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This next topic is a little technical, but I think it's important to understand if you really want to come to terms with modifying image size in Photoshop. It's all about the methods of interpolation. Now interpolation is what Photoshop uses to decide how to rewrite the pixels in the image, either when you upsample or downsample a file. And it's all about taking the existing pixels in the image and somehow comparing them to come up with new pixels. To give you a sense of what I'm talking about, I'll go up to the Image menu and choose the Image Size command.
When the Resample Image check box is turned on, you have access to this pop-up menu. By default in CS6, it's set to Bicubic Automatic, and I'll explain what that means in just a moment. But you can manually override this option if you want to by selecting any of these five interpolation options, Nearest Neighbor and any of the three manual Bicubic overrides. Now to give you a sense of how they work, I created a test file and this is it right here, just these nondescript gray checkers.
We're viewing the file at the 200% zoom ratio so the checkers are actually a little smaller than this. What I did was I went up to the Image menu, chose the Image Size command, changed this option here from Pixels to Percent and went ahead and dialed in a percent value of 72, and notice that, that changes both the Width and the Height values. Now by way of a little tip here; notice that when I switched the option to Percent, both of the values switched to Percent, if that's not what you're looking for, if you just want to change one of the values to something different, you press the Shift key while choosing a new option.
So in this case, I had the Shift key down when I chose Pixels, and as a result I can see that the Height of my image is going to decline to a 184 pixels when the Width is set to 72%. Then I went down to this pop-up menu and chose each of these options. Well just to save us time, let me show you what that looks like. I'm going to switch over to this diagram that I created, and here is each and every one of those manual interpolation settings shown in order. So we'll start right here with Nearest Neighbor. Notice that Nearest Neighbor doesn't attempt to do any real interpolation; it just keeps pixels or throws them away.
And as a result, some of my checkers are shorter or narrower than others, and so we get this kind of patterning effect that's known as aliasing. Nearest Neighbor can work well for highly graphic files, for example, screenshots. If you want to take a screenshot and expand it to 200% of its former size, then Nearest Neighbor would be the way to do it. That's about the only time I use that option, however, except I should say, for magnifying each one of these images. Notice that I have taken them all and scale them to 400% and I did that using Nearest Neighbor so as not to introduce any new pixels.
All right, I'll zoom into Bilinear. Bilinear is ultimately a simple averaging formula. And as you can see it ends up creating soft transitional pixels. But it doesn't do a very good job of maintaining detail, which is why Bicubic was invented. Now Bicubic is much more complicated. It uses a series of derivatives that you don't need to know anything about. But notice that what we get are these halos inside and outside of the checkers. So we get this kind of border pattern around the checkers and we get dark halos inside the dark squares and light halos inside the light squares.
Now if those halos end up being a problem, you can switch over to Bicubic Smoother which downplays the halos significantly. So you can see that we don't have nearly as much halo action going on. It's subtle but it's distinct. If you want more halo action--and the idea is the halos end up emphasizing edge contrast which creates the appearance of crisper detail, then you can bump things up by switching to Bicubic Sharper. Now I should say, when you're working with Bicubic Automatic anytime you downsample an image, Photoshop is going to automatically apply Bicubic Sharper with the assumption that you want crisp detail out of your downsampled image.
I've also created a demo file for upsampling. What I did this time was I switch to a smaller version of the checkerboard image. We're still seeing it at the 200% zoom ratio, and I upsampled the image to 576%. It just happened to work well for my demo file and came up with this composition here. I'll go ahead and zoom in--notice that this time around, things work out pretty well for Nearest Neighbor. Some of the squares are different sizes than others, but it's not nearly so noticeable. Where Bilinear is concerned, we end up with these soft, continuous transitions.
So it's almost like we've turned each one of the squares into a kind of gradient. We have the appearance of sharper detail because we have these dark halos inside the dark checkers and these light halos inside the light ones. Bicubic Smoother ends up resulting with less haloing and then Bicubic Sharper ends up resulting with even more haloing. Now when you're upsampling an image, Bicubic Automatic goes ahead and applies the Bicubic Smoother setting. The assumption being that you still want bicubic detail but you want smoother transitions.
So that's how the various interpolation settings work. In the next movie, I'll show you how they affect a photographic image.
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