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Extracting the foothills

From: Digital Matte Painting: Changing a Scene From Summer to Winter

Video: Extracting the foothills

I am going to use a different technique to deal with these background foothills: one on the right and one on the left. I need to separate the foothills from the background mountain, because I'm not going to handle the mountain the same way. Once again, I am going to look to my alpha channels and see which one has the most contrast between the foothills and the background mountain. The blue channel is the best choice because it offers the most contrast. Duplicate the blue channel and then apply a Curve to it.

Extracting the foothills

I am going to use a different technique to deal with these background foothills: one on the right and one on the left. I need to separate the foothills from the background mountain, because I'm not going to handle the mountain the same way. Once again, I am going to look to my alpha channels and see which one has the most contrast between the foothills and the background mountain. The blue channel is the best choice because it offers the most contrast. Duplicate the blue channel and then apply a Curve to it.

You'll want to make everything on the foreground foothills black and everything on the background mountains white. Pull the white point to the left till the mountains turn completely white. Then pull the black point to the right, until the foothills turn completely black. You won't be able to get it absolutely perfect, but it'll work fine with a little cleanup. Now zoom in on the alpha channel and with the Lasso tool, get rid of this little bit of bleed-through from the background mountains.

I've got black in the background channel. I am going to press X to load in white. That way when I hit Delete, it'll fill the selection with white. Now I'll quickly clean up the other side. Once again, I press Delete to load white into the selection. I need to do some cleanup on the interior of the foothills also. Press X to swap the foreground and background colors and load black into the background color picker. The house still needs to be removed from the alpha channel.

It's helpful to turn on the RGB along with the alpha channel, so you can see what you still need to do. Press Delete to fill the selection with white. While using the Lasso tool, if you press and hold the Option key on the Mac or the Alt key on the PC, it temporarily turns the regular Lasso tool into the Polygonal Lasso tool, so you can lift and click, lift and click, to create straight-line selections. I am having a hard time seeing the RGB through the alpha channel, so I'm going to select the alpha channel I'm working on, then go to the upper-right corner of the Channels panel and choose Channel Options.

You can reduce the opacity of the color overlay of the alpha channel so you can see the RGB channel more clearly. Now remove the house on the far left from the alpha channel. You can turn your alpha channels on and off so you can see exactly what you're getting in the selection. Now zoom out and you can see there is a little bit of cleanup at the bottom of the house and the snowfield. Now select All, Command+A or Ctrl+A, and then hit Invert, which is Command+I or Ctrl+I to invert the entire alpha channel.

Now, Command+Click or Ctrl+Click into the layer icon preview of the alpha channel to load the selection that contains just the background hills. What we want to do is take the black-and-white Information from one of these alpha channels, copy it out, and make a new layer and paste it into the RGB document. But we need to figure out which channel has the right amount of detail and contrast. The red channel doesn't have much detail and is too dark. The green has a nice amount of detail and good contrast. And the blue has too little detail and contrast.

So the green channel is the best one in this instance. Before you copy the green channel out, you need to deal with the ragged edges on the selection. You can do that by Command+Option+ Clicking in the layer icon preview on the Mac and Ctrl+Alt+Clicking in the layer icon preview on the PC. That subtracts the mask from the selection and now it has only the foothills. Making sure that the green channel is selected, do a Copy Merged. The keyboard shortcut for that is Command+Shift+C, or Ctrl+Shift+C. That copies all of the pixels inside of the selection, not just the pixels on the current layer.

Click on the RGB icon in the Channels panel to return to full color and then go into the Layers panel. With the Snow layer selected, paste the information from the green alpha channel into a new layer. The selection from the alpha channel is now pasted into the layer in black and white. Right now it doesn't look very much like a snowy hillside, but watch what happens when I apply a curve to it. Now drag the white point quite a bit to the left, but still try to avoid severe clipping or the tones going completely white.

Also, drag the midpoint up, until it starts to look like the hillside is covered with snow. The white point still needs to be pulled in a bit and the midpoint even further up, but it quickly begins to look like a convincing snow-covered hillside. The one downside of using alpha channel information like this is it comes in completely black and white, and there is none of the natural color variation that is found in nature. We'll deal with that by restoring some of the color. First, name the layer Foothills and zoom out again so you can compare the color to the rest of the plate.

Select the Brush tool again and holding down the Option or Alt key, select some of the color from the snow. Now choose a large soft round brush and Command+Click or Ctrl+Click into the layer icon preview for the foothills to load the selection. Then, from the dropdown menu to choose to the Transfer mode for the brush, choose Color. Now, softly brush that color into these foothills so that they more closely match the foreground snow.

Next up, we will deal with the background mountains and make them look like they are in the middle of winter.

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