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 building 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.

Figure 1.48

Dragging guides to building edges

Figure 1.48

Dragging guides to building edges

Three Point Perspective Drawing

Notice how the vertical walls lean inward in three-point perspective. You can see this lean as the distance between the guides and the upper portion of the vertical walls. The guides help establish a visual vertical reference line.

10. In the Channels palette, click the Alpha 1 channel. Click the Load Channel As Selection button (leftmost button along the palette's lower edge). Click the RGB channel again.

11. Press Ctrl+T to start the Transform command.

12. Right-click anywhere inside the document window to open a context menu that lists specialized transform tools. Choose Skew from this menu.

13. Hold down the Shift key to constrain motion horizontally. Drag the upper-left handle to the left until the vertical building wall aligns with the guide. Do the same by Shift-dragging the upper-right handle to align with the guide on the right, as shown in Figure 1.49. Click the Commit button on the Options bar.

14. Press Ctrl+D to deselect. Press Ctrl+H to hide the guides. Press Ctrl+R to hide the rulers.

15. Press C to choose the Crop tool. Drag a crop window around the image, and click the Commit button on the Options bar. Figure 1.50 shows the completed two-point perspective image. The building walls are truly vertical.

Figure 1.49

Skewing the image

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