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It's been demonstrated that upscaling incrementally improves image quality, Empirical Study: Extreme digital upscaling, and that thread also shows that choosing 105% increments versus 110% increments can produce different results.

It's pointed out by Stan Rogers in this thread How to scale up a photo? that

for users of Ps CS6 (and, I'd assume, above when there is an "above"): the resize options now include a "Bicubic Auto", which does the smoother/sharper thing automatically depending on the new resolution, and does iterative up-rezzing so you don't have to do it manually anymore.

What type of iterative up-rezzing is "Bicubic Auto" doing when it upscales, and is there a way to control this? Such as 105% versus 110%?

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  • \$\begingroup\$ I think you may have misread what @jrista was explaining. The algorithm is Bicubic, but the increments are user controlled in the scaling dialog. So, basically, you resize image to 105% or 110% and then do it again and again. The evidence would indicate that smaller steps produce better results than less steps or one giant leap. It's also possible that I misread it, but I don't think so, tools like Photoshop ask you select scale and algorithm. \$\endgroup\$
    – Joanne C
    Jul 9, 2013 at 19:49
  • \$\begingroup\$ No, "Bicubic Auto" in CS6/CC both automatically selects "Bicubic smoother" and does an iterative upscale when the target size is larger than the source size. As far as I can tell (mostly from Julianne Kost's postings and talks), the increment is 110%, but I refuse to be held to that in a court of law. \$\endgroup\$
    – user2719
    Jul 9, 2013 at 20:25
  • \$\begingroup\$ Ah, cool... I never use it, for upscaling I tend to use Perfect Resize. It looks like the test referenced didn't use Auto, though, so the short answer is to use smoother and do the resize by your own increments. \$\endgroup\$
    – Joanne C
    Jul 9, 2013 at 20:29
  • \$\begingroup\$ You can also still control that by only scaling by 105% all the time, which would tighten the increments. \$\endgroup\$
    – Joanne C
    Jul 9, 2013 at 20:30
  • \$\begingroup\$ @StanRogers interesting, so this can't be controlled then I imagine? Has anyone made PS actions available for manual upscaling at different increments? \$\endgroup\$
    – DavyCrockett
    Jul 9, 2013 at 20:31

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There are some settings that are better for upscaling and others that are better for downscaling. My understanding of the Auto setting is that it simply decides on sharper or smoother based on if you are upscaling or downscaling. The only reference I found for this is the For Dummies article on Photoshop CS6, but it seems to be consistent with the behavior I've seen.

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    \$\begingroup\$ Upscaling is now iterative, whether you select "Bicubic smoother" manually or leave it on "Bicubic auto". \$\endgroup\$
    – user2719
    Jul 9, 2013 at 20:27
  • \$\begingroup\$ @Stan: Do you have a reference for that? I did some searches, and I couldn't find any information about upscaling actually being iterative. As far as I could tell, the "auto" simply means it figures out whether to use smoother or sharper depending on whether you are scaling up or scaling down. \$\endgroup\$
    – jrista
    Jul 10, 2013 at 3:36
  • \$\begingroup\$ @jrista - No direct links to text — watch Julianne Kost's PS CS6 for Photographers videos. (Yes, she actually speaks for Adobe.) I don't know of much commercial software that lays its algorithms bare for public consumption, the info was basically "you don't have to do that manually anymore, wee've built it in". \$\endgroup\$
    – user2719
    Jul 10, 2013 at 6:54
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I did some simple comparisons of one step vs. iterative (many small step) upscaling, which you can find here:

http://wmjaeger.com/William_Jaeger_Photographs/Upscaling.html

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