Pixiq

Single Image Tone Mapping

Single image tone mapping can be done with TIFF, JPEG, and RAW files. Technically, it’s not true HDR imaging, but if the camera sensor has captured the full dynamic range of the scene, this pseudo-HDR technique can look identical to HDR.

Compared to merging several exposures to HDR, single image tone mapping offers reduced processing times, saved disk space, and a less intensive workflow at the scene and in the computer. Combine that with ghost-free images and you could say that tone mapping single images gives you the biggest bang for your buck. As we will see, capturing the full range of light without clipping the highlights or shadows is critical to the success of single image tone mapping. Consequently, the choice to shoot a single image rather than an HDR image set should be based on the contrast of the scene. A low contrast scene is easily captured and tone mapped with a single image, but a medium contrast scene can go either way in terms of quality. Noise will be present in the medium contrast scene, but if it is fairly insignificant in the overall image, it is usually acceptable. If the scene is high contrast, shooting an HDR image set is recommended for a quality image.

The biggest shortcoming of single image tone mapping is that it enhances any noise present in the image. However, a few precautions beforehand can go a long way in minimizing the noise that often ruins an image.

• Shoot RAW files.

• Ensure proper exposure, perhaps even slightly overexposing to capture as much shadow detail as possible (but watch out for blown highlights). You may even consider setting exposure compensation to +1/3 or +1/2EV.

• Use in-camera noise reduction features when available.

• Process the image in a high-end RAW converter and reduce color noise before tone mapping. In the Photoshop CS3 Camera RAW plug-in, there are sliders for Noise Reduction, Luminance, and Color Noise Reduction.

• Unlike pre-processing HDR source images, applying tonal adjustments in the RAW converter for single RAW images you intend to tone map can and should be done. Exposure settings can be used to recover highlights that would otherwise not be recovered during tone mapping. The Photoshop CS3 Camera RAW plug-in has two additional sliders that are useful: Recovery and Fill Light. (Be sure to zoom in and check edge details when using them.)

• After RAW conversion of your single source file, save the image in 16-bits per channel. They are better to tone map than 8-bit files.

Photomatix Pro

Photomatix Pro allows you to tone map a single exposure as long as it’s a RAW file or a 16-bit image file. If you have a JPEG image (which is 8-bit), it will need to be converted to a 16-bit file before tone mapping. There are two methods to begin tone mapping in Photomatix Pro:

1. Go to File > Open to load the 16-bit image or RAW file, then go to HDR > Tone Mapping to open the tone mapping dialog box. (This streamlined method does not involve saving a 32-bit HDR file to your hard drive.)

2. Select Automate > Single File Conversion and check the box From Low to High Dynamic Range to convert the RAW or 16-bit image to a 32-bit HDR file. You can choose a folder for batch processing or a single file for a single conversion to HDR. Click Run and the 32-bit HDR file(s) will be saved to your hard drive. Go to File > Open to retrieve the HDR file, then HDR > Tone Mapping to begin tone mapping.

Comparing Images

The following are a series of HDR images, single tone mapped images, and just plain single images with no tone mapping processed in Adobe Camera RAW for comparison.

The picture of the archway at Princeton University, taken during overcast skies, is well suited for single image tone mapping. The HDR image (A) is indistinguishable from the single tone mapped image (B), and further shows that shooting an HDR image set in low contrast conditions is not necessary. The Adobe Camera RAW image (C) is acceptable, but has less impact as it lacks local contrast enhancements.

A

B

C

D

E

F

A typical medium contrast scene has about a 7 – 8EV range and is near the upper limit of the camera’s sensor. If your camera is set on a low ISO, a side-lit scene on a sunny day can be captured with limited problems in a single image as long as exposure is correct. Nubble Point Light was photographed during the late morning on a sunny day. The scene is side-lit and some of the rock faces are in shadow although shadows are not a dominant part of the scene.

The HDR image (D) is the overall winner when it comes to shadow detail, but if shadow detail were not an issue, I would give the nod to the single tone mapped image (E), which has no ghosting and good color and contrast enhancements. The Adobe Camera RAW image (F) has less punch and vibrancy than the other images and, comparatively, appears unfinished.

G

H

I

Photographers are often drawn to old buildings; the more dilapidated, the better. I’m not sure if we are drawn to their paint-peeling beauty, or we just like to shoot in a place where no one will bother us. This tilted barn scene had me slamming on the brakes and making a u-turn. The clouds were great, and the barn was in an open field. Considering that this was a medium contrast scene, it was hit or miss as to whether a single tone mapped image would be a good bet.

The HDR image (G) has the best shadow detail, the best local contrast, and the best overall look. The area under the roof in the HDR image also has more detail than either the single tone mapped image (H) or the Adobe Camera RAW image (I). In this case, the single tone mapped image shows a significant loss of detail due to noise, and because of this, would not be a good first choice. The Adobe Camera RAW image (I) is comparatively flat and uninteresting despite lengthy tweaking using Levels, Shadows and Highlights, Curves, and Saturation.

The sunrise at Acadia National Park has a 9EV dynamic range as the sun begins to come over the horizon. This is a high contrast scene, and it’s clear that the HDR image (J) has out performed the other two image examples in this comparison. Shadow detail and noise control are superior in the HDR image, and the overall impact is greater. There is some chromatic aberration that could use follow-up processing, but for the most part, image processing is complete. The single tone mapped image (K)

J

K