Effect of scaling on quality of digital images (x Delta-l)

[Mike Dallwitz]

                                                                   17 May 1995

> From: stipoid at cc.usu.edu (Mary Barkworth)
> To: Mike Dallwitz
>
> I am curious as to why shrinking an image makes the picture worse (large
> squares in the image) rather than better.

Many people are puzzled by this, because it is contrary to experience with
optical reduction of image size, which usually makes images seem sharper.

In the context of digital images, `shrinking' usually means reducing the
number of pixels in an image. This always blurs the image, because each pixel
in the reduced image is an average of some pixels or parts of pixels in the
original. In the extreme case where the reduced image has only one pixel, all
structure is lost, and that pixel has the average colour of the whole image.

The subjective impression of the reduced image is complicated by the effects
of the resolution of the screen (or other output device), and the viewing
distance. If viewed at the same screen resolution, the reduced image will be
smaller and seem blurred. If viewed at a lower screen resolution, the pixels
will be larger, and the subjective impression of individually visible pixels
(i.e. `large squares') may dominate the impression of blurring. The size of
the on-screen image may even be larger than the original. For example, if an
820x500 image is displayed at 1024x768, it occupies 80% of the width of the
screen. If scaled to 640x390 and displayed at 640x480, it occupies the full
width of the screen.

The sharpening effect of optical reduction of image size can also be seen with
digital images, provided the number of pixels is not changed. For example, try
viewing a 640x480 image at screen resolutions of 640x480 and 1024x768 (for
example, by using the `lock' facility in VPIC). At the latter screen
resolution, the image will be smaller and sharper.

Mike Dallwitz                                  Email md at ento.csiro.au
CSIRO Division of Entomology                   Fax +61 6 246 4000
GPO Box 1700, Canberra ACT 2601, Australia     Phone +61 6 246 4075