Among the arsenal of image editing aids in Photoshop at your disposal, rulers are perhaps the unsung heroes. They can help you to align content down to a pixel, help create guides, and help control the unit of measure globally—all without so much as a peep. If the rulers aren't visible, you can view them by choosing View ^ Rulers or pressing Ctrl+R (Windows), (Mac OS); the keyboard shortcut is a toggle.
By default, the ruler's zero origin is set to 0x, 0y coordinates (top-left corner of the visible image for us simple folks). However, when you absolutely must set it elsewhere in the document, it's a quick and painless operation. Just click in the box where the two rulers intersect and drag diagonally into the image (Figure 2-4). As you drag, you will see a crosshair temporarily superimposed over the image to help you set the new ruler zero origin. Position the pointer where you would like to set the new ruler zero origin; then let go. That's all there is to it! You can, if you wish, set the new zero origin anywhere in the canvas area and not just the visible image area.
The rulers do not move from their locked position when you drag from the ruler intersection box, but if you look at the zero markers on the rulers, you will see that they have been set to the new position. In the example shown in Figure 2-4, the new ruler zero origin is being set to 100 px horizontal (x) and 100 px vertical (y) from the top-left corner of the image.
To reset the zero origin to the default top left-hand corner, double-click the box intersection.
NOTE The zero origin affects the grid's x, y coordinates and, therefore, also the x, y readings in the Info palette.
TIP To make it easier to set the new ruler zero origin, you can snap it to Document Bounds, Grids, Guides, Layers, or Slices (these options can be set in the View ^ Snap To submenu).
When working on an image, sometimes you need to change the ruler units from their current setting to something more appropriate to help you create or line up an element. Fortunately, you don't need to make a trip to the Preferences dialog box each time you want to make a change. You can do it from much closer: right inside the document window, in fact. Just right-click (Windows), Ctrl-click (Mac OS) the rulers and, from the contextual menu, choose an item (Figure 2-5). Save your energy for something more strenuous—such as making that third cup of coffee of the morning!
Just for the sake of completeness, you can also change the ruler units by double-clicking the rulers to call up the Preferences dialog box; or, in the Info palette, you can click the cross icon in the mouse coordinates section and select an item from the pop-up menu (Figure 2-6).
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