Straightening Crooked Images

The Measure tool is a fantastic tool for measuring distances between two points in a document. You can measure a distance in any of the units set for Rulers in Preferences ^ Units & Rulers ^ Units. If you are not sure of the current units of measure, just double-click a ruler to bring up the Units & Rulers preferences screen.

To use the tool, click and drag out a line in any direction and then read its length and angle in the Info palette or the options bar. Hold down the Shift key to constrain the measure lines to 45, 90,135,180, -135, -90, and -45 degrees.

The tool has a couple of "hidden" features that are not displayed on the options bar when you select it. These include the ability to rotate image data arbitrarily and to measure angles the way a protractor does. Both of the features can be used with great success to straighten crooked images or to rotate objects with great precision.

To invoke the protractor mode to measure angles, take the following steps:

1. Select the Measure tool (hidden under the Eyedropper tool in the Toolbox).

2. Draw a measure line by clicking and dragging with the tool; the line can be at any angle.

3. Hold down Alt (Windows), Opt (Mac OS), hover over the either end point and, when the icon acquires a protractor and a plus sign, click and draw another measure line at an angle to the first (Figure 11-6). You can also double-click the end point to invoke the protractor mode.

4. Read the measurements either on the options bar or in the Info palette.

Figure 11-6: Switching the Measure tool from measure lines to protractor mode.

When you're working with the Measure tool, here are a few pointers to keep in mind:

■ To change the angle of the measure lines, click an end point and then drag.

■ To extend an existing measure line, drag outward from either end point.

■ To reposition a measure line, or the protractor, click it and then drag to a new position.

■ To draw an absolute straight measure line, hold down the Shift key as you draw.

■ To clear a measure line, or a protractor, click the Clear button on the options bar, or drag it out of the document window with the pointer.

When you use the Measure tool to straighten a composite image, it requires a slightly different treatment to straightening objects on a layer. Take a look at the steps required to straighten a composite image first and then objects on a layer.

1. Select the Measure tool.

2. Find a straight line in your image, one that you want to be perpendicular. Draw a measure line beside it by clicking and dragging with the tool.

Figure 11-6: Switching the Measure tool from measure lines to protractor mode.

3. Choose Image ^ Rotate Canvas ^ Arbitrary.

4. You will see the angle by which the canvas needs to be rotated displayed in the angle text box (Figure 11-7). Click the OK button to apply the measurement; the image will upright itself.

Figure 11-7: Straightening a crooked photo with the Measure tool and the Image^ Rotate Canvas ^ Arbitrary command.

If you want to use the preceding method to rotate an object arbitrarily on a layer, first, it's not at all obvious which command you should use (for example, Rotate Canvas ^ Arbitrary obviously won't do because it rotates the whole canvas). Second, after you have found the right command and invoked it (it's actually Edit ^ Transform ^ Rotate), the object rotates counter to expectations. That is to say, if you want it to rotate clockwise, it rotates counterclockwise, and vice versa. To overcome the contrary behavior, you need to use the Measure tool in protractor mode, or enter a negative value in the Set Rotation text field, after the command has been invoked.

To rotate an object arbitrarily using the Measure tool in protractor mode, take the following steps:

1. Select the Measure tool and draw a horizontal measure line; hold down the Shift key to constrain it to a straight line.

2. Hold down Alt (Windows), Opt (Mac OS) and draw out a protractor edge from the start of the horizontal line so that it hugs a diagonal line that you want to be perpendicular (Figure 11-8).

3. Choose Edit ^ Transform ^ Rotate (if the command is not available, make sure that you are working on a normal layer and not a Background layer).

4. Accept the transform by pressing Enter (Windows), Return (Mac OS).

Figure 11-8: A—The Measure tool in protractor mode ready to rotate an object. B—The object rotated by selecting Edit ^ Transform ^ Rotate.

The content will be rotated by the number of degrees described by the protractor. Remember— apart from 90-degree transforms, all rotations result in the content's being interpolated, using the Image Interpolation algorithm set in Preferences to resample the data.

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