Author Ben Long recently asked what, exactly, the Auto Enhance option does when enabled for Photoshop CS3’s new Quick Selection tool. Gregg Wilensky, one of the engineers behind the tool, kindly provided some details:
Auto Enhance enables Quick Selection to generate better quality edges. Without Auto Enhance, the resultant selections are somewhat rough. The focus is on getting a decent selection quickly. However, doing things quickly results in less-than-perfect edges. The edges can be slightly off the mark. Blocky artifacts can arise and are especially apparent where edges are weak. And the selection edges may be sharp even though the actual image edge is soft or vice versa.
With Auto Enhance, we start with the imperfect edges and do several things to improve them. First we flow the imperfect selection mask towards the image edges. It’s kind of like trying to snap to the edges in the image in order to get closer to their true locations. This can correct some of the blockiness.
We also smooth the edge, with a special smoothing which attempts to preserve edge contours. This is the same smoothing that is used in Refine Edge’s Smooth control.
Finally, we capture a more faithful, truer edge profile using the same technology that underlies the Radius slider in Refine Edge. The Radius determines the extent around the edges over which we extract more faithful opacity values for the selection mask based on the image colors and how they change locally near the edges.
The result of all these steps is to produce a much improved selection mask that provides a truer representation of what’s going on in the image. The resultant selection mask follows the image edges better and captures a bit of the color transitions near the edges as opacity variations in the selection mask.
Note that while scrubbing with the mouse, the rough selection is shown; but, when the mouse is let up, the improved results are shown. The difference between making a selection with or without Auto Enhance is usually apparent if you go into Refine Edge and magnify an edge using the Mask preview mode.
So, to paraphrase for my little Arts & Leisures primate brain: turn on Auto Enhance unless you find that it’s taking too long for your images (a function of resolution and complexity vs. processing grunt).
Gregg’s collaborator Jeff Chien says that you might want to work with the option off for high-res files, but that "Once the selection
is done or almost done, you can then enable the option to trigger the edge
smoothing at the mouse up of the next stroke. In case that you have done
with the selection, you can simply paint inside the selected area to trigger
the operation." The results won’t be quite as precise as if you left the option on the whole time, but it’s one way to trade off performance vs. accuracy.