[P]rinters have, based on settings, specific ppi that they need/expect to print and image. If they do not get that ppi, the will interpolate to that size and, usually, that interpolation is much rougher than what would be done in the computer.... Also, the computer interpolation used to get to the expected printer ppi, which is why better results are obtained be Perfect Resize or Qimage vs bicubic.
I may be missing something--if so, please tell me, I'd love to learn!--but it seems to me that
some interpolation is almost unavoidable. (This post is not about how much this some interpolation affects print quality; but if the very negative statements about the quality of printer-based interpolation are correct, then the issue merits some inquiry.) One can say that the printer needs the exact ppi. But the ppi tag does not (AFAIK, in most common software) control the printing behavior; the input file's pixel dimensions and the requested output size do that. It seems to me--again, I'd love to find out that I'm wrong--that we have no reasonably-easy way to determine the
exact pixel dimensions to send for a given print size for a few reasons:
(1) We do not know the printer's exact ppi resolution. We may say that Epsons are 360 or 720 ppi and that Canons are 300 or 600 ppi. But how likely is it that Japanese companies performing most if not all of their engineering in Japan are working in English units? Doesn't it seem far more likely that the exact specification is an even figure in a metric unit, like pixels per millimeter (ppmm)? Some years back by own analysis of a "250 ppi" lab concluded that it was far more likely that the printers were actually 254 ppi--which is exactly 10 ppmm. I'd bet lunch that so-called 300 ppi printers are actually 12 ppmm, which is 304.8 ppi. And what about those supposedly-360 ppi Epsons? I suspect that they are actually 14 ppmm = 355.6 ppi. But I don't
know any of this. And I would be unlikely to trust information on it from any U.S.-based source. Obviously if you send a printer an image that has enough pixels for 300 ppi and the printer needs, 304.8 ppi, the printer (or its driver) has to up-interpolate to generate 1.6% more pixels. And obviously if you send a printer an image that has enough pixels for 360 ppi and the printer needs 355.6 ppi, the printer (or its driver) has to down-scale to generate 1.2% fewer pixels.
(2) We don't know what the printer thinks the media size really is. Have you ever gotten "8x10 inch" prints from a lab that appeared to be slightly less than 8 inches, and maybe slightly less than 10 inches? I have. From more than one place. I suspect that the paper rolls they were feeding into their printers were not exactly 8 inches wide, but in fact were exactly 200mm wide, which comes to 7.87 inches. So if you tell your printer 8x10 inches, does it really expect exactly 8x10 inches? Maybe, maybe not, it's hard to know. (Although I suspect that the U.S.-market printers actually do expect the exact dimensions in English units).
(3) For borderless prints, we do not know how much overspray the printer applies. The paper feed guides are not ultra-high-precision devices; they just slide into place, up to the edge of paper that might be bowing a little. Except where there's paper misaligment in the feed path, I have not seen thin white borders on borderless inkjet prints. To achieve this, the printer has to spray at least
a little ink past / off the edge of the paper. How much? I don't know, but enough that some people recommend not printing borderless to avoid gunking up the printer's insides with overspray ink. If the printer oversprays by even 1/32 inch (a little under 1 mm) on each side, then for a print at (a nominal?) 300 ppi, it needs about 19 extra pixels in each dimension. So you think, '8x10 inches at 300 ppi means I send 2400x3000 pixels to avoid interpolation.' In reality the printer needs 2419x3019 pixels, to give something for overspray, and some sort of interpolation has to occur.
Is Lightroom, or Qimage, or whatever able to query the printer about
exactly how many pixels it needs, to allow them to use their more sophisticated interpolation methods to send the printer the exact number of pixels? I don't know. Maybe. But if you or I can get that information from Lightroom or Qimage or whatever, please tell me how.
Thanks!