Sunday, June 29, 2008

High Dynamic Range (HDR) Imaging

This photo of the interior of John Burroughs’s writing studio illustrates a fundamental limitation of photography.

The view out the window is so bright that it has burned out (or “clipped”) to white, and the shadows in the corner are an impenetrable black. In both instances, the optical sensors are unable to record any meaningful color data. But in real life, our human eyes could see plenty of color and detail in both areas of the view.

The problem is that the camera, whether film or digital, can only record within roughly a 300/1 ratio of light intensities, while the human eye can easily respond to a range of 50,000/1 within a single scene.

The range of intensities is called the dynamic range. The camera is not the only thing with a limited dynamic range. The same 300/1 ratio also applies to digital printers, computer screens, or artist’s pigments.

It’s analogous to the problem you’d have playing Beethoven’s Ninth over a cellphone.
Photographers have found a way around the problem. By taking several different exposures of the same scene (one shot exposed for the bright sky and another for the dark shadows), the photos can be combined, or “tone-mapped” into a single image where every part of the scene is visible in rich, glowing color.

Here is a photo of a room interior with a normal exposure.

With tone mapping in HDR photography, color is saturated and detail is present throughout the image.

The effect of HDR can look a bit garish at first, partly because we’re not used to it, but also because it can take away the excitement of strong contrast. But in artistic hands, it can also be otherworldly and attractive, reproducing the feeling of what an artist might see, with color and detail infusing the entire scene, both in the shadows and in the blue sky.

The real frontier for HDR photography will be the new brighter computer screens, like the new BrightSide technology, which will be able to output HDR image files in their true range of brilliancy, giving the viewer the feeling of standing in a real street in the bright sunlight.

What does all this mean to painters? Although our eyes can see greater dynamic range than the camera can see, we’re still forced to translate our observation through the measly 300/1 funnel of pigments on canvas.

When you're painting subjects in soft, overcast light, it's easy to convert reality into paint. But when you have subjects with extreme tonal contrasts, like views out windows, or illuminated signs at night, a successful painting requires a keen awareness of the distribution of tones.
Flickr HDR groups, link and link.
Wikipedia on HDR imaging, link.


Erik Bongers said...

The high dynamic range of light is indeed more a problem for photographers than for paint smearers. With photographs I find myself constantly darkening the skies in photoshop to get rid of the horrible white.

Two remarks about this post.

1. On our 'dynamic range'. As I though, the human eye doesn't have a 'real' dynamic range of 50000:1. I verified it on this wiki page on the eye which says that the eye can only handle a light intensity range of 100:1 at a single time ! That's less than a photograph/TV/painting !
But we overcome this limit by adjusting our 'exposure' as we scan the image. Thus reaching a dyn. range of a blasting 1 million:1 !!! (probably for mammalian eyes in general- the page doesn't specify)

2. I want to stress the difference between real HDR images and 'tone mapped images', which are being described in this post. Especially vendors of Tone Mapping software love to use the HDR-word because it's currently a buzz-word. But both things are not quite the same.
A real HDR image has a dynamic range that simply doesn't 'fit' on your screen. This means that when viewing HDR, you have the option to show the shadows (and thus burn the highlights to white) or show the highlights and have the shadows rendered pitch-black. Of course you could compress the dynamic range but then you would get a very dull contrastless image.
Whether you create a HDR image with a special digital camera with a high dynamic range sensor or with your consumer camera by combining different exposures is irrelevant. Your result contains 'information' going from very very dark to very very light.
That's real HDR.

'Tone Mapping' however is cut-copy-and-pasting for example a darker exposed sky on top of a lighter exposed landscape.
The result is that your landscape may now contain values that are lighter than some sky values which may not have been the case in the real scene.
If overdone, this looks alien, but if used in a subtle way, the result will look quite realistic because it resembles what we do with our eyes when 'scanning' a landscape and adjusting our 'exposure' as we look at the sky or at the shadows in those woods.

Actually the first landscape photograhers quickly came up with such a 'tone mapping', although the name didn't exist yet : they 'glued' a darker sky on top of a landscape. Quite often the sky had been photographed at a entirly different time and location.

Also, landscape painters will also unconsciencly do some form of 'tone mapping' as our eyes do this for us.
A good example of this can be found in this post by a great landscape painter. The included reference photograph shows the differnce between real values and 'tone mapped' values on the painting.

Erik Bongers said...

Some extra clarification (I may have caused confusion).

Tone Mapping is e.g. adding a darker photographed sky that was photographed at the same time and location on top of a scene.
When you 'glue' another sky on top, that's a heavier form of cheating.

And actually you can moderately 'tone map' within a 'one-exposure' photograph too, by simply darkening the sky a bit in photoshop. However your single photograph has 'limited 'information' for the highlights of the sky and thus the burned-to-white highlights of say, the clouds will not suddenly get detail, but will render dull gray when darkened too much.

Another way to put the difference between HDR and Tone Mapping.

HDR is a linear representation of a high dynamic range of light, while Tone Mapping is non-linear in that the high range is shifted to partially overlap the low range.
(the sky suddenly being darker than part of the landscape)

Erik Bongers said...

Dare I add a 3rd comment ?
Yes, I do !

An adjusted definition:
HDR : linear rep. of high dynamic range at it's full expanded range
Tone Mapping : a non-linear rep. of a high dynamic range compressed to a printable/viewable range.

And to conclude:

A HDR image is the best source to do Tone Mapping.

James Gurney said...

Thank you, Erik, for parsing the buzzwords. I think you're right that a lot of this material is old wine in new bottles. Both photographers and artists have been dealing with this problem for a long time. But I'd sure love to see a real HDR file on a super HDR output screen.

Paolo Rivera said...

I used to have Fuji camera that was excellent at subtly tone-mapping photos all by itself. I bought it specifically for this ability, made possible by a new kind of CCD which was interlaced with 2 types of sensors, one for high range and the other for low.

I've never gotten better color from a point and shoot digital camera. Unfortunately, it didn't prove reliable in the daily use department (and neither did my mom's) so now I own a Canon. Fuji may have worked out the bugs by now, though. I got the camera back in 2003.

As for a true HDR screen, I think that's something I could only handle in small doses.

Anonymous said...

I've always pondered the feasibility of cracking open one of those glowsticks and using that to illuminate a painting. Once, in my childhood, I accidentally broke a glowstick and dribbled glowing radioactive orange all over the carpet. Whoops!


Tom Scholes said...

You know I've never cared for HDR photography. It's always reminded me of bad 3D computer graphics with very poor lighting. It seems to take the life out of so many things.

JP said...

One of my favorite illustration teachers loves to use the phrase "contrast is the meaning of life" as his number one rule. I know that rules are meant to be broken, but it seems to me that losing the contrast in the images sort of dulls the drama. By the same token, one of my favorite photography teachers told me that the mark of a true photographer was not the blacks or whites, but how one handles the gray. I suppose it's a matter of taste, but I tend to lean toward the side of contrast when making images.

Bryan Crump said...

HDR is very fun and interesting for sure. I am using a combination of HDR and editing to bring out different details and effects for portraiture. There is a lot of potential for all sorts of editing and detail that cannot be achieved any other way.

Are any of you using HDR in portraits?

Mark Reep said...

Years ago, when I was learning the basics of photography (most of which, sadly, I've since forgotten, and am now having to relearn) someone told me that the most important thing to remember was that 'Film doesn't see light the way your eyes do.'

It's amazing, as Erik mentioned, how our eyes continually adjust for exposure as our gaze moves across whatever's in our field of vision.
Stare into deep shadow, and then, without transferring your focus, try to note- out of the corner of your eye, sort of- how whited-out a brighter area reads when your eye is still exposing for the shadows.

Both of John-Paul's points are great too. To me, the greatest challenge, whether I'm drawing or photographizing, is finding that ideal balance between consistency and contrast.

Erik Bongers said...

yeah, "balance between consistency and contrast", that's a good way to store it in the attick.

And actually it's something 'we artists' are confronted with every time we scan our latest masterpiece and then find ourselves adjusting things in photoshop and thinking 'why didn't I get this tone or contrast right in the original?'. Perhaps not a masterpiece then...oh well, next time I'll get it just right !

Denis Loubet said...

Oddly enough, in computer games HDR is often used to create exactly the high contrast situation of low range media that the HDR photography seeks to eliminate.

For the indoor scene in a computer game to look more realistic, you need that blooming, blown-out light from the window. It simulates the feel of a movie shot on film better, and that's what the audience thinks reality looks like.

It's the same with other photographic artifacts like lens flares and the Bokeh effect. Computer graphic artists often go way out of their way to simulate these flaws.

James Gurney said...

Thanks, Denis, that's a really interesting insight. I'm always amazed when I look at a video game and get tricked into thinking it's real. Thoe lighting designers in that business are brilliant.

Denis Loubet said...

I'm one of those CG guys trying to imitate the flaws of photography in my work because the presence of those flaws lends my images an undeserved credibility. ;-)

We game artists are trying to convince you that you're watching a film because everyone has been conditioned to view film as a credible depiction of reality, lens flares and all.

And that's a strange state of affairs considering that reality includes all that HDR detail you pointed out, and contains all the immediacy of "being there" that photos far more often than not completely fail to present.

Hence, my lame vacation snapshots just can't generate the excitement of the real Arc de Triomphe.