but wouldn't overlaying images add noise with each layer ? Even tiny misalignment of each image layer would also surely degrade final picture quality ?

Some additions ... I've just found this on reducing HDR noise
http://www.photoacute.com/noise-free-hdr.html

Exposure fusion sounds a great alternative to HDR to avoid noise:
http://digital-photography-school.co...how-do-i-do-it

Last edited by amcam; Feb 15, 2010 at 11:38 PM.
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