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Author Topic: ZERO NOISE technique  (Read 295355 times)
Ray
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« Reply #60 on: July 04, 2007, 11:59:46 PM »
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[attachment=2754:attachment][attachment=2749:attachment][attachment=2750:attachm
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Unfortunately, it doesn't work well at all with the sample raw files provided by Guillermo Lujik. There are nasty highlight artifacts. Granted, the difference is 4 stops, not 3. Adjusting the sliders so that one gets rid of the highlight artifacts also seems to get rid of the reduced noise.
[a href=\"index.php?act=findpost&pid=126439\"][{POST_SNAPBACK}][/a]

Jani,
Jonathan's method seems to be an improvement over other blending techniques I've come across, but it still relies upon post-blending skills in Photoshop.

I struggle with Photoshop techniques. I know I should spend hours and hours pouring over Photoshop manuals, but for some reason I'm reluctant to do so. Is it a fear of becoming a nerd, or is it just I'm a lazy student who wants the glamour but not the hard yakka? I don't know. A bit of both probably.

I mentioned before, in blending images there's often a halo effect around borders between high contrast transitions. I don't have the skills to get rid of these, at least without painstaking hours of work.

I'm thinking that maybe GLuijk's method offers a quick solution to this.

If you've got the patience, I'll go through a processing procedure I used to blend a couple of images that varied by 4 stops, using Jonathan's method. The final result is not perfect, and not to my complete satisfaction.

The following is a scene of a 5 star hotel 'ensuite' in the foothills of Nepal. (5 stars is relative, you understand   ). The cost for one night in this luxurious accommodation was just US$4. That's less than 1$ per star   . I wanted to capture the scene out of the bathroom window, as well as the bathroom itself.

One shot would simply not do. So I brought my tripod into the bathroom and took a series of 3 bracketed shots.

Here's my attempt at blending 2 of those 3 shots, using Jonathan's method.

(1) The initial blend. Jonathan's recommended settings for "This Layer" should be varied according to the circumstances. It might be right for exposures differences of 3 stops, but my exposures differed by 4 stops, consequently my split layer values were not 35/220, as Jonathan suggested. but 28/43, based upon eyeballing, not mathematical calculation.

[attachment=2749:attachment]

Having flattened the image, I then proceeded to use the limited but familiar techniques I normally use to get the image into shape.

First, a curves application to raise the shadows.

[attachment=2751:attachment]

Then a shadows/highlight adjustment.

[attachment=2753:attachment]

Then a levels adjustment.

[attachment=2754:attachment]

After the levels adjustment (ctrl click on RGB channels, reverse selection, layers levels, 80% opacity), the levels histogram looks like this.

[attachment=2757:attachment]

So here is the rather flat image lacking in umph!

[attachment=2758:attachment]

The problem is; how do I get from this rather flat image to the final vision without introducing artifacts around the edges of the window frames?

[attachment=2759:attachment]
« Last Edit: July 05, 2007, 12:11:31 AM by Ray » Logged
dwdallam
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« Reply #61 on: July 05, 2007, 02:57:46 AM »
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I think what a lot of you are missing here is that this technique is not making an image where you actually see the HDR range increased, but reducing noise in the shadow areas, and bringing back detail, like nothing I've seen, like RAW's "lluminance" etc smoothing, or even PS's "Reduce Noise" filter. I may be wrong, but what hit me is the incredibly clean shadows in what appears to be a a typical dynamic ranged image.

As for automation, just wait. I'm quite sure this will become standard in cameras simply because getting rid of noise is like DUH! No noise is good noise, and if you want noise, then simply switch off advanced noise reduction in camera.
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Jonathan Wienke
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« Reply #62 on: July 05, 2007, 03:39:03 AM »
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The problem is; how do I get from this rather flat image to the final vision without introducing artifacts around the edges of the window frames?

Does this work for you, Ray?
[attachment=2761:attachment]
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DiaAzul
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« Reply #63 on: July 05, 2007, 03:42:56 AM »
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I think what a lot of you are missing here is that this technique is not making an image where you actually see the HDR range increased, but reducing noise in the shadow areas, and bringing back detail, like nothing I've seen, like RAW's "lluminance" etc smoothing, or even PS's "Reduce Noise" filter.
[{POST_SNAPBACK}][/a]

I'm not trying to knock what is being discussed but it really isn't new and has been discussed several times e.g.
[a href=\"http://luminous-landscape.com/forum/index.php?showtopic=3581&hl=]Noise Reduction with multiple exposures[/url]

If you seach through the forums there are examples of using HDR to reduce noise as well as image stacking and we have discussed a number of software packages to reduce noise and increase dynamic range on a number of ocassions.

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I mentioned before, in blending images there's often a halo effect around borders between high contrast transitions. I don't have the skills to get rid of these, at least without painstaking hours of work.


If you have halos around high contrast edges that you need to get rid of then I would suggest shooting multiple exposures no more than 2/3 or 1-stop apart. Blending in Photoshop (and I guess any other technique) will mitigate the effect of halos provided you have sufficient exposures from which to extract 'clean' data. Two exposures 4-stops apart doesn't give sufficient information to eliminate all types of artifacts.
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« Reply #64 on: July 05, 2007, 04:31:47 AM »
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I'm not trying to knock what is being discussed but it really isn't new and has been discussed several times e.g.
Noise Reduction with multiple exposures
Yes, this appears to accomplish with a bunch of exposures what this technique manages with two, and at the cost of more than a little bit of extra manual labour. The exception may be the benefits of the image stack program, which appears to introduce additional flexibility.

For those who don't want to follow links twice, here's a direct link to the PDF (1.7 MB).

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If you seach through the forums there are examples of using HDR to reduce noise as well as image stacking and we have discussed a number of software packages to reduce noise and increase dynamic range on a number of ocassions.
From what I've seen and recall, none appear to have the same simplicity as this method.

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If you have halos around high contrast edges that you need to get rid of then I would suggest shooting multiple exposures no more than 2/3 or 1-stop apart.
Why shoot four exposures 2/3 or 1 stop apart, when you can settle for two exposures three or four stops apart?

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Blending in Photoshop (and I guess any other technique) will mitigate the effect of halos provided you have sufficient exposures from which to extract 'clean' data. Two exposures 4-stops apart doesn't give sufficient information to eliminate all types of artifacts.
It seems to work well enough in the examples provided in this thread; I see no artifacts in the images presented to us.

Perhaps more rigorous testing will reveal such artifacts, but so far, Guillermo's method seems better for the sheer simplicity of the procedure.

He's followed the KISS principle.
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Ray
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« Reply #65 on: July 05, 2007, 04:41:59 AM »
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Does this work for you, Ray?
[attachment=2761:attachment]
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Jonathan,
Not quite, but thanks for the attempt. With that amount of light streaming through the window, the eye would pick up a much brighter scene around the vanity basin. Below is what I'm aiming for. But to achieve this, I had to carefully isolate the scene out of the window with the polygonal lasso tool and process the two parts of the image as though they were separate images. The foliage is perhaps a bit too saturated, but this is the sort of effect I want.

However, in a situation like this, if I have to use the lasso tool, I could simply copy & paste the 'window view' from the dark image to the light image. With both methods I have the problem of that transition edge along the window frame, a problem which is not particularly apparent in the jpeg but is definitely there as can be seen in the crop.

[attachment=2762:attachment]  [attachment=2763:attachment]
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Ray
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« Reply #66 on: July 05, 2007, 04:54:46 AM »
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Two exposures 4-stops apart doesn't give sufficient information to eliminate all types of artifacts.
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David,
This is perhaps the appeal of GLuijk's method, when fully developed. Two exposures 4 stops apart should contain all the data one needs.
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Jonathan Wienke
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« Reply #67 on: July 05, 2007, 06:20:36 AM »
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However, in a situation like this, if I have to use the lasso tool, I could simply copy & paste the 'window view' from the dark image to the light image. With both methods I have the problem of that transition edge along the window frame, a problem which is not particularly apparent in the jpeg but is definitely there as can be seen in the crop.

Here's a way to fix your edge blending problem:

1. Have the window view and interior in separate layers, with window view layer on top.

2. Create a layer mask for the window view layer.

3. Assign the original window view exposure (where the room view is very dark) to the layer mask.

4. Do a level adjustment on the layer mask to make it straight black and white with no intermediate values, so that the window view is 100% white and the room view is 100% black. Depending on image content, you may need to do a bit of paintbrushing to eliminate stray specks where they don't belong. The Dust & Scratches filter can be handy for this, but must be used very carefully.

5. Do a 2-pixel Gaussian blur on the layer mask to make the transition soft and fuzzy. The edge will look horrible, but don't panic, there's a method to the madness.

6. Do another Levels adjustment on the layer mask. By adjusting the source white and black point sliders, you will be able to move the outer and inner edges of the blend transition zone independently anywhere within the 2-pixel blur zone, which will avoid matte lines and make the blend as seamless and natural-looking as possible.

Whenever you're compositing images, you always should have a bit of fuzziness at the edge where one layer transitions to another, or you'll either have matte lines or an artificial "cut-out" look to the edge. This technique makes it easy to get natural-looking blends along composited edges.
« Last Edit: July 05, 2007, 06:27:59 AM by Jonathan Wienke » Logged

Ray
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« Reply #68 on: July 05, 2007, 08:06:28 AM »
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Whenever you're compositing images, you always should have a bit of fuzziness at the edge where one layer transitions to another, or you'll either have matte lines or an artificial "cut-out" look to the edge. This technique makes it easy to get natural-looking blends along composited edges.

Yes, I realise this. It's really an issue of how much stuffing around one needs to do to achieve the right balance which looks natural. However, I'll try to go through those procedures you've outlined tomorrow with a clear head.

I noticed that the CS3 demo version had a feature whereby one can enlarge or diminish a selection by a specified number of pixels, as well as specifying a degree of feathering. That could be useful with this particular image.

Out of curiosity, I loaded the 3 RAW bracketed images of this scene into CS2's HDR. The results are at least equally useful. There are many ways to get a desired result. I used a bit of dodge and burn along the window edges. It's probably not an ideal image to demonstrate the halo problem because I see signs of clear silicon sealant that have smeared the edges of the window panes   .

[attachment=2764:attachment]
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Jonathan Wienke
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« Reply #69 on: July 05, 2007, 01:13:18 PM »
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Yes, I realise this. It's really an issue of how much stuffing around one needs to do to achieve the right balance which looks natural. However, I'll try to go through those procedures you've outlined tomorrow with a clear head.

I noticed that the CS3 demo version had a feature whereby one can enlarge or diminish a selection by a specified number of pixels, as well as specifying a degree of feathering. That could be useful with this particular image.

The advantage to my technique is that it allows one to expand or contract the selection, as well as adjust feathering amount of the blend, with a real-time preview.

I noticed the sealant on the window as well. The small upper section of window visible seems to be free of it though.

On a different note, I spent a few hours playing around with some bracketed triples 3 stops apart, experimenting with various blending techniques. I discovered that using all 3 images works best. Blending the +3 and -3 shots only leaves somewhat of a quality gap where the highlights and shadows look good, but the midtones are noisy, and the transition between the highlight shot and the shadow shot is uglier. The comparison crops have all had the same sharpening:

2 frame blend: (+3 and -3 only)
[attachment=2766:attachment]

3 frame blend: (+3, 0, and -3)
[attachment=2767:attachment]

As you can see, the highlights and shadows look equally good (as they should, coming from the same frames), but the midtones of the 2-frame blend are much noisier.

The attached ZIP file has the layer styles I used to blend the frames. For the 2-frame blend, the -3 frame was on the bottom as background, and the +3 frame was on top with the "HDR Shadow Layer Blend" style applied. For the 3-frame blend, the 0 frame was background, then the -3 frame with the "HDR Highlight Layer Blend (3 Frame)" style applied, then the +3 frame with the "HDR Shadow Layer Blend (3 Frame)" style applied. After flattening both, I picked a crop area that had samples of the entire tonal range, cropped, and applied equal sharpening to both.

To use the blend styles, simply unzip and then load the extracted file from the Styles palette in PS. Process all 3 RAWs exactly the same except for the exposure setting. The 0 file is your best guide for things like setting white balance, lens corrections, etc.; try to adjust exposure of the 0 file so that shadows and highlights are about equally clipped. If you only want to improve tonality within a single-frame exposure range, be strict about setting the exposure of the -3 file exactly 3 stops above the 0 file setting, and the +3 file exactly 3 stops below. But if you want to include more DR in the frame (which will of course make it look flatter after blending and require more curves and local contrast boosting), adjust the -3 file for the best highlight appearance (no or only light specular clipping) and the +3 file for best shadow appearance (no or only light specular shadow clipping). Then stack the layers 0 (background), +3 / HDR Highlight Layer Blend (3 Frame), -3 HDR Shadow Layer Blend (3 Frame).

Enjoy!
« Last Edit: July 05, 2007, 01:29:26 PM by Jonathan Wienke » Logged

Alaska
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« Reply #70 on: July 05, 2007, 04:26:41 PM »
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The attached ZIP file has the layer styles

The ZIP file downloads as "index.php".  Did get it to work by renaming to "index.zip".  Then it would unzip to HDR Blend States ASL.

Thanks for the great hints......

Jim
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Guillermo Luijk
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« Reply #71 on: July 06, 2007, 04:15:34 PM »
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Yeahh! we did it! a mate of me and me have modified DCRAW's source code to implement this technique as an extra option. And it works great!!! it has some advantages over the little test program I provided here, as it works straight from the RAW files. I mean it does the image blending before the RAW developing stage and that has many important advantages:

1. Noise free virtual RAW: Bayer interpolation is done over a 16-bit "virtual" RAW where noise has already been minimised. This way we avoid noise propagation into interpolated cells.
2. Process previous to white balance: WB rescales all channels, so the threshold value for the blending algorithm had to be chosen for the most severely corrected channel. Now an optimal threshold is chosen for the three channels at a time as WB has not been applied yet.
3. Only one RAW development: thanks to the internal virtual RAW data generated, no matter how many files are to be blended, only one RAW developing will take place incresing calculation speed.

For those familiar to DCRAW the miX option will be something like:
dcraw -X [threshold] [exposure correction] ... *.cr2 for instance
It will take all indicated RAW files, calculate their relative exposition no matter which it is, and blend them into an optimum noise free image with the same appearance (contrast and tone) as the original but with expanded dynamic range in the shadows and zero noise.
As people are not used to DCRAW perhaps I will write a simple GUI for this tool.


Hi Jonathan Wienke, just to say that I never stated that reducing noise through multiexposure was a new idea; I am sorry if it looked like that. In particular in my website I textually say: "Las ideas descritas hasta ahora no son nuevas, la novedad consiste en aplicarlas con el fin de obtener una reducción de ruido radical y de forma automatizada (...)", that means "The ideas described here [referring to the noise reduction process through overexposition] are not new, the new thing consists of applying them with the goal of achieving a radical noise reduction in an automated way".

If you can get the same noise reduction as this modifed DCRAW with PS, at the same speed (about 10s to process 3 shots starting from their RAW files), and with the same ease of use (one button click), just let me know. Otherwise you are invited to try our tool.
Apart from the advantages of RAW blending commented at the begining, a progressive blending technique as the one you propose in PS is not optimal (nor needed) compared to a more simple but effective pixel selection algorithm for two reasons: first your noise reduction is not maximised as you are mixing noisy pixels with noise-free pixels, while I simply discard the first ones. Secondly, even minimal image overlap errors (less than 1 pixel) would lead to a loss of sharpness due to averaging in your case.


I did a test yesterday over a high contrast scene (about 13 f-stops of real dynamic range) and achieved a completely free of noise histogram with texture recovery in the darkest shadows:
(please note these histograms are logaritmic plots where each vertical division corresponds to 2 image f-stops. The original hardly achieved to represent properly 8 f-stops due mainly to noise and quantization. The blended version clearly increases the histogram precision up to 13 f-stops):




[span style=\'font-size:8pt;line-height:100%\'](what can be seen on the floor is not noise but the tiles texture)[/span]


Please find here the RAW files of my two test images for anyone wanting to try alternative techniques:

Couch:
http://www.guillermoluijk.com/download/sub.cr2
http://www.guillermoluijk.com/download/sobre.cr2

Blue room:
http://www.guillermoluijk.com/download/foto1.cr2
http://www.guillermoluijk.com/download/foto2.cr2
http://www.guillermoluijk.com/download/foto3.cr2
« Last Edit: July 07, 2007, 02:18:45 AM by GLuijk » Logged

pcrov
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« Reply #72 on: July 06, 2007, 05:11:29 PM »
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That's fantastic!

I'm not familiar with DCRAW but don't mind working on the command line. Is there somewhere I can download your version? Will this be able to output a DNG file?
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Guillermo Luijk
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« Reply #73 on: July 06, 2007, 05:23:18 PM »
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I'm not familiar with DCRAW but don't mind working on the command line. Is there somewhere I can download your version? Will this be able to output a DNG file?[a href=\"index.php?act=findpost&pid=126905\"][{POST_SNAPBACK}][/a]

it's still not ready, coming soon.

we were thinking about that, to produce a 16-bit DNG as an output. Would be orgasmic (lol). But need to know a lot about RAW file formats. Perhaps with assistance from David Coffin...
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« Reply #74 on: July 06, 2007, 05:26:32 PM »
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we were thinking about that, to produce a 16-bit DNG as an output. Would be orgasmic (lol). But need to know a lot about RAW file formats. Perhaps with assistance from David Coffin...
[a href=\"index.php?act=findpost&pid=126909\"][{POST_SNAPBACK}][/a]

Well DNG would be a quite welcomed feature!
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« Reply #75 on: July 07, 2007, 03:40:57 AM »
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Well DNG would be a quite welcomed feature!
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I second that!
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« Reply #76 on: July 07, 2007, 04:48:38 AM »
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Hi Jonathan Wienke, just to say that I never stated that reducing noise through multiexposure was a new idea; I am sorry if it looked like that. In particular in my website I textually say: "Las ideas descritas hasta ahora no son nuevas, la novedad consiste en aplicarlas con el fin de obtener una reducción de ruido radical y de forma automatizada (...)", that means "The ideas described here [referring to the noise reduction process through overexposition] are not new, the new thing consists of applying them with the goal of achieving a radical noise reduction in an automated way".
My Spanish is very limited, so I wasn't able to read your web site article. My apologies. The first program you posted about breaks no new ground, and offers little or no advantage over blending techniques that have been available for years, including the method I posted about using layer blend styles.

On the other hand, your DCRaw modification is very interesting and is something new, DR blending the linear RAW data prior to other processing. This approach is much better than blending after gamma adjustment and tone curves have been applied.

If you can create a tool with a GUI that can take 3 RAWs exposed at 3-stop intervals, blend them, and output a blended 16-bit DNG that ACR or any other DNG-aware program can process the same as the original RAW, I'd happily buy it. It would need to meet the following criteria:

1. The highlights would need to be set so that the brightest value in the darkest image is scaled automatically to the maximum usable value in the output file without clipping.

2. The output DNG must process identically to the original RAW. This means that the normal ACR color processing must work identically to an original RAW. I've color calibrated ACR to my cameras, and would not want to get different color when processing a blended DNG than when processing a single RAW. ACR would have to recognize the camera model and serial number from the DNG file for this to work. The complete DNG file format specification is available from Adobe, so this shouldn't be too difficult. The hardest part would be figuring out what shutter speed to put in the metadata when the file is derived from several exposures with different shutter speeds.

3. Some means to manually adjust registration between the RAWs would be necessary, expecially if it could be done in 1/2-pixel increments. This would require that Bayer interpolation be done prior to blending. The output file would have to be a linear-RGB DNG, but ACR's normal color processing would not be affected. Limiting output to 16-bit linear RGB would greatly simplify your work; you'd only have one output format to worry about. It is just tagged with the appropriate camera model, serial number, and other metadata. DCRaw can handle the plethora of input formats, so you wouldn't have to re-invent the wheel for that side.

If the Bayer interpolation routine was modified so that it took all source images into account simultaneously, slight mis-registration would actually be an advantage, because there would be more than one color channel at each output pixel site. Imagine that when shooting, the +3 exposure was shifted 1 pixel vertically relative to the 0 exposure, and the -3 exposure was shifted 1 pixel horizontally relative to the 0 exposure. After registering the RAW data, the Bayer interpolation now has 2 color channels per pixel to use, either red and green, or green and blue. The exposure scaling would have to be accounted for, but increased color accuracy could be achieved. Some medium format digital backs do this (but not at different exposure levels) to improve color accuracy; 3 exposures are taken, whith the sensor being moved 1 pixel vertically or horizontally between frames.

You can download the complete DNG specification from a link at the bottom of this page.

Edit: The Bayer interpolation modification wouldn't really be necessary, the color accuracy improvement happens already when the different exposures are blended together. Never mind.
« Last Edit: July 07, 2007, 05:05:23 AM by Jonathan Wienke » Logged

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« Reply #77 on: July 07, 2007, 07:15:55 AM »
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If the Bayer interpolation routine was modified so that it took all source images into account simultaneously, slight mis-registration would actually be an advantage, because there would be more than one color channel at each output pixel site. Imagine that when shooting, the +3 exposure was shifted 1 pixel vertically relative to the 0 exposure, and the -3 exposure was shifted 1 pixel horizontally relative to the 0 exposure. After registering the RAW data, the Bayer interpolation now has 2 color channels per pixel to use, either red and green, or green and blue. The exposure scaling would have to be accounted for, but increased color accuracy could be achieved. Some medium format digital backs do this (but not at different exposure levels) to improve color accuracy; 3 exposures are taken, whith the sensor being moved 1 pixel vertically or horizontally between frames.

You can download the complete DNG specification from a link at the bottom of this page.

Edit: The Bayer interpolation modification wouldn't really be necessary, the color accuracy improvement happens already when the different exposures are blended together. Never mind.
[{POST_SNAPBACK}][/a]

The program I already posted blends the images prior to any gamma correction, as DCRAW is a linear RAW converter (if used in -4 mode, and I did). The improvement of this new DCRAW version is that the blending is applied even before the white balance, which improves even more the accuracy of the threshold set and in addition to this saves some RAW developings. We can say it's more elegant.
This is a comparision between the results of:
1. My previous program and a conservative threshold decision value: still noise on some areas (wall).
2. My previous program and an agressive threshold decision value: noise is almost gone but incorrect colour pixels appear (couch).
3. Modified DCRAW with a conservative threshold decision value: noise is almost gone and more regularly spreaded, and no colour artifacts.




We don't sell software. All we do if for free and for fun. It's not likely that we will achieve the DNG output as it's an effort beyond our intentions, and we love DCRAW! so we trust it. DCRAW is a really VERY good RAW developer itself. But I will check your links to find out, thank you.


"1. The highlights would need to be set so that the brightest value in the darkest image is scaled automatically to the maximum usable value in the output file without clipping."

This requirement would be fine in theory. In practice it is not so much, and the reason is that the least exposed imaged provided to the program is taken as a reference for the output. The improvement of scaling it so it fits the maximum (65535 level) is not worth as almost ALL valid images have some pixels close to blow or directly burnt (this is the usual case). You can use this program of mine: [a href=\"http://www.guillermoluijk.com/software/histogrammar/index.htm]Histogrammar[/url] that plots 16-bit histograms to check this by yourself on any of your images. If you wait a couple of days, I have developed a new version of Histogrammar which reads directly image files (the one currently in the link needs some mess with a PS plugin and translates the image into a text file. horrible but I didn't have the libraries needed to read 16-bit images) and even plots logaritmic histograms like the one I showed before.
However we will introduce an exposure correction parameter into this DCRAW, so that: dcraw -X 4.0 ... *.cr2 for instance will correct by 2 f-stops up (2^2=4.0) the resulting image avoiding level agregation and mathematical rounding errors if that would be done afterwards.

Moreover the white balance can be very agressive. A scaling of more than one complete f-stop in some channel is very usual and this burns pixels; but few of them. So it is strange to find an image that can strictly be exposed up as most of them have something burnt (and if they don't have, is because were heavily underexposed).
The important thing is to have a good exposure gap between the least and the most exposed images. This really fulls with detail all the shadows thanks to the 12-bit to 16-bit conversion. These images can handle really severe toning curves before showing any banding or posterization.
Our program calculates itself the relative exposures between images without looking at the metadata; and handles N images with N from 2 to what your memory can store, taking always for each individual pixel the best possible value from the whole set of images provided. In fact, this relative exposure calculation is more accurate than using the EV values set in the camera since lighting conditions could vary slightly during the shots (for instance a cloud passing by).

Thanks for taking your time, we will let you know when this DCRAW is ready.
« Last Edit: July 07, 2007, 07:53:46 AM by GLuijk » Logged

Jonathan Wienke
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« Reply #78 on: July 07, 2007, 03:48:56 PM »
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I've been working with sets of 3 images exposed 3 stops apart, +3, 0 and -3. The -3 image usually doesn't have any clipped pixels unless there is a light source in the image, like a bare light bulb or something really shiny with bright specular highlights. Blending these images together with exposure set to -2, 0, and +2 gives a slightly flat-looking initial result, but which still responds well to curves and local contrast enhancement so that you get nice smooth transitions to clipped highlights and shadows, and end up with about 2 stops more DR in the image as well as practically no noise throughout the tonal range.

So I'd like to be able to boost that -3 image to max values below clipping, and scale everything else below that. I'm not talking about changing the WB settings in the RAW, scale all channels by the same amount so the most exposed channel maxes out at 65535 and everything else scales proportionately. When you add 6 stops to the 7 stops or so that most cameras can capture well in a single exposure, using all of the 16-bit linear scale is not a bad idea, especially given that the blue channel can be 2 stops or more below the red channel in incandescent and similar lighting. That means the deepest shadows in the blue channel can be 15 stops below the highlights in the red channel, and an unused stop of highlight room can mean the difference between whether that shadow detail is processed meaningfully or gets posterized into oblivion.
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Guillermo Luijk
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« Reply #79 on: July 07, 2007, 04:40:06 PM »
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oh I understand now. Your way to proceed is to bracket a couple of stops below and above the "correct" exposure. In my opinion bracketing underexposed as a rule is not necesarry. Any underexposed shot (and -3 is VERY underexposed) doesn't provide additional clean information to the 0 and +3 shots.

My concept here is slightly different: take one shot making sure that you capture all highlights, but not underexposing at all, just make sure you don't blow information (a RGB splitted camera histogram is good enough to check this). This is the most important shot of all and after it you can forget about any additional underexposed shot.
If the camera has punctual measuring (don't know if this is the right term in English), the classical trick is measure the highlights and overexpose that measure by over +2EV according to the camera particular metering behaviour.
Afterwards comes the funny part: additional shots with RADICAL overexposures. 2, 3, 4 f-stops... The only care that has to be taken is not exceed a 4 f-stops gap between every two shots or some areas could start to suffer quantization problems or at least not reach the degree of tonal richness we are expecting from the whole image.

For the example of the couch I used:
0 EV, +4 EV
For the example of the blue room I used:
0 EV, +3 EV, +6 EV

Taking this last example I simply made sure not to burn pixels in the 0 EV (in fact I have just analysed and got real {696;501;344} RGB burnt levels which is absolutely nothing in a 8 Mpx sensor. Those few pixels surely were on the ceiling lightbulb.
And I didn't use 0, +4, +8 because a crowd came to the scene and I could not keep on shooting; but 0,+4,+8 would have been desirable for a better recovery in the 13th stop (see histogram).

It works.

In your case, I am sure with -3,0,+3 you will achieve a fantastic noise reduction and DR enhancement in the shadows, but I am not so sure if the DR  in your image clearly represents the DR found in the scene. I mean: my shots are blended and developed in linear mode, and then obviously comes the conversion to the colour profile (sRGB, AdobeRGB,...) that will introduce a non linear gamma correction that is unavoidable. But I feel comfortable to know that right the previous step to this is pure linear data where 1 stop of DR exactly corresponds to 1 stop of DR in the scene since the sensor is workig in its linear range. I think you have more control this way on what's going on with your image than in a PS blending.
« Last Edit: July 07, 2007, 04:44:32 PM by GLuijk » Logged

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