From the course: Photoshop: Creative Lab Color

Lab inversions and L-channel inversions - Photoshop Tutorial

From the course: Photoshop: Creative Lab Color

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Lab inversions and L-channel inversions

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- [Instructor] The inversion adjustment in Photoshop which is accessed via image adjustments, invert is a bit of a one-trick pony, since it only does one thing. But this one thing, channel inversion, turns out to be an incredibly powerful tool in conjunction with LAB because of the color opponent architecture of the LAB channels. To see this in action, let's start by taking an image with a white background such as this foxglove. I photographed this flower by laying it flat on a light box. It's an interesting but totally irrelevant to LAB color that the heart medication Digitalis is derived from the foxglove, and it's the name of the flower's genus. So if you're photographing one and you come across foxglove, don't start munching the petals without a note from your cardiologist. Start by converting the foxglove image to LAB color. Image, Mode, LAB color. Now in the Channels palette, you can see that you have all three LAB channels up. Lightness, A, and B. And this is a good place to verify that you're in fact in LAB color. So now to apply the inversion adjustment to all three channels, I go Image, Adjustments, Invert. As you can see, with all three channels of the foxglove inverted, black is substituted for white, but also the color areas of the image are impacted. A three channel inversion is very attractive and can be useful in it's own right in the context of interaction with other LAB adjustments, but it's not as pure a visual effect as inverting a single L channel, which leaves the color information in the image untouched. To see how this works, I prepared a duplicate copy of the original image, which is still on white, and I can make sure it's active. I convert it to LAB color. Image, Mode, LAB color. And with the lightness channel selected and all channels visible, I can go Image, Adjustments, Invert, and you can now see that only the lightness channel was operated on, and the colors of the petals, and the stalk of the digitalis are basically at the original colors. I said basically, but in fact they're exactly at the original colors, and this is sort of a hard thing to wrap your brain around, because it's not what one's used to in Photoshop, but it's really very important because you've only operated on the lightness or L channel. The distinction within a channel between visibility and being selected is a really important one, and it's easy to overlook it. Keep in mind that to do further work with an image where one channel was operated on, you're going to need to reselect all the channels like this by clicking within the LAB bar in the channels panel. I find that inverting the L channel on an image with a white background such as a high-key image photograph with back lighting like this flower on a light box is a very useful part of post-production. Paradoxically also sometimes the best way to create an image on a black background is the start with a photograph on white.

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