by Julieanne Kost
When working with the vector tools (such as the shape tools), Photoshop has a preference to “Snap Vector Tools and Transforms to Pixel Grid”. This preference is extremely helpful when creating shapes that need solid, straight edges as it snaps the edges of the shapes to be fully aligned to a pixel, preventing soft, anti-aliased edges).
This illustration demonstrates three options. The first shape was drawn with the Snap Vector Tools and Transforms to Pixel Grid preference turned on. As a result, all sides of the rectangle are solid (note, even though the initial rectangle shape might not have been drawn in perfect alignment to the pixel grid, Photoshop automatically snapped the rectangle to that grid because the preference was turned on).
The middle illustration was drawn with the Snap Vector Tools and Transforms to Pixel Grid preference turned off. This resulted in anti-aliased edges (varying levels of opacity) because the rectangle (the vector path) was not perfectly aligned to a pixel edge when the rectangle was drawn.
The final illustration is the same rectangle as in the second illustration ( the Snap Vector Tools and Transforms to Pixel Grid preference was turned off), however the Align Edges option was enabled for the rectangle shape layer (in the Options bar) after drawing the rectangle. Enabling the Align Edges option “jumped” the fill of the rectangle to the nearest pixel grid (you can still see that the original shape (the black path), is not aligned to the pixel grid, but the fill is being forced to Align Edges to the edges of the pixel grid. Align Edge is a convenient way to align objects to the pixel grid on a “per shape layer” basis if you want to turn off the Snap Vector Tools and Transforms to Pixel Grid preference.
However, I expect that most people will leave the Snap Vector Tools and Transforms to Pixel Grid preference turned on as it will help to avoid anti-aliased edges when drawing shapes as well as transforming them making alignment clean and precise.