Real World Adobe Photoshop CS5 for Photographers: Creating an HDR Image from a Photo Set
From the book
You can create HDR documents using Merge to HDR Pro with bracketed exposures; raw files are recommended. Merge to HDR Pro uses each frame's EXIF metadata to determine the exposures and blend them accordingly.
Shooting for HDR. Bracketing by one stop, using enough exposures to cover the entire dynamic range you're trying to capture, often works well. A heavy tripod, mirror lockup, and static scene all help. Objects in the scene that move, such as fluttering leaves on a tree, will result in ghost fragments of the moving objects as they vary across frames; you'll have to try to address these using the Remove Ghosts option in Merge to HDR Pro.
Opening Images in Merge to HDR Pro. One way to start is to choose File > Automate > Merge to HDR Pro, load up files or a folder, if necessary select the Attempt to Automatically Align Source Images check box, and click OK. You may find it more convenient to start from Bridge or Mini Bridge; after selecting images, in Bridge choose Tools > Photoshop > Merge to HDR Pro, or in Mini Bridge click the Tools panel menu and choose Photoshop > Merge to HDR Pro (see Figure 11-2).
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Figure 11-2 Opening selected images into Merge to HDR Pro from Mini Bridge without leaving Photoshop
If you open images lacking EXIF data, such as scanned images, Merge to HDR Pro displays the Manually Set EV dialog (see Figure 11-3). Select the button for the data you know, then for each frame enter the EV or exposure settings. Click OK when you're done. If you don't have the data, there won't be enough information to properly create the 32-bit merged image.
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Figure 11-3 Manually Set EV dialog for images lacking EXIF exposure data
Using Presets. Use the Preset pop-up menu (see Figure 11-4) to apply a preset. To the right of that, use the Preset Options menu button to save or manage presets. Because HDR users tend to fall into two camps—photorealistic and creative (see Figure 11-5)—the presets get you started toward both goals.
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Figure 11-4 Overview of the Merge to HDR Pro dialog
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Figure 11-5 Examples of using Local Adaptation to make both photorealistic and creative images from one HDR set of photographs
Using the Camera Response Curve. When you load images into Merge to HDR, Photoshop calculates a response curve for the camera based on the images you loaded. The response curve is invisible—you don't see it anywhere, only its effect—but if you want to apply it to other HDR image sets you load later, save and apply response curves by clicking the Response Curve menu button (see Figure 11-4) below the Preset Options menu button.
Choosing an Output Bit Depth. Once you get the images open in Merge to HDR Pro (see Figure 11-4), decide whether you want to combine the images into an image at 32, 16, or 8 bits per channel. If you want to store the entire dynamic range of the original images, choose 32 bit, but be aware that editing and output options for 32-bit images are limited. If you're merging to HDR because you want a photograph that you can edit with the widest range of Photoshop features and that you can also print, you must choose 16 Bit or 8 Bit.
Previewing a 32-Bit Image. If you selected 32 Bit in the Mode pop-up menu, you see a histogram with a slider (see Figure 11-6). Because no monitor can show the entire dynamic range of a 32-bit image at once, this slider lets you control which slice of the full dynamic range you want to preview on the limited dynamic range of your monitor. It doesn't change the image data in any way; the point is to be able to inspect details in the content.
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Figure 11-6 32-bit preview histogram and slider; drag the slider to shift the 8-bit preview to see various tonal ranges of the 32-bit image.
Choosing Options for 16-Bit or 8-Bit Tone Mapping. If you choose 16 Bit or 8 Bit from the Mode pop-up menu, an unlabeled tone mapping pop-up menu appears next to the Mode pop-up menu (see Figure 11-7) where you can choose how to tone-map the 32-bit original down to 16 bits or 8 bits of dynamic range. It's a good idea to preview all four conversion methods in the HDR Conversion dialog, because the best choice for one image may not work for an image with a different distribution of tones. The first three options can be useful for creating a source image for further manipulation, while you'll probably choose Local Adaptation if you want the most control and the best shot at creating a finished image straight out of Merge to HDR Pro. Each choice offers different options.
- Equalize Histogram. There are no options for Equalize Histogram—it tries to compress the tones while maintaining contrast.
- Highlight Compression. There are no options for Highlight Compression. The highlight end of the tonal range is compressed to fit the luminance values into the 16-bit or 8-bit version.
- Exposure and Gamma. This option offers two slider controls. Exposure sets the white point, so drag this slider first, stopping when you like where the highlights clip. Gamma sets the midtone.
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Local Adaptation. As the name implies, Local Adaptation (see Figure 11-7) implements a form of controlled local contrast adjustment where the tonal values of areas and edges are related to the surrounding areas.
HDR tone mapping involves an unavoidable contradiction: You're trying to cram a whole lot of luminance values into a much smaller number of values, while at the same time trying to keep tones far enough apart that image contrast doesn't collapse into a featureless gray goo. Local Adaptation often gives you the most usable image out of Merge to HDR Pro because instead of trying to find an overall compromise for image contrast, it also attempts to enhance local contrast. This means the results you get in any area of the image are affected by the surrounding tones.
The Edge Glow section controls the size of the local adaptation zone—it answers the question "How local do you mean?" Radius controls the local adaptation area in pixels. Strength defines what local means for tones; as long as two pixels' values are within the range set by Strength, they're considered part of the same brightness region.
In the Tone and Detail section, Exposure controls overall brightness, so you should set this first; then adjust Shadow and Highlight to set the brightness of the darkest and lightest tones, respectively. Gamma controls overall contrast, while Detail enhances sharpness through microcontrast (very local contrast).
In the Color tab, Vibrance and Saturation work much like their namesakes in Camera Raw (see Chapter 5) and in the Adjustments panel (see Chapter 7).
In the Curve tab (see Figure 11-7), you get to map (redistribute the tones) for the 16-bit or 8-bit converted image using a tone curve. This is important because you can prioritize which areas of the image gain and lose contrast while drawing from the full 32-bit dynamic range. This curve works like other curves in Photoshop such as the Curve Adjustments panel (see Chapter 7), but with a couple of key differences. First, you can convert a selected point to a corner point by selecting the Corner check box; I find this helps prevent the tendency for a curve to create ranges of tones that end up nearly flat. And second, about the only keyboard shortcut that works is nudging a point with the arrow keys.
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Figure 11-7 Merge to HDR Pro conversion controls, and the options for the Local Adaptation method
Ghost Busting. Zoom in, check all edges, and see if there are any partially transparent fragments of images caused by elements that moved between exposures, such as fluttering leaves. To try to take care of these, select the Remove Ghosts check box (see Figure 11-7). If you don't like the results, click a thumbnail in the filmstrip to use a different image as a reference; the currently used image for Remove Ghosts has a green border (see Figure 11-4).
Finishing Up. Once you've got everything worked out, click OK to generate the merged HDR document.
When you want to save the HDR document in another format, Photoshop supports established HDR formats such as Industrial Light and Magic's OpenEXR and the Radiance format used by the open-source Radiance ray-tracing and rendering engine, in addition to Portable Bitmap Format (PBM), Large Document Format (PSB), Photoshop (PSD), and TIFF.