A version of this tutorial first appeared in the September 2004 issue of
SBS Digital Design
Sometimes the real world can’t be captured in one pass. Whether it’s a poster that’s too big for your flatbed, or a horizon that’s too big for your digital camera, there are times when you simply have to stitch images together to get the full picture. For this tutorial, we’ll be stitching together an image that required 3 separate scans.
Before scanning any oversized image, here are some points to keep in mind:
Scan each piece of the image with as much overlap as possible to other pieces
Use only manual exposure adjustments (turn all automated adjustments off)
Use the exact same adjustment and resolution settings used for your first scan for each subsequent scan
Save each scan as a separate RGB TIF or PSD file
Aim for a similar orientation, but alignment is not critical at this point
With the exception of 90° increments, rotation degrades image quality, so try to limit the number of times a given image is rotated
Make a new RGB white-background document with slightly larger dimensions than your original image. Use the same resolution that you scanned at. For this tutorial we’ll use 10” tall x 30” wide at 300ppi and name it scanstitch.psd.
Open scan1.tif, select the entire canvas with Command/Ctrl-A and copy it to the clipboard. Close scan1.tif. Paste the clipboard contents into scanstitch.psd and name the new layer Scan 1. Press V to select the Move tool and use it to position Scan 1 in the middle of the far left edge. Open scan2.tif, select the entire canvas, copy, and close scan2.tif. Paste it into scanstitch.psd and name the new layer Scan 2. Use the Move tool to roughly position it in relation to Scan 1, but do not rotate it yet.
Click on the Scan 2 layer, change its blend mode to Difference. This will highlight any differences between the two layers. Identical areas will appear black.
Select Edit>Transform>Rotate. Rotate by dragging the mouse outside the bounding box, and move by dragging the mouse inside the bounding box. You can fine-tune the position by nudging with the arrow keys. Fine-tune the rotation by clicking in the angle box of the options bar and using the arrow keys (click back in your image to resume nudging). Once the two images are aligned the overlap area will appear as solid black. When finished, hit Enter and change the blend mode back to Normal. To avoid accidental movement, click on the empty square next to the Scan 1 icon and link the two layers’ positions together.
Turn off the visibility of the Scan 1 layer and add a layer mask to the Scan 2 layer. Click in the layer mask icon and paint slightly beyond the overlapping edge with a soft black brush. When finished, restore the Scan 1 visibility.
Repeat steps 2-5 using scan3.tif.
Now you can crop and flatten scanstitch.psd. Zoom in and inspect for any misalignment cues. Use the Clone Stamp tool to repair small misalignments.