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Selecting from a channel


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Selecting from a channel

In this lesson I'm going to show you one of my all time favorite techniques for creating a selection in Photoshop. This method makes use of a color channel as the basis of a selection. Let's take a look at how it's done. The first thing we need to do is to evaluate the individual color channels for the image. So, I'll go to the Channels panel and then click on each of the thumbnails for the red, green and blue channels in turn. What we're looking for is good contrast in the area that we want to select.

In this case, I'd like to make a selection of the poppy. And so, I'm looking for good contrast between the poppy and the sky. Here you can see we have some good contrast for example the flower petal up here at the top has pretty good contrast relative to the sky but down below there's not as much contrast. Taking look at the green channel you can see we have an image that seems to represent a better starting point as a black and white imae But we do have the same problem where portions of the pedals blend very smoothly into the sky. Taking a look at the blue channel, you can see we have great contrast between the Poppy and the sky.

As a result the blue channel is the best starting point for this particular image. In fact that's why I chose this image to demonstrate the technique but I assure you that in many situations even when you don't think it's going to be possible to make a selection this technique can work really well. All you need is a little bit of color contrast. There doesn't even need to be very obvious luminance contrast between the area you want to select and other areas of the image. Since we've identified the blue channel as the best starting point, I'm going to make a copy of the blue channel. If I were to work directly on a color channel, I'd be altering the color appearance of the image, and I most certainly don't want to do that. So I'll drag the thumbnail from my blue channel down to the Create New Channel button, the blank sheet of paper icon at the bottom of the Channels panel. This will create a blue copy channel which is an alpha channel. In other words not a color bearing channel.

It's not one of the red, green or blue channels that determines the color of pixels within the image. I can therefore work directly on this blue copy channel being as destructive as I want knowing it's not going to actually effect the appearance of the image. Now this channel represents good contrast between the poppy and the sky. But I need great contrast so I'm going to choose image adjustments levels from the menu in order to apply a levels adjustment directly to this blue copy channel.

I'll bring the white point inward in order to turn the sky completely white, and I'll bring the black point inward in order to make the poppy completely black. I can also adjust the mid point of the slider as needed. Basically what I want to accomplish is squeeze both of these sliders, the black and white sliders together, so that they are compressed and every pixel in this channel is either pure black or pure white. As you can see, that creates a great silhouette so I've now defined a good outline for my poppy. I'll go ahead and click okay to apply that levels adjustment. Of course in this case I have some problem areas up in the corners of the image.

Perhaps if I had adjusted the sliders in the levels dialog a little more carefully, I might have been able to avoid that. But the important thing, is that we can very easily clean this up. I'll simply choose the brush tool, and then make sure I'm working with white as my foreground color, and then simply paint to clean up these areas. In this case I'll press the right square bracket key to increase the size of my brush, so I can work a little more quickly. And there we go. All of those areas are cleaned up. Similarly if there were some specs inside the poppy, I could switch my foreground color to black and then paint over those areas.

At this point, I have a great silhouette for my poppy so I'm ready to create a selection. All I need to do is to click the load channel as selection button, the dashed circle icon at the bottom of the channels panel. This will cause white areas to be selected, and black areas to be deselected. This happens to be the opposite of what I want, and so I'll choose Select > Inverse, from the menu, in order to get the opposite selection. In this case the selection of the poppy. I'll go ahead and click on the RGB tile, so that we can get back to the full color image, and as you can see we've got a great selection. The individual color channels that make up the full color information in an image often contain unique contrast that can be put to use to create a selection for what might otherwise be a tricky situation.

As a result, using a channel as the basis of a selection is the selection technique I use most often in Photoshop.

Selecting from a channel
Video duration: 4m 50s 2h 33m Advanced

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Selecting from a channel provides you with in-depth training on Photography. Taught by Tim Grey as part of the Photoshop Selections Workshop

Subjects:
Photography video2brain
Software:
Photoshop
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