Straightening Buildings, Part 2: How-To Use Distort And Perspective

Back in April and May I had a couple articles showing how to straighten buildings with the Crop tool and the Lens Correction filter. Now we’ll look at a more basic method that can do the job: Distort and Perspective transformations.

Figure 1: The original image.

In Figure 1 we have the same building used in the previous tutorials: Salisbury House in Des Moines, IA. This technique makes use of Photoshop’s transforming options (found under the Edit menu), which are now augmented in CS2 with the same warp tools as those found with Type tool (more on warping in a future post). Depending on your skill level, you may find using Distort and Perspective more intuitive than Crop or Lens Correction; I used Distort and Perspective early in my career, back when I was a relative novice with Photoshop.

Step 1: Select the image (Select –> All, or Cmd/Ctrl-A).

Step 2: Select Edit –> Transform –> Distort. You’ll see anchor points on the four corners of your selection, which will be familiar to those who used the Crop tool earlier to straighten this building.

Step 3: When working with the Crop tool technique here, the anchor points are dragged so the crop area’s boundaries match the skew of the building. When working with Distort, however, the anchor points are dragged so the building’s edges become vertical. In this case, dragging the upper-left anchor point to the left and the upper-right anchor point to the right will bring the verticals in line with the image boundaries (see Figure 2). Remember that you can hold Shift and drag to keep the anchor point from moving vertically, as you can in practically any other dragging action in Photoshop.

Figure 2: The Distort anchor points and guides are visible.

Step 4: When the image looks straight, press Return/Enter to execute the distort. The image may look a bit stretched out after this, so select Edit –> Free Transform (Cmd/Ctrl-T) and drag one of the side anchors to stretch the picture to proper proportions. Hold down Option/Alt while you do this in order to stretch the opposite side at the same time (see Figure 3). Press Return/Enter to execute the transformation, and you’re done!

Figure 3: Stretching the (final) image to restore proper proportions.

USING PERSPECTIVE

The Perspective transformation is almost exactly the same as the Distort transformation; the only difference is that, while Distort treats the anchor points separately, Perspective treats the anchor points as pairs. If you drag one, the other will move to maintain perfect horizontal or vertical perspective. This is great if your image is skewed upon a horizontal or vertical axis; if the image is more cock-eyed and perspective is more diagonal, Distort is more suitable.

In the case of Salisbury House, the photographer was pretty much in line upon the vertical axis so Perspective may be a helpful tool in this case.

Step 1: Select the image (Select –> All, or Cmd/Ctrl-A).

Step 2: Select Edit –> Transform –> Perspective. You’ll see anchor points on the four corners of your selection.

Step 3: Drag the upper-left (or upper-right) anchor point away from the image horizontally. Because Perspective is based on horizontal or vertical perspective, the anchor point will move upon a horizontal axis. You’ll also see the other upper-left/upper-right point move to match the change in perspective. The image will change to match the new perspective, which you can use to judge when perspective is appropriate.

Step 4: Press Return/Enter to execute the transformation. As with the transformation using Distort, you’ll probably want to use Free Transform to stretch the image a bit to compensate for the change in proportions.