Advanced Range Adjustments - Adobe Photoshop

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…)

  1. Hide the levels layer with which you were just working by clicking on the eye icon next to the layer in the Layers palette.
  2. Create a new threshold adjustment layer by either going to Layer >New Adjustment Layer >Threshold or clicking the Create New Fill or Adjustment Layer button on the bottom of the Layers palette and choosing Threshold, and accept the default name by clicking OK in the New Layer dialog box.
  3. In the Threshold dialog box, enter 128 for the threshold value if it is not already so. This is, in effect, a mask for the areas that have a brightness greater than 50 percent.
  4. The black/white demarcation should go right through your #1 marker.

    black/white demarcation should go right through your #1 marker.

  5. Click OK to accept and close the Threshold dialog box.
  6. Switch to the Channels palette. Since this is a black and white image, all the channels should look the same.
  7. Duplicate one of the channels for calling on later: Click a color channel, click the triangle button to open the Channels palette menu, and choose Duplicate Channel.
  8. I chose the blue channel, but the R and G channels would have been fine, too.

    chose the blue channel, but the R and G channels would have been fine

  9. Give your channel a name—preferably one that is descriptive— and click OK. You see your new channel, called an alpha channel or holdout mask, on the bottom of the Channels palette.
  10. I named my mask >50.

    named my mask >50.

  11. Click the RGB composite channel.
  12. Click the Layers tab to bring forth the Layers palette. Hide the threshold adjustment layer by clicking the Eye icon next to the layer’s name.
  13. The adjustment layers have been hidden and now you can see your original gradient.

    adjustment layers have been hidden and now you can see your original gradient

  14. Go to Select > Load Selection on the menu bar.
  15. In the dialog box, choose your recently made alpha channel from the Channel pop-up menu, make sure this is a new selection, and click OK. This loads the left(light) side of the image as a selection. points out the desirable settings.
  16. Load your alpha channel.

    Load your alpha channel

  17. With the selection still active, create a new levels adjustment layer by selecting Layer > New Adjustment Layer > Levels.
  18. Name your layer and click OK.In the next dialog box, you see the histogram in the middle is chopped off. That is because you only have half of the values selected.
  19. Ensure that the upper limit for the Output Levels is 217
    (85 percent).Notice in the Info palette that the values change in the #1 and #2 markers when you input that value. The #2 marker is perfect at 217, but the #1 marker should come back up to 50 percent.
  20. Slide the triangle under the left side of the Output Level slider slowly toward the center, until the #1 marker in the Info palette reads 128.

The secret numerology.

secret numerology

Great. Now that you have the upper(lighter) range fixed, do the same for the lower(darker) range.

  1. Load the same channel map selection by choosing Select >Load Selection.
  2. Invert the selection by pressing Shift+Command+I(Win:Shift+Ctrl+I) or by going to Select > Inverse.
  3. Now that you selected the other half, go through same methods you just did for the upper half but apply the methods to the lower half.

Do the same technique to bring up the lower sections.

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.

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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