Sharpening using unsharp masks, part 2

To set the scene I recommend you read my earlier post - Sharpening using unsharp masks.

If you work through the procedure detailed above you will create an unsharp mask of your image which, when overlayed with the original image, gives the appearance of sharpening.

Why does this work?

Two desaturated (colour information removed) copies of the image are created, one is inverted (black to white, white to black) and the blending mode (a way of combining two layers in Photoshop) between the two is switched to colour dodge. In this particular blending mode blending with black has no effect on the image contained in the other layer and blending with white gives white. Since the images are the inverse of each other the blended image is white.

Here's the clever bit. The inverse image is now blurred slightly. The new non-white pixels added due to blurring will overlap with the non-white pixels in the original and an edge outline of the original image is observed. These layers are merged to leave the outline which is then combined with the original image using the multiply blending mode. This causes the overlapping areas to become darker (result colour = top colour x bottom colour/255) and hence the contrast in the edges of the image is increased.

This increase in contrast gives the appearance of sharpness.

If you're confused feel free to post a comment. There's more on sharpening to follow.

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