Wednesday, January 28, 2009


Master high dynamic range imaging to enhance tonal range or create hyperrealistic effects

This Article Features Photo Zoom

There are several programs that assist with HDR images; one that many photographers already have is Photoshop, so here are the basic steps to use in that application.

1. It’s best to start HDR generation in Adobe Bridge for a couple of reasons: It works with the RAW files, and Stacks > Auto-Stack Panorama/HDR analyzes the images and EXIF, and can magically stack and sort your source images.

2. Whether working off stacked images (as I am here) or a number of selected unstacked source images, simply choose Tools > Photoshop > Merge to HDR to get the 32-bit party going.

3.  The photos on this page (marked #3) are the same HDR image with different White Point Previews set. This image contains all the information from the nine bracketed source images. But your monitor can’t display the full luminance of the HDR image all at once. It’s sort of like the dial on an adjustable lamp.

4. I always recommend saving your true HDR image, but if you want to jump straight into tone mapping, drop the bit depth from 32 bits/channel to 8 or 16 bits to launch the Tone Map command box. Otherwise, click OK to bring your 32-bit HDR image into the main Photoshop interface.

5. There are a number of indicators, some subtle and some in-your-face, that 32-bit space isn’t quite the same as the 8- and 16-bit space we’re used to working in. The bottom of the window has an exposure slider that functions like the White Point Preview arrow we saw in the Merge to HDR window. Notice the zero (ø) in the histogram window? You’ll also notice a similar “Not Allowed” symbol on many of the toolbar items. And the Color Picker sure looks different, doesn’t it? Notice that the 32-bit RGB values are floating point decimals, which means a whole lot more numbers representing colors and luminance values.

6. There are a number of editing options in 32-bit space, but most of the time, you’ll just be visiting 32-bit space to save a copy of your HDR file in OpenEXR (.exr) or Radiance (.hdr) format before dropping down to 8- or 16-bit space.

7. The only way to launch into tone mapping is to drop the bit depth under Image > Mode > x bits/Channel.

8. Click on the down arrow to expand the HDR Conversion Dialog box to show the Curve control. Select Local Adaptation from the drop-down menu. Radius and Threshold behave similarly to the sliders of the same names in Unsharp Mask, except here it has to do with the local contrast adaptations as your HDR image is downsampled to traditional bit space. You can control the curve to adjust local contrast and boost details throughout the image. Notice the check box marked “corner.” This can turn a point into a straight-line segment, like the last segment of this curve. We like the detail preservation and realism feel of this scene, but it does lack some color “oomph.”

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