What if you need your gray values to be between 10 percent and 85 percent? The range is correct if you enter 26–217 in the Output Level box, but your 50 percent gray is shifted to 122(48 percent). This won’t do for something like a bump map, which must use 50 percent as a neutral point.
What you need to do is first take everything below the 50 percent mark and compress it to go between 10 percent and 50 percent. Then take everything above the 50 percent mark and compress it between 50 percent and 85 percent. Luckily, I happen to know an ancient Chinese secret that solves this problem. (Well, maybe it’s not really ancient…or Chinese…and if I tell you, I guess it won’t be a secret either…)
The black/white demarcation should go right through your #1 marker.
I chose the blue channel, but the R and G channels would have been fine, too.
I named my mask >50.
The adjustment layers have been hidden and now you can see your original gradient.
Load your alpha channel.
The secret numerology.
Great. Now that you have the upper(lighter) range fixed, do the same for the lower(darker) range.
Do the same technique to bring up the lower sections.
This time look at marker #3 and bring in the range from 10 percent to 50 percent by sliding the right slider in.Voilá! Your bump map is now complete.
The same gradient altered to fit the numbers.
Your middle, flat areas are still at 50 percent and your range is between 10 percent and 85 percent gray. Bravo! Using these techniques, you can squash and stretch any grayscale image to the values that you need. The more you do it, the more intuitive it becomes.
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