Page 66 - Enhancing CAD Drawings with Photoshop

4386.book Page 49 Monday, November 15, 2004 3:27 PM

SIZING AND TRANSFORMING 49

2. Choose File Automate Crop And Straighten Photos.
3. A new document window appears with the—you guessed it—cropped and straightened photo.
Photoshop is now “smart” enough to deduce the image’s edges and does the tedious work for
you. This is wonderful if you are scanning dozens of photos because you can forget about try-
ing to carefully align them with your scanner bed as Photoshop takes care of it for you.

4. Close the open documents without saving.
Correcting for Two-Point Perspective
Photos of tall buildings tend to have walls that lean inward, especially if taken with wide-angle
lenses. If you extend lines along all the wall edges, they converge at three points in the distance (two
on opposite sides of the horizon, and one directly above), technically called three-point perspective.
In architectural illustration, it is common to draw tall buildings with straight vertical walls because
we “know” in our minds that walls are vertical. In two-point perspective, objects along the horizon
diminish in size with distance from the camera (as they appear to our eyes), but objects running in the
vertical dimension remain straight up and down (not exactly true to our eyesight but intellectually
pleasing nevertheless).
We can simulate a two-point perspective in a photo by skewing the pixels using the transform tool:
1. Open the file Street.psd from the CD.
2. Press Ctrl+A to select all.
3. Click the Channels palette and click the Save Selection As Channel button (second from the left
along the lower edge of the Channels palette).
4. Press D to set the default colors.
5. Choose Image Canvas Size to open the Canvas Size dialog box. Click the Relative check box
in the New Size group.
6. Select Inches from the Width and Height drop-down lists if these are not already selected.
Enter 2 inches in both Width and Height text boxes and click OK. The canvas is increased in
size so that a 2-inch white border surrounds the image; the overall image size has increased.
7. Press Ctrl+R to turn on the rulers.
8. Drag a guide from the vertical ruler and release it where it meets the bottom edge of the build-
ing wall, as shown in Figure 1.48.
9. Drag another guide out from the vertical ruler, and release it along the street corner on the
right side of the image.