One blending mode that I’ve started to use more and more is the Difference blending mode. Some times I need to see visual changes on a change, especially when some precise control is needed.
For example. I am working with a precise curve at the moment and it needs to be adjusted slightly. The process behind the curve changes is a physical print and is extremely sensitive to any tonal changes on the image.
The curve below is opening up the shadow area (described in the upper most right area of the curve) and giving a separation of shadow tones.
The lower portion (lower left hand area of the curve) is controlling the opening of highlight values.
However, the highlights are not as open as I would like, but too much will flatten the image and reduce any contrast.
One option is to guess the change by duplicating the curve layer and turning on/off each curve adjustment and eyeballing the difference, but it’s very hard to be precise (as you can see below).
Or, I can take a quick screen shot of both curves and open them into another document. For this, I’m going to use the File / Place Linked command and add both screen shots of the curves in to a new document.
You can see, that the top layer is dominant and overriding the underlying layer. To see the difference between the two, just change the top most layer to be have a blending mode of ‘Difference’ (marked in Blue below).
(Close up image below)
You can also see the structure of both curves, relative to each other, it’s then easy to make more informed decisions on the shape of the curve (usually this type of adjustment won’t be noticeable (especially on the screen)), however, when printing to paper (especially with alternative processes), small adjustments can be the difference between a good image and an amazing image.
The beauty of using the ‘Place Linked’ command is that when taking screen shots of subsequent curves, the appropriate layer can be updated, by using the re-link graphic on the properties panel of the linked layer
This process can also works for graphics as well, when used within the canvas (as opposed to making a screen shot)
