Today’s Question: Zooming all the way in [in Photoshop], you can see each individual pixel. But I’m able to place a guide at different locations within a pixel. How can this be? You can’t cut within a pixel, can you? Shouldn’t the Guide snap to the boundary of that pixel?
Tim’s Quick Answer: It is indeed possible to place a guide in a position that does not align with the edge of the pixels in your image, which I agree is a bit unusual at least in the context of a raster (pixel based) image. However, it is possible to have your guides “snap” to the edges of pixels in an image.
More Detail: For those not familiar, Guides in Photoshop allow you to position non-printing horizontal or vertical lines within an image. Those Guides can then be used to align objects, force objects to snap to a specific position, keep track of margins or borders in an image, and more.
There are several steps required in order to force new guides you create on an image to snap to the edge of the pixels in the image. The first is to make sure the “snap” behavior is enabled. To do so, you can confirm that there is a checkmark icon to the left of the Snap item on the View menu in Photoshop. If there isn’t a checkbox shown to the left of “Snap”, that indicates the feature is disabled, and you can select Snap from the View menu to turn this feature on.
Next, you need to make sure that the unit of measure for the Rulers feature is set to Pixels. If you haven’t already displayed the Rulers for the image, you can choose View > Rulers from the menu to turn on the Rulers display. You can also press Ctrl+R on Windows or Command+R on Macintosh to toggle the Rulers display. Then right-click on one of the Rulers (at the top or left edge of the image) and choose Pixels from the popup menu that appears. Note that you can also change the unit of measure for the Rulers on the “Units & Rulers” page of the Preferences dialog.
With these two options set, you will be able to create a new Guide that snaps to the edge of the pixels within the image. A Guide can be created by simply clicking on one of the Rulers and dragging into the image. To force that Guide to snap to the edge of pixels, hold the Shift key while dragging the mouse to the position where you want the Guide created. The new Guide will then “snap” to a pixel boundary as long as you’re holding the Shift key when you release the mouse to place the Guide.
So, in concept all you need to do to snap a guide to the pixel boundary is hold the Shift key while dragging from a Ruler to create a Guide. However, you also need to be sure the Snap option is enabled, and that the unit of measure for the Rulers is set to Pixels.