Does Downsampling Reduce Image Quality?

"Ken Hall" wrote in message
...

I understand that upsampling can result in a poor quality image,
but I have always been under the impression that downsampling
does not reduce the image quality.

It shouldn't, besides the loss of absolute resolution. The original
pixels cannot be represented by proportially smaller ones, so fewer
pixels will remain. That smaller number of pixels will have to be some
weighted average of the original ones. However, since we're trying to
resample in a regular grid, care must be taken to not introduce
aliasing artifacts.

http://www.xs4all.nl/~bvdwolf/main/f...own_sample.htm
describes the effect of various resampling methods and
implementations.
Based on that, here is how that would work out on a normal image:
http://www.xs4all.nl/~bvdwolf/main/f...e/example1.htm

It's a little difficult to be sure this question is clear, so I'm
going to use the following hypothetical scenario.

Assume my printer cannot take advantage of resolutions greater
than 300 ppi and that I have an image which is 1200 ppi.

If I downsample from 1200 ppi to 600 ppi and print it, will the
image
be lower quality than if I had printed it at 1200 ppi?

If the printer would be limited to 300 ppi, its driver would resample
tour resampled image again. That is two possibilities for introducing
aliasing artifacts. All you can do is hope that both pases are done in
an optimal way.

If I downsample from 1200 ppi to 300 ppi and print it, will the
image be lower quality than if I had printed it at 1200 ppi?

The best you could do is a single resampling to final output
dimensions, and sharpen. Your image will in this case have the best
quality (if proper resampling was used), and there are no
"black-boxes" where you have no control over its output.

Bart