The Color Selection Method - Adobe Photoshop

One of the most common methods of creating mattes is to mask using a specific color selection. This is the reason for greenscreens. The green is “pulled” from the frame/image to allow easy replacement. To pull a greenscreen effectively, your subjects against the greenscreen should not have any green shades within them (unless, of course, you want to have that piece of your subject disappear). Oftentimes puppeteers do just that by wearing invisible body suits that are the same green as the greenscreen. Of course, rarely can colors be so controlled. Even greenscreens suffer from spills and other color-contaminating issues that make pulling them less than perfect.When you are dealing with an average photo, the color selection method is rarely used on its own.

Easy Case(Read: Ideal Case)

First start with the simple ideal steps of color selection.

  1. Open GreenScreen.tga, see Figure below
  2. This is a perfect greenscreen, with no spills and an even green backing.

  3. In the upper menu bar, go to Select > Color Range,shown in Figure belowYour cursor now looks like the Eyedropper tool when you take it out onto the image.
  4. This allows you to create a mask based on a color

  5. Click the green area of your image. You see the selection as a mask in the Color Range dialog box. See Figure below .
  6. Select the green areas. The mask is previewed in the dialog box

  7. Move the triangle under the slider down to decrease the amount of fuzziness
  8. Although you could put it to 0 in this case, generally I like to have at least a little bit more fuzziness to avoid aliasing and stray pixel dropouts.

  9. Click OK. This gives you a clean selection of all the green screen areas.
  10. Go back to the menu bar and choose Select > Save Selection.
  11. Name your channel, make sure it says New in the Channel selection box, and click OK. Figure below shows this step.
  12. Saving your selection creates a matte.

You now have what is commonly called an alpha channel. You can see it at the very bottom in your Channels layer and in Figure below.

Clicking your newly formed alpha channel in the Channels palette shows it in your canvas window.

By convention, white is supposed to be the selected area and black is the unselected area. As you can see, your matte is reversed. Invert it before passing it on.

The color selecton method

  1. Click your alpha channel in the Channels palette and press Command+A (Win: Ctrl+A) to make sure that you have the entire canvas selected.
  2. Press Command+I (Win: Ctrl+I), the shortcut for Image >Adjustments > Invert.Your alpha channel should be inverted, with the greenscreen area black. See Figure below
  3. Now the alpha channel has the proper conventions

  4. Choose File > Save, which should save your image with its alpha channel

Difficult Case(Read: Life Example)

Say you need to create a matte for a sky. It’s mostly blue, and in high contrast to the island edge. It shouldn’t be too hard, right? And there isn’t really anything else blue in the scene…except that big body of water called an ocean. You’ll get with that.

  1. Download from the web site and look at TongaVisit.tif shown in Figure below.
  2. This sky needs a matte.

    After you have finished wondering how you can get a tax-deductible vacation to a South Pacific island (be thankful that I didn’t use the one of me and the…well, I’ll stop there), continue on.

  3. Select the Magic Wand by pressing W.
  4. The results of my first click.

  5. Set the tolerance to 8 and make sure Contiguous is not selected.
  6. You want to get all the nooks and crannies between the palm trees without having to go and select them yourself. In this case, being lazy is also faster and better.

  7. Click the sky. Yes, anywhere.Notice that although it selected most of the sky, it didn’t select all of it.
  8. Hold the Shift key and click an unselected area of the sky. I click the lower-left corner.See, you are almost there.
  9. Hold the Shift key and click the cloud—voilá! The entire sky seems to be selected in Figure below.
  10. Here is the selection of the sky.

    By now you’ve probably noticed the little sparkles in your ocean. That would be normal for an ocean, if it weren’t a still image.

  11. Select the Marquee tool by pressing M.I like to have the window opened up a bit to show the edges of the image when I’m using the Marquee tool.
  12. Since you’re trying to subtract any selections in the ocean, hold down the Option (Win: Alt) key and drag around the sparkles.
  13. Take a close look to see how clean your selection is. Figure belowshows the image.
  14. As you can see, your clean selection wasn’t perfectly clean.

  15. Click the Quick Mask Mode button.
  16. Select the Zoom tool by pressing Z; Zoom into the section of the image around the palm trees.You can see some of the stuff still unselected in the sky. Clean this up while you are in Quick Mask mode.
  17. Switch to your Brush tool by pressing B.
  18. Choose a soft brush with 100 percent opacity and diameter of 9 pixels, like you see in Figure below Figure.
  19. This is the brush I started with for clean up. You don’t need to use these exact settings; it is ultimately up to you. This was to provide a guide.

  20. Take a look at your swatches.They would have switched to black and white regardless what colors you had previously. This is because your brush is painting a virtual alpha channel to represent the selection, which is only grayscale.
  21. Your foreground needs to be white, so press X until the white swatch is forward.
  22. Use the left and right brackets ([ and ]) to increase or decrease your brush as you see fit. See Figure below Figure.
  23. The cleaned matte.

Whew! What started out as a simple color selection with an easy subtraction didn’t turn out so simple after all, did it? In actuality, an experienced digital painter would probably not use a color selection tool on a virtually all blue image. The point is that you can still use this technique, regardless of whether it is optimal.


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