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Resizing and Resampling in Photoshop

Last week, we started played around with the Image Size dialog in Photoshop, looking at what it can do when the "Resample Image" box remains unchecked. As you've probably already guessed, the box can be checked, and it's now time to see what effect that has. For now, let's leave "Constrain Proportions" and "Scale Styles" checked.

When you check Resample Image, the first thing you will notice is that the Pixel Dimensions boxes at the top of the dialog become editable. Previously, if we increased the document size in inches the resolution went down accordingly, or if we increased the resolution, the document got smaller. The two were linked. With Resample Image checked, we now have three independent sets of inputs: Document Size, Resolution, and now Pixel Dimensions.

They're only mostly independent though as Photoshop will still try to guess what your intentions are and will change things to match. Change the resolution for instance, and the pixel dimension will change to match. If you then adjust the pixel dimensions, Photoshop will inversely change the document dimensions. It knows not to mess with the resolution any more since it remembers you've already changed it. Whichever one you change first will be held stable if you then change one of the other two.

If you change all three values, it can't leave the first two alone as you adjust the third since these fields are mathematically linked. So many pixels divided by so many pixels per inch yields so many inches.

As we saw last week, an image consists of just pixels. The whole concept of resolution only has meaning with regard to some sort of output medium that itself has a physical size. But if resolution is somewhat ephemeral and pixels are real, what happens if you change the pixel dimensions? What does Photoshop do if you tell it you need more, or less, pixels that you started with?

The Image Size dialog in Photoshop CS2That's where Resampling comes in. To cut to the bottom line, "resampling" means "making things up," or perhaps "guessing." Granted Photoshop makes educated guesses, but especially when adding pixels to increase resolution, that's what they are, guesses. If the camera didn't capture it, or the scanner didn't scan it, the information to fill in those new pixels has to come from somewhere, even if it is out of thin air. Unlike on TV, crime detectives can't extract details from film that were never captured in the first place.

Photoshop has several different ways of guessing or resampling pixels. Some are quick and dirty, others require more computing power to execute but produce more realistic looking and perhaps more accurate results. If you click on the Resampling dropdown list at the bottom of the Image Size dialog, the top entry on the list is called "Nearest Neighbor." This method is on the quick and dirty end of the spectrum and merely looks at what is adjacent to the pixel being worked on and copies values from whatever is nearby. This is essentially useless for photographic work as it is almost sure to result in artificial computer-like jagged edges or "jaggies." Bilinear is the next entry on the list and is somewhat better than Nearest Neighbor in that it averages the values from surrounding pixels rather than simply copying them. Adobe claims that it produces medium quality results, but for photographic work, it's still mostly unacceptable. In versions of Photoshop up through 7, the one you want is called Bicubic. This resampling method works by evaluating even more adjacent and nearby pixels, performing complex calculations on their values in order to predict what the pixel being worked on should be. For moderate size changes, Bicubic works quite well on most images.

Photoshop CS and above add two new resampling algorithms: Bicubic Smoother and Bicubic Sharper. These alter the calculations used for standard Bicubic to optimize it for a desired effect. If you are "upsizing," or increasing the pixel dimensions Bicubic Smoother can work amazingly well to preserve a photographic appearance for images. When "downsampling" or decreasing pixel dimensions, Bicubic Sharper can sometimes work better than straight Bicubic for retaining the sharpness of fine details. Sometimes though it results in oversharpened images, so use it with caution. Smaller images such as web graphics tend to be less forgiving of Bicubic Sharper than do larger images sized more for printing. After using Bicubic Sharper, examine your image at full size and simply undo the resampling (Edit >> Undo, or go back one step in the History Palette). If Sharper produces unacceptable results, just fall back to traditional Bicubic for that image.

So how big can you make images by resampling? That depends a great deal on the particular image, where it came from, and how close people are likely to look at it when you are finished with it. A billboard is obviously huge but people only look at it from down below so they never notice how low the resolution actually is. Good originals can be enlarged more effectively than can images that aren't quite sharp to begin with. Digital camera images (especially those shot at low ISO settings to minimize noise) can be upsized better than scanned film images due to their lack of grain structure. Upsizing a scan also upsizes the film grain. Make the grain big enough and it becomes so obvious that it'll spoil even your best slides. The surest way to find out how big you can make an image is to simply try it and see.

There are also third-party programs for upsizing images that utilize still other algorithms involving such esoteric things as fractals and S-spline curves. Other techniques involve repeatedly upsizing in ten-percent increments until you reach your desired size. The truth is though, even if perhaps they once may have, with the advent of Bicubic Smoother, such tools rarely if ever produce better results than Photoshop itself can. Since not all images are created equal, some may upsize better one way while the best results for other images may come from other tools. As such, if you are determined to blow up a troublesome image to fill your living room wall, give some of these third-party programs a try, but for most purposes you're likely better off saving your money and sticking with Bicubic Smoother.

Now to take care of the "Constrain Proportions" and "Scale Styles" checkboxes.

So long as Constrain Proportions is checked, the aspect ratio of the resulting image will match that of the source. If you uncheck it though, you will be able to change height and width independently. Squishing and pulling images this way isn't generally all that useful for photographic work, but you may find a use on rare occasions if used in moderation. It is more likely that someday you will accidentally uncheck this and wonder why Image Size no longer seems to work correctly.

Scale Styles determines whether or not Layer Styles such as drop shadow, bevel and emboss are scaled along with the layers they modify. While this can be quite useful for creating buttons, banners and other web graphics, it has no meaning if you upsize an image that does not use Layer Styles.

Image Size works essentially identically in Photoshop Elements as it does in Photoshop itself. The only real change is where Adobe chose to hide it. Rather than being simply Image >> Image Size, you'll find it at Image >> Resize >> Image Size. Go figure.


Date posted: September 4, 2005

 

Copyright © 2005 Bob Johnson, Earthbound Light - all rights reserved.
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How Can You Have "Dots Per Inch" if You Don't Have Inches?
Bicubic Smoother versus Bicubic Sharper, and Which You Should Really Use
 

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