What size image do I need?

davidjones8418

My head is spinning a bit from dpi, pixels and oversampling.

I want to generate images that are as sharp as possible when printed, which in my case means 300 dpi. Which is high, obviously. Fortunately the print areas are small.

I have worked back from the image size (2.5"x 2.5") to determine how many pixels that equates to and the answer is 750x750.

I understand people go higher than this though, to improve the image when resized back down. The question is.. how much higher? A bit? A lot? Just trying to get a feel for whether we are talking +20% or +200%.

Or would you just plough on at 750x750?

 

Richard Haseltine

If you want to use the resizing to denoise trick, assuming you still have noise and  don't want to increase the render quality setting, then I would think double it - then each final pixel will be the average of four pixels. This isn't sharpening per se, though, if your render settings are reasonable - it's a way to get rid of noise

Gordig

Here's the same scene, re-rendered at 750x1000 and downscaled from its original resolution (2700x3600) to the same dimensions. I have no idea what happened to the father's eyes in the re-render, but otherwise the two images are illustrative of the difference between the two approaches.

Re-render

Downscaled

Endlessdescent

David, you only really need to worry about DPI when planning for print images (magazine, photo prints etc) in which case of course 300dpi is best. 

Otherwise it all really comes down to the size of your audience's screen they will be viewing the media. Mostly you will want to work in the following ratios: 1:1, 4:3 and 16:9
1:1 is good for social media like Instagram etc. 16:9 is for widescreen HD displays. 4:3 is the good old fashioned TV resolution many films are shot in. 
For oversampling you generally only need to go up to 4k for the highest quality. 4000x4000 1:1 which obviously you would never release other than to a paying client.
So 4000x4000, 4000x3000, 3840x2160 ( <- this is the actual res for a 4k 16:9 digital display monitor). 
You would then resample your render down to 1080 for public release. 

From there 1080 seems to be the golden standard to generally release media in, it is still extremely common for consumer grade PC displays. So in those ratios, 1080x1080, 1440x1080 and 1920x1080
Finally to answer the question direction. For oversampling or a paying client I would send 4k or 2k. For general audience 1080 and for a "preview" 720.

3141592654

In my personal experience, if the image is sharp and clear at 300 dpi - it will print well and not need oversampling. If your image suffers from a problem like a low-light situation with brings speckles, dots, fireflies (whatever you want to call them), then oversampling and reduction can help a lot. I have not had to experiment with this much, I just typically double thte image size (in your case 1500 x 1500) and resample down.  This has come about, in part, because resampling quality varies greatly from program to program. Higher end programs like Photoshop can handle all sorts of percentage reductions ... but other programs do best with simple ratios like 50% reduction.

davidjones8418

Gordig, that is a very interesting comparison! I have been oversampling and downsizing but I dug myself into a hole I think with 100% quality and x3 oversampling. It was too much. I have pushed this down to 99% and x2 and that alone has made a world of difference.

Endlessdescent, this stuff is for print and so I have been aiming at 300 dpi. Thank you for the info on the ratios though. I appreciate it. I'm kind of constrained there by the print sizes but if 4000x4000 is the highest than that gives me hope. It also confirms my belief I was aiming too high. Trying to make quaklity with my machine, not with my eyes !

3141.. Yes, x2 is the way forward for now I think. I import into Affinity.

Richard, thanks as always. I locked myself into a quality fetish I think. Got a big render queue here and wasn't getting anywhere. Am now, so thank you.