Using Subsampling To Your Advantage
I don't mean to sound as though I'm against subsampling. Used correctly,
it's actually a helpful feature. Keep in mind that when you shoot a photo,
you have no definitive way of knowing whether subsampling will affect the photo's
visual quality. However, when you're preparing images in Photoshop, you
have the advantage of Photoshop's side-by-side previews to determine if
a given level of compression/subsampling is affecting an image's quality.
In a case such as this, you may be able to apply subsampling, reduce file size
even more, and do so with the photo's visual quality completely untouched.
In a situation such as this, subsampling now becomes a useful feature.
No Subsampling vs. Subsampling. Image 4 (above) was saved on
a setting of 90 (on a scale of 0-100) with Subsampling turned
off. Image 5 (below) was also saved on a setting of 90, but
with Subsampling turned on. The difference may not be obvious
to the naked eye, but Images 6 and 7 will show that the damage
is still there.
It may surprise you to learn that, just as hardware vendors are secretive
about subsampling, so are many software vendors. Over the years, Adobe and Macromedia
have both refused to discuss when subsampling is applied. They'll only
echo what is said by hardware manufacturers: At some point, subsampling is turned
on, and at some point, subsampling is turned off.
If you're interested in fine-tuning your JPEGs with the perfect combination
of subsampling and quality, there are a small handful of software vendors that
do provide direct control over subsampling. Corel's PHOTO-PAINT (the photo
editor in Corel's Graphics Suite) enables two sampling options: "444,"
which is the equivalent of no subsampling, and "422," which applies
mild subsampling. Ulead's PhotoImpact also makes the 444 and 422 options
available, along with 411, a stronger form of subsampling.
I've found that the best way to use subsampling is to first decide on
a quality setting with subsampling turned off. Once this is done, try 422 or
411 subsampling. In many cases, this will cause absolutely no visual change
in your image (check the side-by-side previews to be sure), and will result
in a smaller file size.
These images are crop-outs of Images 4 and 5, respectively.
Although zooming in on an image always causes some damage, the
difference in these images will be clear. Image 6 (above) shows
clean-cut lines with only minor zoom blurring where abrupt color
changes occur. In addition to the zoom blurring, Image 7 (below)
shows clear JPEG artifacts along the edges of each of the white
and green color blocks. These artifacts are the result of the
JPEG subsampler averaging the color values of the black lines
with the white or green color blocks, and coming up with color
values that don't match the original values. As your eyes
move away from the black lines, you'll notice the black
lines have less and less influence on the color averaging, and
eventually the green and white color blocks return to their
normal values. You could print Images 4 and 5, or post them
on a website, and you may never see a problem. The risk comes
in using them for long-term storage. If, at some point down
the road, you wanted to do some additional post-processing on
a subsampled JPEG, these artifacts may well become visible.
The bottom line here is, for that one original digital image
you plan on keeping long term, as a replacement for a film negative,
or a replacement for a TIFF, you should always use non-subsampled
JPEG to ensure a high-quality original.
The bottom line with JPEG is that if you avoid the pitfalls (using it on computer-generated
images, using subsampling on originals, and adding subsampling/extra compression
without using side-by-side previews), there is absolutely no visual difference
in a properly saved JPEG, and in a much larger file such as a TIFF, PSD, or
Anthony L. Celeste appreciates feedback from his readers. You may contact him
via e-mail at firstname.lastname@example.org.