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Frame-Grab Tutorial
Written by Jeremy Butler   
Thursday, 02 November 2006
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Using Your Captures:
Screen & Print Formats

Getting from Screen to Print

The most common use of frame captures is printing them--whether in a book, magazine, or word-processed term paper. This, obviously, was our goal while preparing the frame captures for Television. The process we used and the format/specifications we chose would be appropriate in many contexts. As always, we looked for the cheapest method that resulted in good images.

[Tech note for nerds: the specs for Television's images.]

Print Formats

The world of image formats is a confusing and complex one. As we saw on page 4, Premiere Elements and iMovie give you the following options when saving a frame grab:

  • Bitmap
  • JPEG
  • Macintosh PICT
  • Targa
  • TIFF
  • Windows Metafiles

For our purposes, it's not worth going into the differences among all these formats, but, basically, each stores image data in its own way. (Wikipedia can tell you more about image file formats.)

And, although this is a gross over-simplification...

  • JPEG is the best format to use for images that will appear on a computer or television screen.
  • TIFF is the best format to use for images that will appear in print.

But what if you want to both print a frame grab and use it in a Web page?

  • Save it as a TIFF file first and then use an image-processing program to convert it to the JPEG format.
  • Or, you could also save the image twice--once as a TIFF file and once as a JPEG.
  • If you're using iMovie on the Mac, then TIFF is not an option. Just start with JPEG and convert the file to TIFF later, if you're going to print it.

A Tip About Formats and Word Processing

Many word processors will accept JPEG files and do their own conversion of them to a format they like. So, starting with a TIFF file is not entirely necessary. However, professional printers will almost always prefer a TIFF file to a JPEG.

Dots and Dots

First, some fundamental principles:

  • Images on TV monitors, computer screens, and in print are all made of dots.
  • The more dots in each inch of an image, the sharper it will appear.

If we examine a detail from this frame capture taken from a WorldCom commercial, we can clearly see the pixels (short for "picture element") that comprise the image. Note that they're square and not round dots.

Compare this with the same part of this capture as it appears in print in Television (p. 400; 3rd edition).

In print, the image is made up of irregularly sized dots (not really squares). And, more significantly for our purposes, it takes more dots in print to make an acceptable-looking image than it does on a computer or television screen. In an inch of computer-monitor real estate, there are typically 72-85 pixels, but each inch of the images printed in Television contains 220 dots. And, further, many magazines and books cram even more dots into each inch.

Consequently, a frame grab that is, say, 10 inches wide on our computer monitors might be 800 pixels across. If these 800 on-screen pixels were converted to a printed image using 220 of them for each inch, then the resulting image would be less than four inches wide (800 divided by 220 = 3.63 inches).

From Television to Computer to Print

These pixel numbers are further complicated when one considers that our frame grabs were television images before they were computer images. Analog television images consist of a very limited number of pixels. Indeed, the number of pixels in NTSC images is much lower than that of a computer monitor--although high-definition television is changing this. (NTSC is the North American standard for television images.)

[Tech note for nerds: when a pixel is not a pixel.]

Basically, the NTSC television image offers you a width of 720 pixels and a height of 486 pixels. If you're quick with a calculator, you'll notice that this is an aspect ratio of 1.48 to 1, which does not match the actual aspect ratio of standard-definition television (1.33 to 1). The reason?

NTSC pixels are not square!

They're actually taller than they are wide. These tall pixels compensate for the relative lack of pixels in the height of a TV image and give us our 1.33-to-1 ratio.

[Tech note for nerds: counting pixels.]

What real impact do all these confusing numbers have on frame capturing?

  • Most importantly, there are only so many pixels in a TV image. On a computer screen or in print, a frame-grab image will never be as sharp as a photograph.
  • When capturing an image, software such as Premiere Elements and iMovie (see page 4) must convert rectangular, non-square TV pixels into square computer pixels. And, further, they must add (interpolate) pixels in this process--attempting to do so in a way that will not betray the original image.
  • Frame-grab software will often let you choose a size--in pixels--in which to save the image.

Click here to see the WorldCom frame grab at 640x480 pixels in a new browser window.

When this frame was printed in Television, we kept its width under 3 inches, which resulted in a tolerable looking image. Click here to see the WorldCom frame grab as it appears in Television (3rd edition), on p. 402.

Of course, a 640x480 image may be enlarged when printed, but the quality will deteriorate noticeably. For this reason, we chose to limit Television's frame grabs to 2.9 x 2.2 inches (or 2.9 x 1.7 inches for widescreen images).

If you're curious about the actual file we used to create this image, you may download it. Just right-click here (Mac users: Control-click), choose Save Target As, Download Image to Disk or something like that (your browser may vary), and then save the file somewhere on your computer that you can find again. Open this file with one of the image-processing applications we recommend below.

Enlarging Your Frame Grabs: Going Beyond 640x480 Pixels

The frame grabs in Television are of acceptable quality, but only because we kept them fairly small. What if your project needs to print an image larger than 3x2 inches? The answer is to somehow generate more pixels, more dots, from your original image; but how do you do so without having the image degrade in a big pixely mush?

Essentially, you have two options:

  1. Increase the number of pixels while doing the capture--using, for example, Topaz Moment (explained on page 2), or
  2. Increase the number of pixels through image-editing software (see below for recommendations).

Software such as the GIMP, Photoshop and IrfanView have gotten smarter and smarter about increasing an image's size while minimizing its distortion. There will always be some distortion, but there's much less now than, say, five years ago.

Moreover, there is software that you can add to Photoshop (a so-called "plug-in") that is expressly designed to make enlargements look as good as possible. One such plug-in is onOne Software's Genuine Fractals, which boasts of being able to "Scale up to 800% [8 times!] without image degradation." In practice, you can use it to increase images four or five times with good results -- if you don't mind shelling out $159.95 for it ($100 for students; as of October 2006).

Here's what it looks like in action; click the image below to see its full interface:

Detail from Genuine Fractals' interface.

I'm not quite a true believer in Genuine Fractals' abilities, but DV magazine printed a tutorial titled, "Repurposing Video for Print" (read it online) that relies on it to make a printable image out of a video frame grab. Plus, this favorable review of Genuine Fractals has almost convinced me. Still, if you feel like experimenting with other image-enlarging software, you should check out the free trials of:

  1. BenVista PhotoZoom Pro ($149)
  2. STOIK Smart Resizer ($49)

Converting Formats and Modifying Images

To shift files among formats and to modify images (e.g., bumping up the brightness) requires image-processing software. Fortunately, there are inexpensive options available.

The image-processors that come built into Windows and Mac computers, unfortunately, are crude at best; but a quick download can solve that problem. Any of the following applications--ranging from no cost to high cost--will perform the necessary image processing required of TIFF and JPEG files. All prices are current as October 2007.

Further Reading


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