The Digital Eye

Figure 1: The Photomerge dialog box enables you to choose images from a list or from images opened in the workspace.

A few years ago, I wrote an article on panoramic images to illustrate an exciting new feature in Photoshop CS—the ability to merge or “stitch” images together into impressive panoramic composites (see Digital Graphics, Feb. 2005, page 54). Ideal for large-format applications, the Photomerge feature was in its infancy and required a bit of elbow grease to construct a credible seamless panorama.

Figure 2: The Auto Align option intuitively positions the images horizontally or vertically

After three subsequent versions of Photoshop, (CS2, CS3 and CS4) the feature has been improved and simplified, and now works better than ever. Also, a couple of new features have been added to the impressive list of commands that streamline the process of merging image content. Merged images saved to PSB (large document) format and printed to a large format printer is a marriage of two highly compatible technologies.

The Photoshop CS4 version of Photomerge has been refined to automatically blend images and reduce the amount of post-merge editing. There are also several options that map images into specific shapes. Furthermore, the Photomerge automation works equally as well on horizontal and vertical panoramas.

AUTO MERGE
In Photoshop, Photomerge can be applied to a group of open images or to images selected from a directory by choosing File Automate > Photomerge. It can also be applied to selected images in Adobe Bridge by choosing Tools > Photoshop > Photomerge. In the Photomerge dialog box (see Figure 1), choose images in the Source Files field either by browsing or adding files currently opened in the workspace. The Layout options on the left side of the interface merge images in specific shape configurations, including Auto, which intuitively positions the images horizontally or vertically, as in Figure 2. The Vignette Removal and Geometric Distortion Correction checkboxes use the image’s metadata to adjust color and shape.

Figure 3: The geometric options configure the merged images into specific shapes.

The Layout options including Perspective, Cylindrical, Spherical and Collage bend the merged images in specific shapes as shown in Figure 3.

Figure 4: A comparison of two images to illustrate how the application of Blend Images Together seamlessly merges images with variations in exposure.

Check the Blend Images Together checkbox to adjust the exposure of the merged images so that the areas of overlap blend seamlessly (see Figure 4).

When the images are merged, the layers palette displays the images in a sequence of consecutive overlapping images whose edges are defined by layer masks (see Figure 5).

Depending on how well the images were captured, additional editing may be required to adjust unaligned areas and remove ghosting. If the exposures vary too much you may have to create an adjustment layer or two to make corrections using Levels, Curves, Color Balance or any of Photoshop’s color correction features.

Figure 5: This is the Layers palette for the image displayed in Figure 2. The images are stitched together in consecutive overlapping layers with layer masks.

PREPARATION HELPS
Here are a few initial tips that will facilitate the capturing and printing of images that will be merged with the Photomerge feature.

Plan your shots. Always use a level tripod and look through the camera lens to determine where the best breaks will be. Perform a dry run sweep before actually taking the pictures.

• Observe a feature on each exposure and then use that feature as a marker to overlap your images by at least 25 percent. Some cameras have built in features to help overlap images accurately.
• Shoot with a normal or slightly telephoto lens if possible. Wide-angle lenses exhibit too much distortion that Photomerge tries to compensate for with barrel distortion to make the adjacent images fit.
• Shoot at the highest resolution possible with your digital camera in Camera Raw format and in 16-bit mode if possible to enable you to collect the maximum amount of information for each shot.
• Prior to opening your images, choose the Enable Large Format (PSB) option from the Photoshop File Handling preferences to enable you to produce documents up to 300,000 x 300,000 pixels.
• Open the images on the desktop and make general color corrections and color matching prior to applying Photomerge.
• To maintain a dynamic workflow use Adjustment Layers to make your final color corrections. After color correcting, inspect the image very carefully, zooming in to observe misalignments, small seams, ghosting and artifacts. Use the retouching tools to correct these flaws.
• Flatten your image and convert it to 8 Bit (Image > Mode > 8 Bits/Channel) to reduce file size before printing the final panorama to your large format printer.

Figure 6: The application of Auto Blend to previously auto aligned layers creates a seamless composite.

AUTO ALIGN AND AUTO BLEND
Two features in Photoshop CS4’s Edit menu merge overlapping layer content. Manually compositing the images to layers, while more labor intensive can be advantageous if the images vary too much in exposure or composition. Very similar to the Photomerge automation that aligns the content of separate documents, the first of these two features—Auto Align Layers—analyzes layer content for similar pixel values and repositions the content to form a panoramic image. The application of Auto Blend (see Figure 6) will then smooth the overlapping areas to form a perfect seamless composition. Auto Align offers the same geometric options as Photomerge and what results from applying these two commands is a layered document with a sequence of layer masks that seamlessly blend overlapping content.

PREP FIRST
Before applying these manual features, observe a few basic tips that will lighten the load on the software. Remember that many thousands of mathematical calculations have to be made to determine where to place and how to blend the images together so that in the interest of saving processing time prepare your images first.

• Duplicate the image so that you are not affecting the original. The Auto Align feature can distort layer content to fit it into a composition and you’ll want to experiment with various geometric features without hurting the originals. To make a copy of the image choose Image > Duplicate.
• Place the layers in a consecutive sequence in the Layers stack.
• Roughly position the layers in the picture plane as they will appear in the finished image.
• Edit specific Layer content before applying the commands. You can then apply more global tonal adjustments after the layers are merged.

Figure 7: Content Aware Scale command preserves the proportion of the critical image content when the image is enlarged.

CONTENT AWARENESS
One other feature I should mention is the Content Aware Scale command (see Figure 7) that can come in handy when creating wide-format panoramas. Content Aware Scale analyses and determines the important elements of a composition and magically preserves the proportion of the critical content when the image is enlarged. You can flatten an image and make a selection, or you can target specific layer content. If you’re planning to enlarge the entire image, add canvas first (Image > Canvas Size…). The content aware Scaling command is found in the Edit menu and when selected, a bounding box appears around the content. Drag one of the anchor points to scale the image. Press the Option/Alt key to radiate the scaling from a center point. It is possible to over-stretch the content too much and distort it. Within certain limitations this feature works nicely.

QUALITY RESULTS
Photomerge, Auto Align, Auto Blend and Content Aware Scale are significant improvements to the Photoshop arsenal. They can be real time savers when combining images into large-format panoramas. Remember however, that these features are not ends unto themselves; and as with any editing process, good image capture and initial preparation are critical to producing quality results.