Stairstep Interpolation in Photoshop

Article and Photography by Ron Bigelow

Photoshop CS4 Used in this Tutorial

Sooner or later, most serious photographers end up interpolating their images in order to make larger prints. While there are various interpolation methods from which photographers can choose, one of my favorites is stairstep interpolation. Besides producing excellent interpolation results, stairstep interpolation is free. That is because Stairstep interpolation is a process that uses one of Photoshop’s interpolation methods.

Figure 1: Image Size

Before starting the interpolation, it is important to understand what stairstep interpolation is all about. Usually, if a photographer was ready to interpolate an image, he would start by choosing Image/Image Size. The Image Size dialogue box would appear (see Figure 1). Then, he would select the interpolation method (Bicubic is currently selected). He would make sure that the three boxes above the pulldown menu are checked. Last, he would enter the resolution and final dimensions of the image. For this interpolation, that would be 300 PPI and nineteen inches on the long dimension. Since the Constrain Proportions box is checked, Photoshop automatically enters the short dimension. If the OK button were clicked, Photoshop would interpolate the image in one step.

On the other hand, stairstep interpolation doesn’t do it this way. Instead, stairstep interpolation interpolates the image in multiple steps. Each step increases the image by a small amount (usually about ten percent). In many images, this produces a better quality interpolation than interpolating in a single step.

Stairstep Interpolation Procedure

Figure 2: Wildflower Image

The wildflower image shown in Figure 2 will be used for this interpolation.

Figure 3: Info Panel
However, before starting the interpolation, it is necessary to set up the workspace to make things easier. One of the things that must be done is to track the pixel dimensions of the image. Now, there are a few different ways to do this. One way is to use the Info panel. If the Info panel is not set up, it can be displayed by clicking Window/Info. Next, the Panel Options is accessed (see Figure 3).
Figure 4: Set Ruler Units to Pixels

The Document Dimensions box is checked, and the Ruler Units is set to Pixels (see Figure 4).

Figure 5: Info Panel
The image dimensions, in pixels, can now be seen in the Info Panel (see Figure 5).
Figure 6: Actions Panel

Next, the Actions panel is set up by choosing Window/Actions. So, the workspace is ready to go.

Now that everything is prepared, the interpolation can begin. As mentioned earlier, the interpolation is going to be carried out in multiple steps. Of course, it is undesirable to have to come back to the Image Size dialogue box over and over again as the interpolation is carried out. So, an action needs to be set up to make the work simpler. To do this, the Create new action button on the Actions panel is clicked (see Figure 6).

Figure 7: New Action Dialogue Box
In the New Action dialogue box that appears, the action is named Stairstep, the function key is set as F2, and the Control box is checked (see Figure 7). This action can now be launched anytime by pressing the F2 key while holding down the Control key. The action can now be recorded by clicking Record.
Figure 8: Image Size Dialogue Box
The Image Size dialogue box is now launched by choosing Image/Image Size (see Figure 8).The Bicubic interpolation method is selected, the three boxes above the pulldown menu are checked, and the resolution is set. Next, the pixel dimensions are changed from pixels to percent, and 110% is entered (this will result in a 10% increase in image size). Clicking the OK button interpolates the image. Last, the Stop playing/recording button is clicked.
Figure 9: Image Dimensions

The dimensions can now be checked on the Info panel (see Figure 9). The image is now 3,379 pixels along the long dimension. Of course, it is obvious that, if a nineteen inch image at 300 PPI is needed, an image that is 5,700 pixels long is required. No problem, pressing the F2 key while holding down the Control key will interpolate the image again.

Figure 10: Image Dimensions
The image is now at 3,717 pixels. This is still not large enough. So, the F2 key is pressed while holding down the Control key again.
Figure 11: Image Dimensions
The image is now at 4,089 pixels. This is still too small.
Figure 12: Image Dimensions

Two more interpolations bring the image to 4,948 pixels (see Figure 12).

A final pixel dimension of 5,700 along the long side is needed. So, the image needs to be increased a little over 10% to reach the final print dimensions. One more interpolation step is required, but this step will be manual so that the exact final dimensions can be entered.

Figure 13: Final Image Size
To do this, the Image Size dialogue box is launched by choosing Image/Image Size. Now, the final dimensions of the image are entered. Clicking OK starts the last interpolation.
Figure 14: Final Image Dimensions

Figure 14 shows that the final image dimensions have been achieved.

Now, the question arises about when the image is ready for the last, manual interpolation instead of using the interpolation action that was created. Generally, if the image needs to be interpolated an additional 15% or less to reach the final print size, it is time to perform the last, manual interpolation step.

At this point, some readers might be thinking that a single action that would automate the entire procedure could be created. Then, the action would only need to be launched once, and the image would be interpolated to its final size.

This works great if a photographer is primarily interested in interpolating images of one specific, original size to one specific, final size. For instance, if a photographer needed to interpolate a bunch of images from a 15 megapixel camera and all the images were to be interpolated to the same final size, a single action created for that purpose would simplify the process. On the other hand, many photographers have images from many different sensor sizes and print them in many different sizes. This procedure will work with all of these images and print sizes.

Okay, there is one last question that should be addressed. Is this procedure really worth the extra work? After all, a single step interpolation would be quicker. My experience is that the stairstep interpolation almost always creates a better interpolation than using the Bicubic interpolation that is available in Photoshop.

When comparing stairstep interpolation to the Bicubic Smoother interpolation, that is also available in Photoshop, I have found that stairstep works better with some images and Bicubic Smoother works better with others.

So, in this case, what is a photographer to do? Well, here is what I do. I create two copies of the image and interpolate one with stairstep and the other with Bicubic Smoother. Then, I keep whichever one created the better image. That way, I am assured of having the best possible interpolation and, thus, the best quality image.

There is one final issue. When comparing interpolation methods, it is better to compare them after the images have been sharpened. Without the sharpening, it is often difficult to see much difference between the methods.