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Let's say you want to reduce the print size of an image. By now you should be getting comfortable with the idea that as you do that, resolution will increase. So for example, if I have an image with a print size of 8 x 10 at 300 pixels per inch and I resize it down to 4 x 5 inches, my resolution will go up. It will go from 300 pixels per inch to 600 pixels per inch. Don't worry about understanding that math. Just trust me, that's how the resolution will increase. The important thing to understand is that as print size goes down, resolution goes up.
But let's say I'm giving this image to someone who says they need the image to be 300 pixels per inch, but they need it to be 4 x 5 inches. Now I have a problem, because my image has too many pixels. At 600 pixels per inch, a 4 x 5 inch image has 2400 x 3000 pixels; at 300 pixels per inch, a 4 x 5 inch image has 1200 x 1500 pixels. So I need to throw some pixels away. Resampling is the process of taking my collection of pixels and selecting a new sample of pixels from that set to come up with a new image.
So to get my 4 x 5 at 600 pixels per inch down to a 4 x 5 at 300 pixels per inch, I need to resample the image. That is, I need to go through and take, say, every other pixel from the image and just throw them out. What I'll be left with is a smaller set of pixels that can then be spaced at the resolution that I want. Now let's say I have to go the other way. Let's say I have an 8 x 10 inch image at 300 pixels per inch and I want to go up to 11 x 14 inches. Expanding to that size will drop my resolution to 218 pixels per inch, because to get from 8 x 10 to 11 x 14, the printer has to print each pixel a little bit larger, and with that larger pixel size, only 218 of them will fit in an inch.
So let's say again that you're giving this image to person who insists on having a 300 pixel per inch image. At that point, I need to employ interpolation. In the case of digital images, interpolation is simply the process of making up new pixels based on the pixels that are already there. For example, let's say that I said you 2, 4, 6, 8, blank, 12. You would interpolate 10 to fill in that blank. That may seem simple, but you have actually just performed an analysis. You've recognized a pattern, you've understood that pattern, and you've figured out how to interpolate that missing value.
Photoshop, and most every other image editor, have sophisticated algorithms for interpolating new pixels. So if you want to go from, say, 8 x 10 to 11 x 14, but still at 300 pixels per inch, then your image editor will need to interpolate some new pixel data to get you to a pixel count that will allow you to have the resolution that you want at the given print size. Now, this kind of interpolation isn't an easy, and some of these algorithms are closely guarded trade secrets. In fact, there is an entire industry built around resizing images.
You'll find several Photoshop plug-ins that claim to do a better job than everyone else at upscaling images. The problem is simply that it's just hard making up a new image data. When you interpolate an image upward, you run the risk of softening the image, of introducing stair-step patterns on diagonal lines, of creating noticeable repetitious patterns, and other visible artifacts. Therefore, you need to be very careful about how much you interpolate. I'll have more to say about that throughout the rest of this chapter. Photoshop uses the term resampling to refer to both scaling up and down.
When we are resampling down, we are throwing pixels away; when we are resampling up, we are interpolating new pixels. So I'll sometimes be using the term resampling to refer to either scaling up or down.
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