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Author Topic: HDR using one shot  (Read 16245 times)

Marc Verschueren

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HDR using one shot
« on: August 16, 2011, 07:22:50 am »

I have a question about HDR using only one shot. Normally you would use bracketing on the camera to shoot say tree images which are then blended together afterwards on the computer. But could you do it with one shot if you f.i. use lightroom and shoot raw?

Could you make tree copies within lightroom of the picture; then adapt the f-stop manually for the first copy to say -1 and adapt the thirth copy to +1 and then use those tree images to blend together?

Would it be the same as tree bracketted images coming out of the camera?
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Chairman Bill

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Re: HDR using one shot
« Reply #1 on: August 16, 2011, 07:32:22 am »

I've heard people talk about single exposure HDR, and most of it is nonsense. Use of ND grad filters extends the dynamic range within a single exposure, but simply taking a straightforward exposure & extracting whatever detail one can, doesn't amount to any extension of the dynamic range, so isn't HDR. Blending over & under-exposed copies from the same exposure isn't HDR.

PeterAit

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Re: HDR using one shot
« Reply #2 on: August 16, 2011, 08:25:51 am »

I think Bill is right. The whole basis of multiple exposure HDR is that the over- and underexposed images will contain image information that the "correctly" exposed image does not. For example, the underexposed image might contain highlight details that are blown out in the others, and the overexposed image will contain shadow detail that has gone to black in the other images. Your single image technique does not add any additional image information, and whatever information there is in that one image should be accessible using other techniques, such as fiddling with the curves.
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Bart_van_der_Wolf

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Re: HDR using one shot
« Reply #3 on: August 16, 2011, 08:43:40 am »

Would it be the same as tree bracketted images coming out of the camera?

The previous posters are correct, the purpose of HDR shooting is to increase the quality of the information in an image file. By manipulating a single file, no real data is added to manipulate, only manipulated data ...

Cheers,
Bart
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Guillermo Luijk

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Re: HDR using one shot
« Reply #4 on: August 16, 2011, 10:19:01 am »

I have a question about HDR using only one shot. Normally you would use bracketing on the camera to shoot say tree images which are then blended together afterwards on the computer. But could you do it with one shot if you f.i. use lightroom and shoot raw?

You can, the only difference between bracketing or not bracketing is the amount of visible noise in the shadows (assuming you preserve the highlights in the single shot and in at least one of the shots in the bracketing). The point is that with a regular camera and a high contrast scene, noise in a single shot will usually be so high that shadow textures will become invisible.

But conceptually there is no need to bracket for HDR tone mapping. If noise is not a problem in your single shot, make as many copies of it at different exposures as you will, and feed them into your favourite monstruous mapping application.

Regards
 
« Last Edit: August 16, 2011, 10:21:49 am by Guillermo Luijk »
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Marc Verschueren

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Re: HDR using one shot
« Reply #5 on: August 16, 2011, 10:41:07 am »

Ok! Many thanks to you all for this valuable information. I understand it now: it is the lack of information in the original shot that's killing my idea of being able to do HDR with copies of this on shot.

Now that you've all explained it is clear to me!

Thanks again.
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RFPhotography

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Re: HDR using one shot
« Reply #6 on: August 16, 2011, 11:00:35 am »

You can, the only difference between bracketing or not bracketing is the amount of visible noise in the shadows (assuming you preserve the highlights in the single shot and in at least one of the shots in the bracketing). The point is that with a regular camera and a high contrast scene, noise in a single shot will usually be so high that shadow textures will become invisible.

But conceptually there is no need to bracket for HDR tone mapping. If noise is not a problem in your single shot, make as many copies of it at different exposures as you will, and feed them into your favourite monstruous mapping application.

Regards
 

Conceptually there's no need to bracket to tonemap an image; that is true.  But you're not getting HDR by doing so.  Multi-processing a single file (irrespective of noise) isn't adding any brightness range.  That's really the point of HDR (or other extended exposure methods) so technically to get HDR (or extended exposure) you do need to bracket. 

If you're going to tonemap a single image file, just toss the single image into the tonemap cooker.  Creating +/- brackets from the single file won't make a difference.
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Guillermo Luijk

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Re: HDR using one shot
« Reply #7 on: August 16, 2011, 11:40:59 am »

That's really the point of HDR (or other extended exposure methods) so technically to get HDR (or extended exposure) you do need to bracket.

That is your photographic-oriented opinion Bob. But HDR algorithms were born in the computer rendered graphics world, as a tool to display big contrast gaps in low dynamic range output devices through global contrast reduction plus local contrast enhancement methods. Obviously it is nonsense to talk about any bracket in computer rendered graphics since there is only one 'picture' to tone map and render. HDR formats used in digital photography are single image formats as well, they simply put up together the best parts (shadows, highlights and mid tones) of the bracketing series.

So bracketing is just a consequence of the limited dynamic range of digital cameras because of sensor noise. As long as sensor designs are less noisy, bracketing will become unnecesary. But HDR tone mapping techniques will have exactly the same validity they have today, we'll simply need a single capture (with all the advantages this means: quicker shooting, no alignment nor ghosting issues, storage and CPU savings,...).

Regards
« Last Edit: August 16, 2011, 12:30:48 pm by Guillermo Luijk »
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RFPhotography

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Re: HDR using one shot
« Reply #8 on: August 16, 2011, 01:26:10 pm »

I understand what the origin of HDR is/was.  But this is a photography forum.  The vast majority of the people here are photographers.  The vast majority using HDR will be doing so for photography related reasons, not CG.  I'd venture that very few are doing 3D CG rendering.  So in that context, I think my comments are valid.

Lower noise should be a result of a properly bracketed HDR merge, sure.  But if the scene DR fits within the sensor DR then there's no technical reason to use HDR tonemapping (again, speaking from a photography standpoint).  As sensors are manufactured with even broader DR HDR, as a technical method of extending brightness range, will be obviated.  That doesn't take away from an artistic reason to use HDR tonemapping tools.  But to say the validity of using HDR tonemapping methods where not necessary if the sensor can capture the entire scene DR will be no different I don't think is accurate. 

Your definition of 'single image' as it relates to bracketed photographic HDR is somewhat specious.
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ErikKaffehr

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Re: HDR using one shot
« Reply #9 on: August 16, 2011, 02:49:12 pm »

Hi,

I'm doing some studies on a Stouffer wedge with density range of 4.1 right now. My impression is that Lightroom cannot extract all dynamic info from a raw image without either:

- Using extreme amount of fill light
- Using the built in graduated filter

In my experience, doing two separate developments and combine in Photoshop using some decent masking technique is actually a very good idea. A description of one such a method is described here:

http://echophoto.dnsalias.net/ekr/index.php/photoarticles/46-fixing-sky-with-luminosity-mask

Best regards
Erik

Best regards
Erik

I've heard people talk about single exposure HDR, and most of it is nonsense. Use of ND grad filters extends the dynamic range within a single exposure, but simply taking a straightforward exposure & extracting whatever detail one can, doesn't amount to any extension of the dynamic range, so isn't HDR. Blending over & under-exposed copies from the same exposure isn't HDR.
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aduke

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Re: HDR using one shot
« Reply #10 on: August 16, 2011, 03:03:38 pm »

I have used the OT technique with a few ETTR images. In some cases, the sky is very bright, but not blown. I've been unable to darken the sky and separate the tones in the sky, without unduly darkening the rest of the image, WITHIN Lightroom alone. In these cases, I develop the image twice, once with normal tones in the non-sky portion and letting the sky go where it is, and the other with the exposure lowered and contrast raised, as well as other Lightroom adjustments.

I prefer to use LR over PS for this type of work.

Alan

Edit: I would not call this HDR, however.
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Jonathan Ratzlaff

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Re: HDR using one shot
« Reply #11 on: August 16, 2011, 03:15:33 pm »

If you are having trouble with lighroom resolving the dynamic range of the image, is the sensor you are using capable of resolving the entire dynamic range of the stouffer wedge?  There were some examples posted a day or two ago and from my perception either the highlights or the shadows were blocked up depending on the level of exposure.   I could get more dynamic range in the image by reducing the contrast setting in lightroom.  I often find that the contrast setting is too high for my liking and by reducing contrast the available dynamic range is more workable.
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Bart_van_der_Wolf

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Re: HDR using one shot
« Reply #12 on: August 16, 2011, 03:18:55 pm »

Edit: I would not call this HDR, however.

Hi Alan,

Correct, neither would I. It's tonemapping, and extracting 2 (or more) images from different parts of the same tonescale doesn't make it an HDR. But it does allow to make a nicer blend/tonemapping due to the lacking tools to do it otherwise or as efficiently.

Cheers,
Bart
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ErikKaffehr

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Re: HDR using one shot
« Reply #13 on: August 16, 2011, 03:19:25 pm »

Hi!

I would not argue with your statement. On the other hand HDR is used for two related processing:

1) Achieving extended dynamic range by combining several images
2) Mapping the extended range into something that can be shown on screen or print

Real HDR is by necessity a combination of both techniques. The discussion in the original topic is more like tone mapping.

Edit: I would not call this HDR, however.
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Doug Peterson

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Re: HDR using one shot
« Reply #14 on: August 16, 2011, 04:28:03 pm »

If you're shooting a camera with good dynamic range like a Phase One or Leaf/Mamiya Digital back then you easily get the benefits of HDR without multiple images. Much of the time HDR only requires a few stops outside what is normally used, and digital backs have enough reserve dynamic range to do this in a single shot.

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RFPhotography

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Re: HDR using one shot
« Reply #15 on: August 16, 2011, 04:35:17 pm »



1) Achieving extended dynamic range by combining several images
2) Mapping the extended range into something that can be shown on screen or print

Real HDR is by necessity a combination of both techniques. The discussion in the original topic is more like tone mapping.


Actually I'd say not so, Erik.  HDR is, by definition, High Dynamic Range.  The 32 bit image before tonemapping is an HDR image.  Once tonemapped and dropped into an 8 or 16 bit space it no longer is High Dynamic Range but rather LDR - Low Dynamic Range.  Real HDR doesn't require the second step.  Real HDR (and to keep GL happy I'll say real HDR in standard photography terms) is only the first step.  The second step is required because the devices we view images on and the devices we print on can't replicate the HDR image.  At least not yet. 
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AlanPezzulich

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Re: HDR using one shot
« Reply #16 on: August 16, 2011, 07:37:22 pm »

If you can use a single image to produce a "HDR picuture" depends on the dynamic range of the scene. A printed image has a dynamic range of around 200:1. A jpeg image has a dynamic range of around 400:1. HDR software is not required to map the jpeg into the printed image. If you use HDR software in this case you are just adding local contrast to porduce an "artistic effect". The same effect can be achieved with the unsharp mask with the radius set to a very high number.

A DSLR can capture a dynamic range af around 8,000 to 1 in RAW. If you have a scene with a dynamic range close to but less than 8,000 to 1 you will be able to produce two images from the RAW image, one of which will show the highlights but have the shadows clipped and the other will have the shadows but will have the highlights blown. It will be impossibel to use the recovery and fill sliders to get a composit. If this is the case then the HDR software can be used to tone map the image.

If the dynamic range of the scene is greater than the dynamic range of the camera sensor then multiple images are required to capture the full scene.

Using multiple images may improve the quality of the tone mapped image when the scene falls within the dynamic range of the sensor.
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madmanchan

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Re: HDR using one shot
« Reply #17 on: August 16, 2011, 09:40:45 pm »

There is HDR, and there is tone mapping, and they are related but separate things.

Tone mapping is what is needed to map the tone and color range from one device or space to another.  Perhaps the more photographic term is "development" or "process."  If your camera can capture 10 useful stops of dynamic range, but your printer/paper can only print 6, then you will need to tone map your captured image to reproduce nicely on the print.  This concept is true regardless of how one captured the image (single capture or multi).  Tone mapping can be done in any number of ways.  The simplest is using a global curve, and more complex methods include local adjustments (e.g., dodging & burning).

HDR is a somewhat arbitrary term to describe the idea of "more DR than usual" but ultimately (as Guillermo points out) all it really means is less noise.  There are multiple ways to get more DR (i.e., less noise).  For example, you can either use a camera whose capture capabilities inherently offer less noise (e.g., Canon 1Ds III compared to Canon S90), or you can use multiple captures, or both. 
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Guillermo Luijk

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Re: HDR using one shot
« Reply #18 on: August 17, 2011, 03:41:09 am »

I understand what the origin of HDR is/was.  But this is a photography forum.  The vast majority of the people here are photographers.  The vast majority using HDR will be doing so for photography related reasons, not CG.  I'd venture that very few are doing 3D CG rendering.  So in that context, I think my comments are valid.

As others pointed, we can talk about HDR imaging in digital photography without the need for bracketing (a sensor with enough DR to capture the entire scene in a single shot, use of ND filters to reduce the effective DR hitting the sensor,...), so it's nonsense to say bracketing is a part of the definition of HDR. Anyway I think this is a rather semantic discusion since we all know what we mean.

However I would like to make a consideration that is seldom made: since capturing DR in a sensor is all about noise in the shadows, and visible noise in the shadows is reduced (because SNR increases statistically) when an image is downsized, the ability to capture a given DR by a camera strongly depends on the ouput size of our application.

Quick numbers example: Canon 5D2, let's assume it has an effective DR of 9 stops at full resolution (21Mpx). If in our application we just want to upload a 800x533 image to our website, the effective DR the camera is capable of capturing for that size is larger:

  • Original resolution: 5616 x 3744 = 21Mpx
  • SNR slope in the shadows for 5D2 (SNR curves): 6dB/EV (typical in read noise limited cameras), this means DR will increase by 1 stop for each 6dB in SNR improvement
  • SNR improvement for rescaling 5616 x 3744 (21Mpx) to 800 x 533 (0,43Mpx): 20*log10[(21/0,43)^0,5] = 16,9 dB = 16,9/6 stops = 2,8 stops
  • Effective DR of 5D2 at web sized images: 9 + 2,8 = 11,8 stops

If our real world scene has 11 stops of DR, the 5D2 will need bracketing to produce a sufficiently clean of noise image for a large print at full res. But if we just plan to upload the resulting HDR image to a website, one shot will be enough. Both images are HDR, they simply have different requirements.

The formula to calculate SNR improvement when downsizing is simple statistics. See Emil's article about noise to know more.
« Last Edit: August 17, 2011, 11:30:26 am by Guillermo Luijk »
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JohnBrew

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Re: HDR using one shot
« Reply #19 on: August 17, 2011, 07:39:09 am »

Marc, you can tone map a single image with Photomatix. It's not HDR, but it is different.
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