Why upres images for printing?

[hjulenissen]

...there are other reasons to upscale than to facilitate proper sharpening.  Upressing fills in the gaps intelligently, giving a smooth, soft tonality when viewed up close, instead of pixellation.  (This is also why sharpening first looks so much worse--you're interpolating fake detail, haloing, aliasing, etc.)

What you are describing sounds like regular interpolation using different filter kernels. If inserted into that framework, there is a lot of math and empiry to help us out.

An "allpass" filter would equate nearest neighbor. A triangular filter would equate bilinear interpolation. Etc. The point is that the response of such functions can be predicted from their frequency response - deviation from a flat passband and attenuation of aliasing in the stop-band.

http://www.worldserver.com/turk/computergraphics/ResamplingFilters.pdf
Turkowski wrote a nice whitepaper for Apple on this in 1990.

I am aware that some exotic image scaling algorithms does not fit into this framework, typically being non-linear, content-adaptive etc. But those are exceptions.

-h

[Bart_van_der_Wolf]

I am aware that some exotic image scaling algorithms does not fit into this framework, typically being non-linear, content-adaptive etc. But those are exceptions.

Are they exceptions? They probably are in non-specific workflows, but we are talking about preparing for print output here. Here all 'tricks' are allowed (e.g. adding noise after upsampling, and enhancing edges while avoiding stairstepping), as long as the output quality 'improves' for a given output magnification and viewing distance(s).

There is a number of commercial solutions available, such as Qimage, Photozoom Pro, Blow-up, and perhaps Lightroom. There are even more commercial programs available, but their output quality varies (e.g. Genuine Fractals). Not all that exotic afterall, I would say, but perhaps you were thinking of other proprietary solutions.

Cheers,
Bart

[Policar]

Do you have some before/after scans or images of prints that exhibit the quality improvements you've outlined?

Nope!  I always sharpen after scaling so I haven't eliminated that variable.

If upscaling matters at all beyond giving you per-pixel flexibility with sharpening, adding grain, etc. it's not much.  But that's only because any decent printer that prints really large has built-in upressing software.

Your point about sharpening is spot on:  sharpening for the print helps make up for the printer's softness, not just softness inherent to the file, whereas sharpening at capture resolution (usually) only increases pixellation because you're pushing the frequency response way past 100% right before it cuts off to 0%, resulting in a very "digital" image without that soft "film-like" response I described...

I think you may be reading way more into my comment than you should be...  I basically just said, issues of sharpening aside, bicubic looks better than nearest neighbor...  If you want to see a print that proves that I'll go ahead and make one.

[hacimd]

Does anyone have experience using Noise Ninja or Photo Kit Sharpener?  I've heard some good things about Photo Kit Sharpener, but not too much about Noise Ninja.  Would you recommend using either  of these if you are upscaling for large prints on a 9800?  Many Thanks!