Hi Bart, just out of interest, which program are you using/recommending to upsample?
I use different routes, different pograms, depending on the rest of the workflow.
I have a good MF digital file to make a large print (about 147x190cm). Uninterpolated it is a sweet 120dpi exactly, upsampling this to 360dpi before applying my sharpening routine and sending to the printer, is what is suggested here, but PS does a so-so upsizing.
Actually, Photoshop's Bicubic Smoother isn't all that bad, because it doesn't create too many artifacts which sharpening will only 'enhance' further. It does require, and can stand, some additional sharpening, Smart sharpening (Lens Blur, more accurate) at least. Better deconvolution sharpening would be ideal, but it might take a while to execute (processor intensive).
Another option is to use ImageMagick which by default uses the Mitchell Netravali type of reconstruction filter for upscaling, and it allows to optionally modify the settings depending on how sensitive the source data is for generating (e.g. ringing) artifacts. However, ImageMagick's command line driven interface isn't everybody's cup of tea.
Another option is to use Photozoom Pro which allows to boost sharp edges as rasterless vector data (a bit similar to the Blow-up plug-in) but less expensive. One needs to exercise restraint in applying the edge enhancement, because it can lead to a mental disconnect between the representation of edges and surface detail, which IMHO looks unnatural.
However, these may all may cause file size issues further down the chain, except Photozoom Pro. Photozoom Pro also works as a Photoshop plugin, so it could be used when one prints remotely direct from Photoshop, and PhotoZoom Pro 4 enlarges images up to 1 million by 1 million pixels.
I could use Qimage with 'output to file', and then do my special sharpening, but it balks at such large output, hitting 32bit memory limits, even with an 8bit file. So that's off the list.
I have understood that it's not a direct Qimage problem, but a potential problem with the TIFF library specification. TIFFs are limited to 4GB file sizes due to its 32-bit adressing method. Some libraries are written with signed integers, and are therefore limited to 2GB file sizes. The upcoming BigTIFF standard is not final yet. It would be a nice addition if Qimage could write Photoshop PSB files, but that would probably lock the output files to Photoshop only as printer driver, which wouldn't seem to benefit Mike Chaney much especially since it forces him to change his code when the PSD proprietary (?) format is changed, so I can understand his hesitation. A 64-bit Qimage version appears to be scheduled for later in 2012, but that is not necessarily coming with a large file output option.
Which leave us with printing directly from Qimage, and that should work fine given the fact that all conversions (colorspace and interpolation) are handled on a block by block basis, which are then immediately fed to the printspooler. Since Qimage also allows to tweak the amount of Smart Sharpening it applies after resampling, it seems like a good compromise. For remote printing it may not be possible to persuade them to use Qimage, which would lead us back to one of the earlier options mentioned.
Cheers,
Bart