Some useful Photoshop Tricks
Anti-aliasing, Feathering, use of the Quick Mask and Refine Edge tools are the subjects of this tutorial.
Anti-aliasing refines the edge of a curved selection to make it look relatively smooth. Anti-aliasing, like other selection refinements, doesn't show up in the default marching ants’ view of a selection boundary. To demonstrate anti-aliasing in Photoshop, we need to use a workaround.
Ask a random Photoshop user what anti-aliasing means and you may get a blank stare. I recommend teaching this topic by creating an anti-aliased selection and then comparing its appearance to that of an aliased selection, demonstrating that anti-aliasing refines the edge of a curved selection to make it look relatively smooth. Anti-aliasing, like other selection refinements, don't show up in the default marching ants’ view of a selection boundary. To demonstrate anti-aliasing in Photoshop, we need to use a workaround.
Create a new document (File>New), select the Elliptical Marquee tool, and leave Anti-alias checked in the Options Bar. Click-and-drag to create an anti-aliased oval selection. These aliased and anti-aliased selection boundaries appear identical when they’re represented by “marching ants”.
To see the difference between them, click on the Edit in Quick Mask mode icon at the bottom of the Toolbox (or press Q). The anti-aliased selection looks smoother than the aliased selection. Zoom in for a closer comparison. The aliased selection looks jagged because all of the square pixels at its edge are fully selected. The anti-aliased selection appears smoother because its edge has some partially selected pixels that appear at varying levels of opacity.
To soften a selection boundary, we can use Feathering, which blurs the pixels on both sides of the boundary. To demonstrate how feathering differs from anti-aliasing, draw another elliptical selection, choose Select>Feather, enter a number of pixels in the Feather field, and click OK. Alternatively, enter a number of pixels in the Feather field in the Options Bar before creating the selection. The feathering effect isn’t visible in the marching ants of Standard mode; however, if you enter Quick Mask mode again, you can see that the blurred edges of the feathered selection are different than the anti-aliased, non-feathered edge of your other selections.
The problem with feathering is that it only allows you to guess at the number of pixels to enter into Feather field. One workaround is to apply a Gaussian Blur in Quick Mask mode. This creates the same soft edge you’d get by feathering a selection but offers the advantage of a live preview.
Here’s how: Create a non-feathered selection, and enter Quick Mask mode. Double-click the Quick Mask mode icon to open the Quick Mask mode to open the Quick Mask Options dialogue, choose Selected Areas and increase the Opacity slider until you clearly see both the edge of the mask overlay and the object. If necessary, choose a more contrasting colour in the Colour field then click OK. Apply the Gaussian Blur filter (Filter>Blur>Gaussian Blur) to the Quick Mask. As you increase the Radius slider in the Gaussian Blur dialogue, check the greyscale preview in the dialogue and the live preview in the image to judge how much blur to apply to the mask to soften its edge. (If you can’t see the edge, click in the preview and with the Hand tool, move the selection.) Click OK and press the Standard mode button to exit Quick Mask.
Contacting a selection boundary
An object you’ve selected and brought into another image often carries a fringe of surrounding colour from the first image. You can use the Contrast slider in the Select>Modify menu to try to eliminate a fringe by “choking” the selection boundary. Unfortunately, this is another selection refinement that requires guesswork. Choosing the number of pixels by which to contract a selection is a hit-and-miss affair. In Photoshop, Quick Mask offers a workaround for this. Open an image, make your selection and use the Quick Mask Opacity and Colour options to get a good view of the mask and the underlying object. In Quick Mask mode, apply the Maximum filter (Filter>Other>Maximum). As you increase the Radius slider in the Maximum dialog, the mask contracts in size. Use the live preview in the document window to decide how much to increase the Radius to eliminate a selection fringe. Choose OK and exit Quick Mask Mode.
Using Refine Edge
The workarounds described above are no longer necessary in Photoshop CS3. The new refine Edge feature offers a better way to apply selection edge refinements that eliminate much of the guesswork. Let’s try it. After making a selection with any tool, click on the Refine Edge button in the Options Bar to open the dialogue.
(If a selection tool isn’t active, you can access the Refine Edge dialogue by choosing Select>Refine Edges.) Use the intuitive sliders in this dialogue to control the amount of edge refinement, similar to feathering or contracting. As you move the sliders, you’ll see a live preview of the selected object against your choice of backgrounds: Standard (marching ants), Quick Mask, On Black, On White or Mask.
In the Refine Edge dialogue, you can easily demonstrate anti-aliasing by showing selections against the mask background. Or drag the Feather slider to set the number of pixels by which to blue a selection, judging the results against a live preview.
If you’re selecting an object to copy-and-paste onto a light-coloured background, try previewing it against a white background to check for the fringe. Then move the Contract/Expand slider to the left to contract the selection and minimise the fringe. The new Radius slider does a nice job of refining selection edges where there are soft transitions, and the Contract slider helps firm up and remove fuzzy artefacts along a selection edge. You can toggle through the various preview backgrounds by pressing F, and you can temporarily view the image by pressing X. Click OK when you’re done.
Refine Edge features are faster and more intuitive than the workarounds of old.
- Other ways to demonstrate anti-aliasing are to compare the edges of filled or deleted selections; or view selections as greyscale images in an alpha channel. After creating the anti-aliased and aliased elliptical selections, click the Load Selection as Channel icon at the bottom of the Channels palette to view the alpha channel in the document window. The anti-aliased oval looks smoother than the aliased oval.
- Another alternative to feathering blindly is to create an alpha channel from a selection, view the alpha channel, and apply a Gaussian Blur with a live preview to the alpha channel.