Printing images: resizing and/or resampling?

[djforbes]

Hi All, I'm new to this forum and hope you can help.

I have to collect a series of images from a number of people where I work, to print 16x24 (inches). Different people have used different cameras etc., so I have a variety of ppi values for the different pictures. When I resize an image (without resampling) to 16x24, these images often come out at between 100-150 ppi.  Are these images ok to give to a professional printer, to print at 16x24 or is the quality too low?  Should I be upsampling these images to 300ppi?  Some images are obvioulsy too low quality (one is 42ppi without resampling), but I got one printed as a sample at 104ppi which looks fine (despite me thinking it would be too low quality!)...

Basically: how do I judge whether an image is good enough quality to print at 16"x24"? Do I need to resample and resize, to achieve 300ppi?  And if I do, I have heard people recommending sharpening images that have been upsampled - is this correct?

Many thanks in advance - your help would be MUCH appreciated.

[neile]

You're approaching this the right way, by trusting what your eyes see. Personally I will print 150ppi images and let Lightroom automatically upsize those to 300ppi on printout. If I'm in Photoshop, I will manually do the reszise to 300ppi using the Image Size dialog and then print.

I'm sure you'll get lots of other opinions from folks here.

Neil

[The Doc]

This is where Perfect Resize 7 (the old Geniune Fractials) can really help. Very good product for upsizing. Product also has sharpening if needed.

[Garnick]

As well as my own images and customer base, I also print for a couple of labs in the area, mostly larger sizes that they can't handle. Obviously I have to deal with quite a variety of images, file sizes, resolution etc. Generally, if the res is below 150ppi I will upres to 180. However, anything beyond 150ppi usually works well with today's printers. And of course you have to keep in mind that a 16x24" print will probably be hung and viewed at a distance that will not show any small artifacts. Also, know that upresing an image file will be a positive step in one way only. It can help to make the gradient transitions somewhat smoother when doing any sort of corrections for printing. It will NOT make the image appear sharper, nor will it add more detail. All it's doing is adding pixels where they didn't previously exist. This is a process of resampling and adding information based on existing pixel information. And yes, it's very likely that you will have to add some sharpening after upresing, but you should be very careful not to overdo it. The best way to handle these images is to view them in PS at "Print Size" under "View" in the menu. Also try at 100% screen magnification. Good luck.

Gary

[Jeremy Roussak]

You could do a lot worse than to read Jeff Schewe's article on the subject here.

Jeremy

[bill t.]

Recently had to print some old 6mp images up to 30 x 45.  Was very pleased with the way Resize 7 handled the interpolation.  It rather subtly recast the images from their fuzzy original forms to much cleaner looking images made up of tiny, relatively smooth edged polygons.  At normal viewing distances the Resize7 image holds together a whole lot better than the unprocessed version, and the polygonal transformation is not obvious.

Here's a really unlovely example.  Both images shown much bigger than print size, the original on the left prints at 75dpi, the one on the right was resized to print at 300dpi.  It's probably best NOT to sharpen the images either before or after resizing, Resize7 seems to produce a very subtle sharpening-like quality in the images it processes.  The improvement is less dramatic on sharp 21mp originals, but still worthwhile for really huge blowups.  I'm not sure the attached screen image really tells the story very well, it's one of those things where you have to compare prints.

You can get very similar results by fiddling with SmartSharpen, but the nice thing about Resize7 is that you give it a desired final size, and it figures out how much processing is appropriate for that size, which gives you consistent result over a large group of images.  Glad to have it in the toolbox.

[Robcat]

I concur with the Perfect Resize recommendation. The current version is almost witchcraft in what it can do. However, I suspect you don't have that program in your arsenal and would hesitate telling you to go buy it for a printing project you're sending out anyway. Given that you plan to involve a pro printer, I'd suggest you have them resize the file for you using whatever parameters they prefer for their printer and that print size. If they're just some print-on-demand place that blindly runs your work through and can't/wont deal with resizing, then my 2 cents is resample in Photoshop with bicubic smoother to 240 dpi and 16 x 24 and then do some judicious sharpening with unsharp mask. As per Garnick, 180 dpi is probably ok too, though I don't personally use it. Good luck.
Rob P

[ErikKaffehr]

Hi!

My toughts on the issue:

Reading Jeff Schewes article is an excellent idea, in addition:

- It seems that 180 PPI are needed for a good print, so if you have less it's probably worth upscaling
- Whenever you scale you need also to sharpen, downscaling can cause aliasing artifacts.
- The image needs to be sharpened for printing, to compensate for the expected degradation of image quality on printing, like error diffusion dither and ink creep. The sharpening needed is depending on the printing process.

Regarding downscaling, the right way to do it is to blur the image very slightly before downscaling and than sharpen to compensate. Both blurring and sharpening would be made using small radius.

I would assume that a lot of engineering has been done to Lightroom to make it pretty much optional. The Photokit Sharpener Toolkit implements most of Bruce Fraser's and Jeff Schewes ideas about sharpening.

Best regards
Erik

Hi All, I'm new to this forum and hope you can help.

I have to collect a series of images from a number of people where I work, to print 16x24 (inches). Different people have used different cameras etc., so I have a variety of ppi values for the different pictures. When I resize an image (without resampling) to 16x24, these images often come out at between 100-150 ppi.  Are these images ok to give to a professional printer, to print at 16x24 or is the quality too low?  Should I be upsampling these images to 300ppi?  Some images are obvioulsy too low quality (one is 42ppi without resampling), but I got one printed as a sample at 104ppi which looks fine (despite me thinking it would be too low quality!)...

Basically: how do I judge whether an image is good enough quality to print at 16"x24"? Do I need to resample and resize, to achieve 300ppi?  And if I do, I have heard people recommending sharpening images that have been upsampled - is this correct?

Many thanks in advance - your help would be MUCH appreciated.

[Bart_van_der_Wolf]

- It seems that 180 PPI are needed for a good print, so if you have less it's probably worth upscaling

In my view, it always helps to 'upscale' to the native output resolution. It will allow to do sharpening at the pixel level, and thus maintain smooth gradients, yet sharpen edges (and add pseudo detail in the shape of noise).
 

- Whenever you scale you need also to sharpen, downscaling can cause aliasing artifacts.

Deconvolution sharpening also works to reverse some of the blur that's introduced by upscaling/interpolation.

- The image needs to be sharpened for printing, to compensate for the expected degradation of image quality on printing, like error diffusion dither and ink creep. The sharpening needed is depending on the printing process.

These degradations all take place mostly at the output pixel level. Yet another reason to upscale before sharpening.

Beyond what can be recovered by deconvolution sharpening, it can be beneficial to use dedicated software to do the upscaling. I've never liked the posterized look of Genuine Fractals (now renamed to Perfect Resize), but at least it now allows to add noise to cover up the posterization and tune the exaggerated edge sharpness (there's a disconnect between feature sharpness and edges pretty easily).

I do like the capabilities of Qimage for on the fly upscaling as it feeds the printer driver (or writes to an output file), and Benvista's Photozoom Pro, although with the latter one also needs restraint with the edge sharpening (which is possible because the amount is tuneable). Another possibility is Blow-up, but I don't like the artifacts from rounding off sharp corners.

Regarding downscaling, the right way to do it is to blur the image very slightly before downscaling and than sharpen to compensate. Both blurring and sharpening would be made using small radius.

When the downscaling is significant, the pre-blur radius must increase proportionally. When e.g. pixels in the resulting image are a weighted average from say 3x3 pixels, then the blur radius can affect at least 9 pixels in a semi-circle without hurting resolution. Unfortunately one needs to consider that the effect of a less than optimal blur like Gaussian blur extends beyond it's radius. My rule of thumb is that for each downsampling factor (e.g. input/output=3x) the Gaussian pre-blur radius is multiplied by that same factor (e.g. 3x0.25=0.75).

Sharpening after downsampling is then done with a very small radius, since we probably have pixel perfect detail.

Cheers,
Bart