HDRIHigh Dynamic Range Imaging

using Dynamic Photo HDR by www.mediachance.com

By Cranston Reid

Overview of HDRI• In computer graphics and photography, high dynamic range imaging (HDRI) is a set of techniques that allow a far

greater dynamic range of exposures (i.e. a large difference between light and dark areas) than normal digital imaging techniques.

• The intention of HDRI is to accurately represent the wide range of intensity levels found in real scenes ranging from direct sunlight to the deepest shadows.

From Wikipedia, the free encyclopedia

HDR techniques are not new• From the 1800’s, photographers have been

using darkroom techniques to extract or present greater dynamic range in their images. Dodging and burning are two time-tested techniques that bring out details in shadows or highlights. For example, Ansel Adams worked wonders in his darkroom as he changed and expanded the dynamic range of images to express his artistic interpretations.

Digital photographers owe special thanks to:

Greg J. Ward for creating the original Radiance RGBE HDR image format that made the high dynamic

range imaging accessible to everyone since 1985 and is still the most commonly used file format for HDR

imaging today! Two leading programs, “Photomatix Pro” and “Dynamic Photo HDR” bring these

techniques to digital photographers.

So, why bother with HDRI?Our eyes see many times more that the best cameras can

record. Remember that each increase in f-stop setting is a doubling of light. While our eyes can see almost 24 f-stops (contrast range of almost 16 million to one), most cameras can capture a dynamic range of 8 -11 f-stops at best.

Of what the camera can see, printers and monitors can’t accurately represent even this small subset of contrast. HDRI simply compresses the original scene’s wide dynamic range into something that can be viewed on paper or a monitor, despite its very limited dynamic range.

Over process the HDRI technique and the result will look very bad, just as any image can be over or under exposed. Used correctly, an HDR photograph won’t be identifiable as a processed image; it will be enjoyed as a more accurate 2D representation of a real life 3D scene. It is a merely process, and as such, is certainly no substitute for artistic vision or interpretation.

I’ve heard that HDR images use are 32 bits deep. What’s that mean?

The short version is that a bright, sunny day can present a dynamic range greater than 100,000 : 1. Since digital cameras express images in binary, everything is referenced by the powers of two. For example, 2 to the second power is 4. The bit-depth of 8 bits means that an 8 bit resolution is expressed as 2 to the 8th power, which is 256:1. This limitation can hardly describe a scene greater than 100,000:1, so greater bit depth is needed. 16 bit images are typical of TIFF files. (TIFF files can contain 8 or 16 bit-depth and are non-lossy and non-compressed) However, it is still not enough to capture all that is visible in a bright daytime scene.

HDR processing creates 32 bit-depth images, which equals 2 to the 32nd power and provides 4,294,967,296 steps. Over 4 billion steps is quite an improvement from a few hundred and far exceeds the capability of monitors and printers to reproduce such subtle gradients of color or contrast ratios. This will certainly do a reasonable job of describing a high contrast scene such as a sunny day. However, there is still a problem. Our monitors and prints can’t begin to handle such a wide dynamic range!

The secret of HDR processing is that it compresses (called tone mapping) these 4 billion steps (32 bit-depth images) into something that printers and monitors can display.

This means that we can now see detail in dark shadows and yet the brightest details are also visible. This processing could, for example, allow you to photograph the interior of a building while still preserving the shadowy details of the interior while also recording the highlights of a brightly lit outdoor window scene. This approximates what our eyes are capable of seeing.

The Powers of 2

Because .jpg compression is a non-linear progression, the dynamic range of an 8-bit .jpg is about 11 f-stops. Also, each pixel is described by 24 bits, consisting of 8 bits each for red, green, and blue. Remember, we can see 24 bit-depth. It’s no wonder the files are so large!

Does HDRI processing (both Photomatix Pro AND Dynamic-Photo HDR) do more than enhance colorful

skies and open up shadow and highlight details?

Yes, HDR processing can add a significant perception of texture to some objects. This might be distracting or could really add some interest to the photo. The trick is to determine which scenes are good HDR candidates. Frequently, high contrast scenes work with HDR but low contrast ones don’t change much with HDR processing. Also, remember that sometimes we do need shadows to define depth. Remember, light illuminates, but shadows define. HDR processing can easily over saturate some colors (especially yellow and orange) and wash out the shadows to the point that the image is too flat, so photographer beware!

Is a tripod needed?

Not necessarily, but if you are going to shoot a set of multiple exposures, it certainly makes your work easier. Both HDR programs don’t’ handle different images (due to camera or subject movement) well. You can also create a pseudo-HDR photo from just one RAW image file, but it does not create more detail...it only compresses a wide dynamic range for viewing on a monitor or printing. Remember that the camera in raw format can sometimes capture more range than a monitor or printer can display, so this can help visualize all that the camera has recorded, but nothing more.

HDRI programs refer to EV and LV.

LV is simply how bright a scene is. For example, outdoors in full sunlight is illuminated at LV 15 and a moonlit scene is perhaps a -5 LV. We can’t adjust LV but EV is what is set on a camera by many different equivalent EV combinations of shutter speed and aperture settings. For example EV 14, an exposure time of 1/250 second at f-8 is the same EV as 1/125 second at f-11 (one step slower speed and one f-stop higher aperture setting).

How many photos are required to make a stack for HDRI?

A reasonable number might be five or fewer. I’ve seen very good results with just three, but it is preferable to have more exposures available and discard a few later if appropriate. A compromise is to shoot a series of three to five images that are 2 exposure time steps (or 2 EV steps) apart unless the scene has unusually wide dynamic range, then add a few more steps. Just be certain that all the highlights or shadow details have been correctly exposed somewhere within the series of captures. If your camera has the capability of blinking on excessive highlight exposure, use it here.

Some cameras can shoot a series of images in rapid succession, automatically adjusting the exposure time to record images separated by 2 EV (Exposure Value) steps. The Canon 5D can automatically shoot three of these, and the Nikon D700 can shoot nine in a row. Check your camera’s manual to find references to Auto-Exposure Bracketing or “AEB” settings. When shooting images for HDR processing, be sure to adjust only the time exposure and never change the aperture. Changing the aperture will modify the depth of field, which should be avoided, as it confuses the HDR program. To avoid as much noise as possible, shoot with the lowest appropriate ISO.

Can a single RAW or JPG file produce a good HDR photo?

Yes, especially in the case of RAW files, this is possible because the RAW images contain more detail in the shadows and highlights. RAW images support 12 to 14 bit-depth of data, while JPG only supports 8 bits, so RAW contains more information. However, the result is not a true HDR image, so it might be called “pseudo-HDR”. Paper has the limitation of a narrower dynamic range compared to a RAW file, so the HDR compression (tone-mapping) will display these embedded details more effectively. However, HDR processing can’t create detail where none exists.

Will HDR help every photo?

Definitely not! A good candidate for HDR processing is a scene that contains a very wide dynamic range that would otherwise lose much of the details in shadows or bright highlights. If you are shooting a scene that is low in contrast, HDR processing will have little effect, other than add undesirable noise to the image.

Once a series of photos has been recorded, the set is processed by the HDR program to create a high dynamic file. This expanded 32-bit dynamic file won’t look good on any monitor, since it far exceeds the monitor’s dynamic range. There is, however, a software preview window that allows a hint of what the finished product will look like. Once the HDR file is created, it must then be “tone-mapped” into something that is viewable or printable. There are various algorithms that handle the actual mathematical processing, but I’ll leave those details for a future episode of television’s “Numb3rs” show. The important thing is to blend all these differently exposed images into something that is pleasing and shows just the right amount of details and color without looking like something from a comic book.

What does HDR processing software cost?

I believe that the best and most flexible HDR program is the $99 “Photomatix Pro” from www.hdrsoft.com , but “Dynamic-Photo HDR” http://www.mediachance.com/hdri/index.html is less expensive, although it seems to be less capable than Photomatix Pro. “Dynamic-Photo HDR” costs $55 each or somewhat less if three or more licenses are purchased in bulk. A detailed feature comparison of the Photmatix Programs can be found here: http://www.hdrsoft.com/order/features_compare.html

How do I create an HDR image and tone-map it?

• Remember to use your tripod• Set the camera for aperture priority (you don’t want the

depth of field to change)• Select RAW, and the lowest practical ISO• Shoot at least three images of different EV settings

One should be properly exposed, one should be exposed to +2 EV and one should be -2 EV. Once you have three images, don’t tweak them yet. It is easier and faster to adjust one image later than change three now.

How can I go wrong?

#1 error: over-processing HDR to make the image unbelievable. I suggest that you go easy on the “details enhancer” tab, as too much of this can produce a comic-book look. Also, keep the saturation slider to less than 50% saturation. If the finished product lacks any real shadows or highlights, you might try reintroducing a limited amount of these with Lightroom or Photoshop. Gently applied, clarity, contrast, and perhaps selective saturation work well here. Avoid adding noise to the image, especially where there are wide areas of low contrast such as clear skies.

This describes DPHDR, not Photomatix Pro.“Create new HDRI screen”

• Import images by selecting create HDR file, then drag and drop or browse to your stack of images. Use “Auto Developer” if you are directly importing RAW files.

• Click “Guess EV” and verify they are reasonably close. If you need to change any of the EV settings, you can enter the numerical value or use the slider.

• There is a rough preview of what the finished HDR shot will look like in the right window. The preview window is useful to see the end result if some of the RAW images are deleted or the EV settings are tweaked.

Even if the images look pretty well aligned, click “Align Files in Next Step” just to be sure. It is very important to verify that the images are actually aligned correctly, as sometimes there is movement, even when using a tripod. I’ve found that selecting the default value of “Auto-adaptive” and “Auto Update” work well.

Align Files screen.

On this screen, you will find the combined image assembled from the EV=0 change image and one of the EV+ or EV- images. Each image is sequentially aligned using this feature and if you’ve moved the camera very much between exposures, these changes will drive you crazy. Click the As I mentioned, I’ve found movement even when using a tripod, so check these very carefully. There is a 1X for full view and a 2X magnifier button. I suggest that you verify alignment by using the 2X and the “Diff” button. This will present a difference or subtraction of one image from the other. If you see white lines outlining high contrast boundary lines, use the arrow keys to gently nudge them into alignment.

If you need to make coarse adjustments first, you can hold down the control key and tap the arrow buttons. However, if the images have moved very much, I’ve had very limited success making an HDR image when there is excessive movement. When moving the image around, you can simply left click the mouse and drag the image, or you can use the X and Y wheels to make adjustments of the preview window. However, it is the arrow keys only that will actually align the images. There are shortcut keys labeled “Center”, “LT” for left top corner, and “RT” for the right top, etc.

If one part of the image is aligned, but other areas have shifted, it is because the camera moved, rotated, or both. It will be necessary to go to the advanced functions. Select under the Exposure Weighting function the button labeled “Sharper Output”. This will make the boundary edges sharper and easier to align properly. Since the image is most likely rotated slightly, establish a “pin” at the center location (press the “Center” button and click “Add Pin”. You will see a red pin at the center. Go to the corners and when a high contrast part of the image is roughly in the center of the preview screen, click “Add Pin” to allow moving the image towards exact alignment using the arrow keys. Do this for the four corners and continue to the next screen by clicking “ok”.

Tone Map HDRI file

A Quick Guide window will open, so press “Tone Map HDR file” or “ok” to start the compression algorithm. The large view is an approximation of what the compressed or “tone-mapped” image will look like. Some interesting effects can be chosen under the “Method” panel. The choices of “Eye-Catching”, “Ultra-Contrast”, “Smooth Compressor”, “Photographic”, etc. will render the HDRI with different presentations. The first three choices are local in nature and the last four are global. Each interpretation is up to the user, but I’ve found little use for the first three choices, as they produce very heavily modified images. The “Smooth Compressor” and “Photographic” choices seem to work better for me.

The tone-mapped HDRI file is now visible in their default viewer called, “Photo-Bee”. Each output can be saved as a .JPG, PNG or as a .TIF for later processing in Photoshop or Lightroom. Since the .TIF file is a 16 bit image, I prefer to save it in that mode or .PNG instead of an 8-bit .JPG file. You can return to the Tone Mapping window and select a different mode under the “Method” panel. I suggest that you process and save a copy of each one, with emphasis on “Smooth Compressor” or “Photographic”. If you find that the sky is grainy or noisy, select the “Sky” button to have DPHDR do less aggressive processing of the sky. The program also has a “Skin” mode that has even less processing.

Noise

It seems that HDR processing can easily create noise in the finished product. It helps to use the NR (noise reduction) and the “Sky” option, but you might also try deleting the darkest image in the stack and run the algorithm again. The example stack that I used contained several darker images that were responsible for an excessive amount of noise. I believe that Photomatix Pro is much better than DPHDR when dealing with noise, but direct comparisons should be made to verify this.

More Options

The Tone Mapping window also has many additional tweaks on the right side of the panel that should be explored with the detailed, 93 page downloadable manual. For example, the button labeled “Filter Color” will provide thumbnail files that preview Black/White, Orton B/W, Sepia, Each adjustment has a square with an “R” in it. This will Reset each section to the default value. The controls labeled “curves”, for example, is similar to the curves feature in Photoshop, and will have a dramatic effect on the HDR image.