I’ve received several inquiries about channels and color adjustment, so I thought I’d make some additional information available. Using channels for color correction is one of the oldest Photoshop tricks in the business. And you can use most of the same techniques in PhotoImpact.

One of the pitfalls of using Tone Map, Hue and Saturation, or Brightness and Contrast for color correction is that any changes made to an image without a mask are global. The Tone Map provides the most control in this respect, but, still, an adjustment made with the Shadow slider affects every color across the image. If you give the image some pop with the Midtone slider every pixel except those at the far ends of the curve gets at least somewhat brighter. Often, this is all the image needs to be usable. But what if you are happy with the shadows and midtones and only want to adjust the true highlights? You can move the Highlight slider a bit left or right, but if it’s not enough you have to move it a bit more. And soon you’re affecting more and more pixels other than the ones you’re trying to fix.

The solution, of course, is a mask. If you are lucky, you can isolate the crucial area with one of the Selection tools, such as the Magic Wand or Lasso tool. We are rarely that fortunate with most images, so let’s check out another method – our old friend, the Calculation dialog box.

If you read Part 2 of the composite editing series, you’ll remember that we used the channel information of an image as a mask for color correction. It’s truly the best way for many photos. You only need some foresight and ingenuity. Open the Rural Town image.

A quick inspection reveals an overall lack of contrast, and this photo could use a bit more color saturation also. If you click on Histogram from the Format menu (and make sure it is set at Luminosity), you can see a graphic representation of this image’s problem.

If you’ve never looked at a Histogram before, you only need to know that the left side of the scale is pure black (valued at 0) and the right is pure white (valued at 255). The graph shows the number of pixels at all possible 256 values of luminosity. You can move the slider left and right to see the actual pixel count at each value under Statistics. Notice the big spike at about the 210-214 range. And also notice that are practically no pixels above the 230 value. This picture needs some bleach!

If you’re in quick fix mode, do this: use the Tone Map and crank up the Highlights to +15. It’s definitely a better picture now, and if you reopen the Histogram you’ll see we now have pixels at all values. But that spike is still there (not always a bad thing, but for this type of image, it is), and, if you press CTRL+Z and CTRL+Y to compare the new image with the old, you’ll see the color depth is already a bit washed out. Undo the Tone Map.

Choose Format/Calculation to open the dialog box. Usually, to make a decent mask from a channel, we need to find the channel with the most contrast, and it is almost always the Blue or Red channel. Click on the far left Channel drop-down menu and choose Red. You can now use your up and down arrows to scroll through the Red, Green, and Blue channels, looking for the one that offers the greatest contrast. In this case it’s the Blue, so choose it and click OK.

Now it’s time to decide what we want to accomplish. If you flip back to the original photo for a minute, you’ll see that the greens and browns are quite good. So if we can isolate those areas and just correct the highlights, we should be well on the way. Remembering mask logic, we’ll make the dark areas very dark, so that no changes can be made to those areas when we correct the others. Go back to the Blue channel image and open the Tone Map dialog. Set the Highlight value to +25 and the Shadow value to –50. These settings should change the necessary mask areas to white and black, and the gray areas will be only slightly affected when we use the Tone Map again on the overall image. Of course, these settings will vary from image to image, and it always takes some experimentation.

The next step is to change the image to grayscale so we can use this as a mask. Choose Format/Data Type and choose Grayscale. Now when we switch to the original color image, and choose Selection and then Import Selection, our mask will be there, ready to load. Make sure “Open grayscale images” is selected, then click OK.

Open the Tone Map dialog again and try these settings: Highlight +25, Midtone +25, Shadow –25. Click OK. If you open your Layer Manager, you’ll see you have a new object, which always happens when you color correct a selection. In this case, it’s quite handy, because if you click the Eye icon on and off you can see the changes you’ve made. Notice that the greens and browns have not been altered and retain all their color depth. Press SHFT+M to merge this object.

Take a look at the new Histogram now. Looks pretty good, although if you investigate there is slight spike at value 255. (The sky is too white, but that’s an easy fix.) Open the Hue and Saturation dialog and change the Saturation to about +20 or 30, depending on your monitor, and click OK. It’s looking better already. One click with the Magic wand set at 25 will select the whole sky. Load up the Paint Brush with some light blue and paint away. Close the image when you’re done, or save it for comparison, if you like,

Part 2

You have to realize that the variations of this method are endless. Sometimes it’s a lot of work, and sometimes things fall into place immediately, as they did with that last image. Mostly it’s a lot of work- trial and error, this channel, that channel, or a combination of two channels. But if done often enough, it becomes second nature, and you often know instantly what needs to be done and how to do it. For something a bit more complex, open the Desert Bush image.

Here’s a picture with a different kind of problem. Too many dark pixels dominate the image. Open the Histogram to see for yourself. I do love that deep blue sky and want to keep it, although the right hand portion below the moon could use a tad more drama. I’ll show you one of the more legendary quick-fixes that works surprisingly well on so many photos. Open the Calculation dialog box. Make sure the Master (or Composite, it should be called, because it’s the combination of all channels) Channel is chosen for both Foreground and Background. Check the Mask box. If you scroll through the three channels, you’ll notice the Red channel is darkest in the sky area we want to protect and also the lightest around the bush area. So choose Red and in the Operation drop-down menu choose Addition. Click OK.

If you are familiar with Blending modes (and if you’re not, go straight to Stephanie’s Tutorial Pages and don’t come back till you are), you’ll remember that Addition adds the value of each pixel to the one beneath it, always resulting in a “brighter” value. And our mask protected the sky (except notice that new contrast under the moon!) while it brightened the bush. You can close the original image.

For some further brightening, reopen the Calculations dialog. At this point, it’s no longer necessary to protect the sky, and we’ll look for the greatest contrast channel. Once again it’s the Blue, so choose it in the Foreground section and click OK. Open the Tone Map. We will really crank up some contrast by setting the Highlight to +75 and the Shadow to –25. What we have now is the reverse of what we want as a mask. No prob. Choose Format/Invert. Change the Data Type to Grayscale.

Switch back to our new color image and once again import the selection. You can see that the sky is preserved, and only the bush and ground space will be corrected. Open the Tone Map and move the Highlight slider to +20 or so. Click OK. Use the Layer Manager to toggle this correction off and on. You can always delete the Object and begin again, if you’re unhappy with what you’ve done.

Conclusion

Fortunately, neither of these images needed serious adjustment. But you can fix some very, very bad photos using Calculation and channels. If PI had a Channel Palette ala Photoshop, it would all be easier, but, hey, it’s a $100 program. And has some very powerful tools.