Tuesday, September 30, 2008

XDR, Part V

HDR with Photoshop

This Article Features Photo Zoom

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5) Exposure and Gamma 6) Equalize Histogram

Creating An HDR Merge
Creating a 32-bit HDR merged file in Photoshop is straightforward.
1. Select /images using different exposure times, set one stop or more apart.
2. Turn off all adjustments in ACR and sync these settings to all selected /images.
3. Go to the Bridge menu; choose Tools > Photoshop > Merge to HDR. Check Attempt to Automatically Align Source /images.
4. After you click OK, the Merge to HDR dialog box will attempt to display the merged image, which is impossible to do on standard computer monitors. Set the slider beneath the histogram to preview “slices.” While this dialog is open, you can select and deselect included /images to adjust the final merge.
5. Click OK to render the 32-bit image and save it as a Portable Bit Map (.pbm).

Render The HDR Merge To LDR
Once a 32-bit HDR file is created, you can render it into an expanded dynamic range LDR image. Build a safety net. Render a copy of your file. That way you can always return to your original merged file and re-render it if necessary.
1. Convert a copy of an HDR image into a normal LDR image by copying it, opening it and selecting Mode > 16 bits. Choose a preferred HDR conversion dialog. Photoshop provides four methods: Exposure and Gamma, Highlight Compression, Equalize Histogram and Local Adaptation.
2. Refine the file as you would any file in Photoshop (i.e., local adjustments, sharpening, retouching).

7) Highlight Compression
8) Local Adaptation
9) Merged Layer Stack

Rendering Options
Photoshop offer four rendering options for HDR files. Each favors a different aspect of the tonal structure. Exposure and Gamma offers two sliders. Exposure controls brightness; Gamma controls contrast. It’s straightforward and unsophisticated.

Highlight Compression compresses and preserves highlight detail, often sacrificing shadow detail.
Equalize Histogram attempts to map extreme highlight and shadow points to the normal contrast range of an LDR file, typically compressing midtones aggressively.

Local Adaptation offers the most control with three variables: Radius, Threshold and a Toning Curve and Histogram. Here’s a quick way to approach the tool. Start with a low Threshold. Adjust the Radius slider to create a pleasing effect. Modify the Threshold slider to fine-tune the result; be careful not to produce unwanted halos and lines, most visible around high-contrast contours. Then click on the triangle to reveal the Curve/Histogram and create a curve to enhance contrast.

Old Look, New Look
HDR expands the aesthetic range of photographic possibilities. HDR /images can produce photorealistic effects or hyper-real effects. As with an LDR conversion, HDR compression produces artifacts. Some find them exciting. Others find them distastefully unnatural. It’s a matter of taste. You can choose how to handle the look of your image. Here’s what to watch for.
1. Midtones may become compressed, creating low-contrast effects bordering on posterization.
2. Saturation may be mismatched to corresponding levels of contrast.
3. Contours may become exaggerated—bright halos and dark lines.
Some double-process their HDR /images to exaggerate these artifacts further and create a compelling new look. Often described as a painterly look (reduction of dynamic range, flattening of tonal separation, exaggeration of contours), HDR offers new possibilities. Keep an open mind. And, remember, properly handled you can achieve almost any visual appearance you desire.

In the future, HDR capture will eliminate the need to create Raw merges. Then, LDR will be a thing of the past. In fact, it already may be.

When you work with HDR /images, you’re the master of the dynamic range in your /images. HDR techniques fundamentally change photographic practice. LDR /images fit the dynamic range of a scene first into the dynamic range of a capture device and second into the capture range of an output device. HDR /images capture all of the tonal information in a scene and later render an exposure of your choosing. Not a slave to your tools, with HDR you become master of the dynamic range in your /images.

John Paul Caponigro is an internationally respected fine artist, author of Adobe Photoshop Master Class and the DVD series R/Evolution, and a member of the Photoshop Hall of Fame. Read more of his column R/Evolution online at www.digitalphotopro.com. Get over 100 free downloads and his free e-newsletter Insights at www.johnpaulcaponigro.com.



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