DC Conspiracy[dots pattern]

4.07.2005

dpi, lpi, resolution, &c.

Based on the discussion the group was having yesterday, thought it might be helpful to post some notes on preparing artwork for offset lithography printing. Note: this covers black and white artwork; the same principles of grayscale art apply to four-color work.

Basically, printing is binary: there's either a black dot or not. The frequency of the dots (benday dots) is measured in dots per inch (dpi), and in computer programs it's how we measure resolution. Traditional offset lithography printing is 2400 dpi; laser printers usually output at either 300 or 600 dpi; Web graphics are 72 dpi.

In dealing with a simple black and white image, the goal is to have the dots close enough together that a viewer will not see the "stairstepping" that occurs in a curve. 2400 dpi is ideal, 1200 dpi is usually adequate. In raster image editing programs (such as Photoshop) this mode is called "bitmap." Below is an example of bitmap art; because of the low resolution of the Web, the stairstepping is evident.



If an image needs to have continuous tones (varying shades of gray), as in a photo, it needs to be treated differently. In an image editing program this mode is called "grayscale." The illusion of tonal range is created by varying the size of the benday dots — smaller ones read as lighter gray; one that half the size of a full dot will read as 50% black, and so on. This is halftoning and is most obvious in newsprint. The quality of a halftone is measured in lines per inch (lpi) and is referred to as a line screen (even though it's measuring dots). A line screen of around 133 up to 150 is standard for offset lithography, some high quality printing can be 175 or above; newspapers print with an 85 line screen. Because the edges of objects get fuzzy and flow into each other, the eye is less likely to pick out ragged edges, and the dpi of grayscale images can be lower. The resolution of a grayscale image needs to be at least twice the line screen; 300 dpi will work for just about every purpose.



To use some examples on this page, artwork that is pure b/w line art, like a traditionally inked page, can be done in bitmap, as Jake's Johnny Public illustration, or Evan's Le Corneille Noir — if there wasn't that gray in the border around the title or on the paper. Both could also be done as grayscale, probably just as well, and might be easier to work with. Deb's illustrations, even the line art, rely on shading and on the variations in tone of the lines, and so would have to be done in grayscale (or in color).

Because this grew out of a scanner discussion, these are the numbers to keep in mind: 1200-2400 dpi for bitmap images, 300 dpi for grayscale (or color). Images will always be limited by their output device — a high-resolution image printed on a laser printer will only ever look so good — but it does no harm to have more resolution than you need,* especially if you plan on ever printing. Because, keep in mind, you can't increase resolution after the fact. There will only be as many dots in an image as when it was first created, and typing in a larger value later on will not actually increase the quality of your image. A scanner needs to be able to capture an image at the resolution you want, preferably without "interpolation" (which is kind of faking it).

I hope this explanation doesn't make this all sound more complicated than it really is. Experimenting with an image in Photoshop (or another image editing program) should illustrate some of these points, as well as giving an idea of the limitations of working in bitmap vs. grayscale.


* Web graphics are kind of an exception: they will only ever display at 72 dpi, but the file size of a high-resolution image can cause problems. Always resize Web graphics to 72 dpi.

Bram at 3:21 PM  |  link to this     

7 Comments

Great explanation. I'm a web designer who's been working in the field for about 7 years or so and you did a great job explaining the difference. Only thing else I would mention is the difference between Raster and Vector art.

Shane Bailey at 4/07/2005 4:12 PM   

Thanks for the kind words. I thought about including the whole vector/raster thing, but was worried it was getting too dense — and most folks in the group are drawing on paper and scanning.

But, here goes, way oversimplified:

Raster art is pixels; everything is edited dot by dot. This allows for a great deal of fine detail and retouching, and is most akin to working with paint or ink. It's really the only way to things like photos, and what you get when working with a scan. Photoshop is the most widely used raster program.

In vector art, everything is an object defined by a mathematical formula. This means that objects can be scaled without any degradation of quality, but they are manipulated through points and bezier curves. It is well-suited for mechancial and certain types of illustration. Illustrator, Freehand, and CorelDraw are vector programs.

(I notice that you mention Elk's Run on your page. That's edited by one of our own, and we got free issues at our gathering the other day. You're right — you do need to pick that up.)

Bram at 4/07/2005 4:58 PM   

Hey, this is all very clear and easy to understand. I'm also glad you mentioned the web dpi thing, as I only learned it a year ago.

Would seem to be common sense, but...

Deb at 4/07/2005 7:55 PM   

1) Great info, Bram. Nicely done.

2) Did someone say Elk's Run? Tell your retailer to order you up some, you won't be let down.


I'm like a pimp robot. Or a robot pimp. Either way...

Jason at 4/07/2005 11:11 PM   

pimp robot... drop in the quarter

BLEEP... pimp

BLEEP

BLEEP BLEEP

pimp

oh god, this was much funnier in my head. Happy Friday all!

Jacob at 4/08/2005 11:00 AM   

Is there a way I can get issue #1? I see that it's sold out from Diamond on the Elk's Run site. I really would love to read it.

Shane Bailey at 4/08/2005 1:16 PM   

Well, you can tell your retailer to get it through reorders (just make a reorder through Diamond) or you can get it from our store:
http://www.hoarseandbuggy.com/shop/
As can your retailer, we sell 5 packs for half off specifically for retailers.

Jason at 4/10/2005 1:05 PM   

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