Photoshop LAB -- Ch 8 / The Impossible Retouch
The meat of this chapter is a technique for fixing blown highlights using LAB's impossible colors and some advanced blending techniques. Since I already knew about LAB steepening, color balancing, and sharpeneing, this has been the most useful chapter of the book so far. I finished the chapter last Friday and already a collection of 4 examples has grown up just from questions on dgrin.
As you might remember, LAB can represent some interesting but physically impossible colors because it expresses luminosity (brightness) independently from color. Thus in LAB it is possible to have a in intense blue which is as bright as any color. We know this is impossible in light based devices (monitors, slides) because that blue would be brighter if we also turned on the red and green lights. We know this is impossilbe in pigment because no ink on white paper gives us the brightest possible color. In both cases, white. For the mathematically, inclined, no RGB color can be as bright as R=255, G=255, B=255 which is white. To get blue we have to lower the rad and green values and the result won't be as bright. Ditto, CMYK, where all zeros specify no ink and thus the brightest possilbe color.
LAB is a horse of a different color. L=100, B=(127) is as bright as any color and also as blue. Since this is an impossible color, you wouldn't think it would actually be useful and perhaps it wouldn't be except that photoshop maps it into a real RGB color in order to display it while it is still in LAB and to get it within gamut when converted to RGB. You can experiment in photoshop with the look of these impossible colors by using the color picker. Here you can see that it has decided to show this impossibly bright blue color as a light cyan. Try more positive B values and you will get increasingly unsaturated very light blues. The same goes for the other extremes of A and B as well as combinations of the two.
What this means in practice is that very bright, even blown, colors can take on a very light color in LAB and that the transitions from less bright areas will look natural and gradual. In LAB there is no sudden wall of impossible color to hit, no single point where our bright fleshtone hits its head against the dynamic range barrier of RGB and has to be white not pink.
In essence the technique for getting some color into blown highlights is:
- Convert to LAB if not there already
- Create a duplicate layer
- Do something to the duplicate layer to get the right color into the blown areas, without worrying too much about how the rest looks.
- Use the blending options' blend-if sliders to limit the new color to just the blown areas (or perhaps a little more in some cases.)
- Depending on where that color came from in the step above, use Color blending mode to preserve the detail from the original while substituting color from the layer.
- In some cases, we may need to further limit the blend with a layer mask. But if we have done the blending right up to this point, this can be very crude and not require any careful painting.
I've been vaguely aware of this technique for more than a year. The most obvious application is to get some blue into blown skys. At the top of tis post is a very simple example of this. I took the original at the left and applied the following steps to get the version on the right:
- Convert to LAB
- Duplicate layer
- Curves -- The goal is to construct a nice blue sky. Don't worry about the rest of the shot. I used these L and B curves:
and then the layer looked like pretty bad except for the sky which was a feasible color for a blue sky:
- Open the blending options dialog box for the layer. Mac: Control-Click on the layer, choose blending options.
- Now use the Blend-If slider to keep only the blue sky from the topmost layer and the rest of the image from the background layer:
I started out by moving the black slider from the "Underlying Layer" to the left until only the sky was showing from the top layer. This hapens because pixels from the top layer are blended (shown) only if they lie within the range defined by the slider, in this case, only if they are extremely light.
- At this point the edges of the blend, where they sky meets the rest of the image might probably won't look right. Split the slider you just moved: Mac Option-Click on the slider, and pull the right side of it all the way to the right. This tells PS to gradually grade over from showing the bottom layer to showing the top layer using partial transparancey though the range of the split slider. Pixels as dark or darker than the left side of the slider come from the underlying layer. Pixels as light or lighter than the right side of the split slider come from the top layer. In between the two sliders, pixels get some of each, more underlying layer if they are on the dark side, and more top layer if they are on the light side.
Now to be honest, the example just given doesn't really use the impossible colors of LAB at all. It really just a great showcase of the blend if feature of the blending options. The transition between subject and sky is very well defined (though too detailed for easy manual masking.) Dan's blue sky example is much more challenging, with mountains meeting sky. He wants some of the color of the sky to bleed into the tops of the mountains to make the transition smooth and natural looking. In his example, both the sky and some parts of the mountains are blue but impossibly light.
Here is an example that does combine the blend if option with impossible colors. Aviator327 posted this today:
There are a lot of problems with this shot. It has a distinct blue cast; this could be fixed with an unsymetric B curve. The roses on the girls' heads are dead; this probably requires some sort of selection. The balance of magenta to yellow in the fleshtones is way too high; this will be fixed at least in part by the LAB curves that address the blue cast. But what about the blown areas in the cheeks of the girl on the left and on both of their hands?
I managed to get this result:
Here are the steps:
- Convert to LAB
- Make a duplicate layer
- Use the color sampler to choose a reasonable color from one of their necks.
- With the duplicate layer selected, paint some big splotches to cover their flesh:
- Use LAB curves on the duplicate layer to fine tune the color balance and saturation of the brush strokes so they are nice healty flesh tones, more yellow than magenta and plenty of both.
- Set the blending mode of the duplicate layer to Color. This means that only the color comes from the duplicate layer, but all the luminosity and detail come from the original.
- Bring up the blending options for the duplicate layer by right-clicking on it (I think control-click on the mac).
- Now I needed to limit the blend, so that the color doesn't spill over onto the dresses or hair or teeth. I did this with the Blend If sliders.
- First I limited to blend to exclude the darkest parts of the image:
Note the split slider. This defines a range of blending. The right endpoint defines the end of the range. Colors lighter than this are blended, i.e., come from the duplicate layer. The left endpoint defines the start of the range. Colors darker than this are not blended, i.e., come from the background layer. In between, there is more blend as the colors lighten. Split the slider with option-click (Mac).
- This excluded a lot of what I didn't want from the blend, but there were still places on the dresses which were within the brightness range and were undesirabley blended. So I also adjusted the blend if settings for the A channel.
Here I excludes colors with some green. Again note the split slider to make the transition areas look more natural.
- Here is the image at this point:
Looks better, but I found one bad side effect. I'd like those lips to be redder, instead of flesh toned. At this point, I gave into temptation and used a layer mask on the duplicate layer (with the paint strokes) to reveal the color of the lips from the original.
Now the impossible colors of LAB are really doing some work. In RGB, the blown aras have to be white. There really is no other choice. With the Color blend we get the L value from the background layer and the A and B values from the top laer. Now we get L=100, but we also have positive A and B values, so the blown highlights seamlessly take on a very light flesh tone. The two Blend If restrictions limit the fleshtone smear to just the parts of the image we need.
I used these techniques for one more dgrin thread this week: http://www.dgrin.com/showthread.php?t=21433
The chapter also contains some very useful background about how exactly Photoshop treats impossible LAB colors when converting to RGB or CMYK. The headline here is: always convert to RGB first, even if you are ultimately headed for CMYK.
There is also a parable about duotones and why LAB is the wrong place to make them. In short, it won't make truly white whites or truly black blacks when color blending the second color because it splits the difference just as in the blown faces above.
And for the rare color nerd theory who is also a Shakespeare fan, well there is a treatment of a dialog from Henry IV Part 1 that made me laugh out loud.
In sum, I found this chapter immediately useful. I learned a technique that has found 3 or 4 applications in almost as many days. It's not required for every picture (whew!), but I know of no better substutute when it is required.
If not now, when?