Arbitrary dpi value for prints

Question

I need to print images on large format paper (B1, B2). The image-source/art I'm trying to print is scalable so I can actually export them at any resolution (max 16k per side for technical reasons). The original format is not vector, instead it is 3d graphics that need to be first rendered to a rasterized image format. I understand quality prints start at about 300 dpi/ppi and I can do the math and figure out what resolution I need the images to be exported at.

I picked 300 because it seems to be a common/standard value. 600 seems to be a common one as well.. Are there any other values in between? 450? Can large format printers use arbitrary dpi values? Is there any list of industry standard dpi values? (for me I'm looking in the 300-600 range for practical reasons.

Answer 1

Just a quick correction. You are referring to PPI, not DPI.

DPI is Dots per inch, and it states how many droplets of ink, or laser-generated dots are produced on a laser printer or the generation of plates per inch. These numbers normally vary from 600 on a home laser printer to 3200 on a plate. This number is dependant on the specific hardware. You can sometimes reduce it for some specific reasons, for example, speed or quality of the substrate.

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PPI is Pixels per Inch is how you distribute the pixels present on your image on a real-life print, which has a specific size. This is not a fixed number, it is a relationship. The number is "elastic".

Take one typical 24Mpx photo. This has 6000x4000 pixels. If you print it on a B1 paper you can enlarge it and the result is around 152 PPI.

100 cm to inches => 39.37

6000px / 39.37 = 152.4 PPI.

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300 PPI is "standard" but more by "convention" than as a "rule". When you are doing commercial print on coated paper, the standard is to use 150 of another unit called LPI. And to smooth things on the conversion of the image (by screening it) the standard is double that number on the original file.

On a print made on an inkjet plotter, you do not screen it. The gradients are achieved by dithering them.

This means that you could use a 150 PPI image to have similar "quality" as a printed magazine, or go from there.

200PPI can be considered a good quality for a photographic print, but you can go for the 300PPI simply because it is a number we are used to work with.

B1 is a big file but still workable enough.

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On larger prints, you can forget the PPI, and work with the same file, for example, a 24Mpx photo. The reason is that the bigger the print, the further away you are normally from to see it.

This means that the PPI needed for a billboard, can be 30PPI or even 1PPI for example.

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In short. No. You do not need to use more than 300PPI in almost any case.

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Here is a graph showing this relation between a file of defined pixel size, and the print size.

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I am adding this because of the specific comment. I will go more in-depth here:

I just wanted to know if the typical printer can print at any dpi values in between 300 and 600

Again, the correction. PPI, not DPI. <= (This is really important)

A printer can print, let's say 1200 DPI. If the specific data on your image is 1 bit only, then the printer could use the data on a 1to1 base. 1 1bit px => 1 dot of the printer. So you could use a 1200 PPI 1 bit file.

If the image is a standard 8-bit per channel file (a grayscale image, a 24-bit color image, or a 32-bit CMYK file) then the dots of the printer need to somehow translate the possible gradient values into a visible printed gradient. This is where the additional LPI unit is useful. This conversion process gives us the relationship between how many variations of tone I can reproduce maximizing the resolution.

The relationship is normally 16:1 this is. A 2400DPI printer can print 256 grayscale tones using a 150LPI. 2400/16=150.

and if there are standard values in between. I understand that 300 is enough but I will aim for the highest value I can

Some specific resolutions for Art prints use 200 LPI. This normally uses a 400 PPI file. But the result is similar to a 200PPI dithered print (as I said before). So, a 300PPI is in fact more than you actually need.

So, a "standard" value is 400PPI. But again... this is not a standard. It is not even conventional because too few cases use it. You can also think of 450PPI as a nice number. Just making a 300x1.5 which is a nice number.

In your case, a nice number would be the resolution of the file. Forget for now the print dimensions.

• 16k px? Nice.
• 6,000 pixels on the longest side? This is a normal 24Mpx photo
• 12,000 on the longest side? I like this number as the biggest number I would work with just because it rounds my graph nicely. There is no need to go beyond this number unless you are using a zoom-in feature on an electronic media.

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One additional question would be if humans can see more than 300PPI on normal conditions. Take a look at these discussions on Wikipedia: https://en.wikipedia.org/wiki/Retina_display

Yes, we can talk about some retina resolutions of 366PPI or 400 ish or more. But printed resolutions have a fuzziness due to the printing itself, the absorption of the paper, etc, which makes it difficult to notice the difference beyond 100-150PPI for a photo. We are more perceptive on some specific elements like text. But the limitations are dependant on the print device, paper, contrast, etc.

Answer 2

The printing machine prints dots of inks. A dot is either ON or OFF. The dots are small, but their density (depends on the used print machine and its settings) is so high that a respectable range of apparent brightnesses can be produced by controlling which dots are ON or OFF and the the image still is not looking too grainy.

Your vector images are converted in the print process directly to dot patterns. If you have raster images, say photos, they are at first converted to pixel dimensions (not changing the image size, but resampled to different amount of pixels) which allow every pixel to be presented with a good pattern of dots.

An ultra-simplified example:

Let's assume you have 1 x 1 inch size image. The print process may have for ex. 4000 x 4000 dots in that area. It may divide your image 10 x 10 dots squares and fill each square with a pattern that has 0 to 100 dots ON and the rest are OFF. The process can make pure paper + 100 different darkened shades of it and the maximum useful original image resolution is 400 x 400 pixels (=400 px per inch). Having originally 600 pixels per inch doesn't harm, but having originally only 300 pixels per inch leaves a part of the reproduction capacity unused. In practice 600 pixels per inch can be better than say 401 ppi because all resampling generates some calculation rounding errors. Unfortunately I do not know enough of existing advanced image resampling math, so I must skip that subject.

You should ask from the printer what's the highest useful pixels per inch resolution of the raster images in their process and see some examples of the prints made with original pixels per inch resolutions that you can have.