Better (And Faster) Panoramic Images; Problem Panos Now Have A Solution
Panoramic stitching is not new. In fact, I can still remember my first attempt involved shooting film, processing and then scanning the photos before importing them into an obscure stitching utility that produced reasonable but not stunning results. One of the best-kept panorama secrets is hidden in the midst of all the big and prominent features in Photoshop CS3—it is a totally revamped Photomerge (Adobe’s pano stitching program) utility. Now if you have tried Photomerge before, don’t skip to another article; this is totally different from previous versions. In fact, the program is so different, in terms of its functionality and ability, that it might have been fairer to rename the tool rather than let us all assume that it is the same feature.
How has it changed? Well, apart from breaking the tool into two different processes—Aligning and Blending, and making these processes available outside of the Photomerge feature (see Edit>Auto-Align and Edit>Auto-Blend) the tool now ships with several different auto options, the ability to stitch HDR source files and a cool new technique for the removal of problem parts of source images before stitching. It is this last change that we will concentrate on here as I believe that this one change will dramatically alter the success that many photographers are having with difficult panoramic shots.
How do I know this? Well I’m one such photographer. The example image that I am using here has been hanging around my hard drive for a good few years now. Photographed on a trip to Nepal, the Kathmandu street scene is full of life and interest. This aspect of the subject bodes well for an interesting panorama but makes the process of stitching the source files a nightmare. As it is impossible to halt the street action while you photograph (why would you want to anyway?), the resultant images are full of problems in the important overlapping edges of the photos. While the background remains consistent, and easy to stitch seamlessly, people in the foreground at the edges of the frame change for each photo, creating ghosting effects from image parts that appear in one photo, but not the next.
Now before the release of Photoshop CS3, correcting these problems meant cutting problem parts from the original source file and pasting them into the stitched panorama. It then required some tricky editing to ensure that the pasted section blended into the background. In some cases it was also necessary to deform the pasted section to better match the perspective of the panorama.
Now you know why I haven’t completed the Kathmandu street scene panorama. With previous attempts, the pain of the complex editing quickly outweighed the pleasure of the finished photo, consequently the project was never completed.
One of the changes made to Photomerge in Photoshop CS3 is the ability to stitch masked source files. This provides the opportunity for problem areas of the picture, such as people moving near the edges of the frame, to be removed before stitching. In the case of the street vista, this means
opening the source images, changing the background layer to a standard image layer, adding a mask and then painting out the problem section. The pictures are then saved and used as replacement source files in the final stitch. Your aim with the masking is to remove only those picture parts in the overlapping areas that differ from one source file to the next. Adding a mask forces Photomerge to use the pixels in the non-mask image for that part of the stitch. Cool!
How does this work in reality? To ensure that I knew where the problem parts were in the panorama, I stitched all the original source images together, and then carefully examined the results. I identified three main areas where inconsistencies in the source files were causing ghosting in the stitched result. I then proceeded to open three of the source photos, masking the conflicting areas, save off the files and restitch the panorama.
The resultant wide vista photo contained none of the problem ghosted areas displayed in the first panorama. Next time you find yourself trying to stitch a panorama with ghosting problems try using some deft masking to help Photomerge know which bits of the photo are important and which should not be included in the final picture.
Step-By-Step Masked Panoramas
Multi-select the source files inside Bridge and then choose Tools>Photoshop>Photomerge. Alternatively you can open all the pictures into Photoshop and then choose File>Automate>Photomerge and click the Add Open Files option.
Next Photoshop opens and the new Photomerge dialog is displayed. Notice that the pictures selected in Bridge have been automatically added to the dialog. Choose the type of stitching layout you want to use. Here I selected Cylindrical as the images covered a very wide vista. Also make sure that the Blend Images Together option is selected. Press OK.
When the completed panorama is displayed in Photoshop carefully examine all parts of the image but especially the areas of overlap between source files. Look for ghosting. In the example, there are three blatant examples that need some work. Select the source files for these areas and open them into Photoshop.
Double-click on the Background layer in the Layers palette to change it to a standard image layer. Click the Add Layer Mask button at the bottom of the palette to attach a mask to the image layer. Click D to set the foreground/background colors to the default (black and white) and make sure that black is the foreground option.
Click on the mask in the Layers palette to make it active and then with a soft-edged brush paint out the problem areas on the source image. If you paint over an important detail, simply select white as the paint color and repaint this section to restore. Save the file labeled “filename-mask.” Repeat for all problem source files.
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