photo printing

[Jamie Christy]

OK, I think I am getting pretty good at doing the vector based stuff but not the rasters.
Could someone please give me an overview of dpi vs. size, halftone, ppi......i really am kinda lost.

Thanks
Jamie

 

[Ian Stewart-Koster]

here's an attempt...

dpi is what you get from inkjet printers when you print a picture on paper of some kind. 300 dpi is pretty fuzzy for a photo, 600 is OK depending on the original of course. It refers to how fine the print head nozzles are I think.

ppi is pixels per inch which is the real stuff of photos. A picture of 72 ppi looks ok on most monitors but will look fuzzy printed out on paper, regardless of whether the inkjet printer you are using is printing at 300 or 1200 dpi, as the information just isn't there.


A picture of 250 ppi looks pretty good printed on most good quality papers. That is of course providing it is printed at original size with 250 ppi resolution. If you take a pic of 250 ppi which is say 5" x 2", that's 10 sq inches, multiply that by 250 ppi in length and by 250 ppi in height and you have 625 000 pixels, or a 625k picture, crudely put.

You can enlarge that picture to twice that size-20 sq inches, say 8 x 2.5 inches- and as long as you halve the resolution to 125 ppi, you still have the same original picture. The computer hasn't had to invent any pixels to fill any gaps. You've just stretched it, like printing it on a balloon and blowing the balloon up a bit more.

If you want it at double size - 20 sq inches- and want to maintain the 250 ppi resolution, the computer only has 125 ppi that it knows of at that size, so it fills in the gaps by averaging the colour of adjacent pixels. You have some control of this process by selecting bicubic interpolation, or other kinds of interpolation. However, you're getting it to create something from nothing, so it won't be as good as the original was.

If you knew you wanted it to be twice the size, you ought to have scanned it in at a finer scanner resolution in the first place, so that after enlarging it, the quality is maintained, and nothing is invented.

Scanners are a different thing. Most have a setting in dpi that you choose to have them scan at. However some actually mean ppi when they state dpi. Others say dpi as that is what they expect you to use for your inkjet output setting. I can't enlighten you there. You'll just have to figure out your own machine.

lpi is lines per inch, also called the frequency (of lines on a white screen originally used to create bromides for reducing a photo to half tones for say newsprint or magazine output). A laser printer can only print black, not grey. A photo to look realistic needs shades, so half tone pictures are the solution to get black to appear grey, by altering the amount of black on each dot. These dots are set a certain distance apart - 80 lpi frequency means 80 laser printer dots per inch. You can have a tiny pin prick of black, or none at all, or a big blob in each 'eightieth of an inch' of picture.

On laser printers with Post Script drivers, you can alter the frequency (lpi) down to 45 (crude but OK), 60, 80, 120 or more. 120 is awfully fine and not much use on plain paper- you won't see the difference much past 100 on standard paper. 80 is pretty ok.
You can also set the blob shape - square, diamond, elliptical or circular- for various reasons I won't explain here. You can also set the angle of the lines of the screen- vertical (90 degrees), 45 deg, or any other angle of choice.

There are reasons for altering this, principly to avoid moire, which is an interference pattern appearing and dominating the picture. With colour printing on laser printers, or screen printing, you set each colour at a different angle, to prevent this problem. You can do a search for more info on that.

So.... does that help?

Why are there these differences? Because of the different output 'machines' & technologies. What do you have, & what do you want to print it onto (cloth a coarse t-shirt or fine silk, mugs, glass, vinyl, standard paper, photo paper, newsprint, magazine, brochure...) and how big is it & how big is it going to be, and what equipment will be used for the intervening processes (which has its own limitations of quality & puddling), and how much is the customer's budget & how fussy are they & you about the quality of the end product... and what software do you have... etc.

There are lots of controls aimed at helping the pilot get a good landing on whatever's ahead!

That relates to bitmaps, or photographic stuff mostly, which is how things get printed. Vectors are mathematical, and help you create huge physical pictures while keeping the working file size small. They can be scaled to all sizes without loss of clarity because they are a mathematical formula for an outline, plus a fill inside that boundary. Bitmaps are individually memorised pixels which are independent.

A plotter plots or cuts outlines, but any photographic picture to be printed needs to be converted to pixels & dots, even if it was a vector pic to start with..

Clear as mud?

 

[Jamie Christy]

thank you a lot.....i will be using an edge, which i should have mentioned earlier. I am printing what you wrote.....to study.

Jamie

 

[Fred Weiss]

First off, I would say that if anyone is involved in digital imaging they should own a licensed copy of Adobe Photoshop. In addition to being the industry standard for image creation and editing, it also includes what is probably the best documentation, user support and help files on the planet.

Here is one article from the help files of PS 6 that comes up by doing a search on "PPI"

About image size and resolution

In order to produce high-quality images, it is important to understand how the pixel data of images is measured and displayed.

Pixel dimensions The number of pixels along the height and width of a bitmap image. The display size of an image on-screen is determined by the pixel dimensions of the image plus the size and setting of the monitor.

For example, a 15-inch monitor typically displays 800 pixels horizontally and 600 vertically. An image with dimensions of 800 pixels by 600 pixels would fill this small screen. On a larger monitor with an 800-by-600-pixel setting, the same image (with 800-by-600-pixel dimensions) would still fill the screen, but each pixel would appear larger. Changing the setting of this larger monitor to 1024-by-768 pixels would display the image at a smaller size, occupying only part of the screen.

When preparing an image for online display (for example, a Web page that will be viewed on a variety of monitors), pixel dimensions become especially important. Because your image may be viewed on a 15-inch monitor, you may want to limit the size of your image to 800-by-600 pixels to allow room for the Web browser window controls.

Image resolution The number of pixels displayed per unit of printed length in an image, usually measured in pixels per inch (ppi). In Photoshop, you can change the resolution of an image; in ImageReady, the resolution of an image is always 72 ppi. This is because the ImageReady application is tailored to creating images for online media, not print media.

In Photoshop, image resolution and pixel dimensions are interdependent. The amount of detail in an image depends on its pixel dimensions, while the image resolution controls how much space the pixels are printed over. For example, you can modify an image's resolution without changing the actual pixel data in the image--all you change is the printed size of the image. However, if you want to maintain the same output dimensions, changing the image's resolution requires a change in the total number of pixels.

When printed, an image with a high resolution contains more, and therefore smaller, pixels than an image with a low resolution. For example, a 1-by-1-inch image with a resolution of 72 ppi contains a total of 5184 pixels (72 pixels wide x 72 pixels high = 5184). The same 1-by-1-inch image with a resolution of 300 ppi contains a total of 90,000 pixels. Higher-resolution images usually reproduce more detail and subtler color transitions than lower-resolution images. However, increasing the resolution of a low-resolution image only spreads the original pixel information across a greater number of pixels; it rarely improves image quality.

Using too low a resolution for a printed image results in pixelation--output with large, coarse-looking pixels. Using too high a resolution (pixels smaller than the output device can produce) increases the file size and slows the printing of the image; furthermore, the device will be unable to reproduce the extra detail provided by the higher resolution image.

Monitor resolution The number of pixels or dots displayed per unit of length on the monitor, usually measured in dots per inch (dpi). Monitor resolution depends on the size of the monitor plus its pixel setting. Most new monitors have a resolution of about 96 dpi, while older Mac OS monitors have a resolution of 72 dpi.

Understanding monitor resolution helps explain why the display size of an image on-screen often differs from its printed size. Image pixels are translated directly into monitor pixels. This means that when the image resolution is higher than the monitor resolution, the image appears larger on-screen than its specified print dimensions. For example, when you display a 1-by-1 inch, 144-ppi image on a 72-dpi monitor, it appears in a 2-by-2 inch area on-screen. Because the monitor can display only 72 pixels per inch, it needs 2 inches to display the 144 pixels that make up one edge of the image.

Printer resolution The number of ink dots per inch (dpi) produced by all laser printers, including imagesetters. Most desktop laser printers have a resolution of 600 dpi and imagesetters have a resolution of 1200 dpi or higher. To determine the appropriate resolution for your image when printing to any laser printer, but especially to imagesetters, see "screen frequency."

Ink jet printers produce a spray of ink, not actual dots; however, most ink jet printers have an approximate resolution of 300 to 600 dpi and produce good results when printing images up to 150 ppi.

Screen frequency The number of printer dots or halftone cells per inch used to print grayscale images or color separations. Also known as screen ruling or line screen, screen frequency is measured in lines per inch (lpi)--or lines of cells per inch in a halftone screen.

The relationship between image resolution and screen frequency determines the quality of detail in the printed image. To produce a halftone image of the highest quality, you generally use an image resolution that is from 1.5 to at most 2 times the screen frequency. But with some images and output devices, a lower resolution can produce good results. To determine your printer's screen frequency, check your printer documentation or consult your service provider.

Note: Some imagesetters and 600-dpi laser printers use screening technologies other than halftoning. If you are printing an image on a nonhalftone printer, consult your service provider or your printer documentation for the recommended image resolutions.

File size The digital size of an image, measured in kilobytes (K), megabytes (MB), or gigabytes (GB). File size is proportional to the pixel dimensions of the image. Images with more pixels may produce more detail at a given printed size, but they require more disk space to store and may be slower to edit and print. For instance, a 1-by-1-inch, 200-ppi image contains four times as many pixels as a 1-by-1-inch, 100-ppi image and so has four times the file size. Image resolution thus becomes a compromise between image quality (capturing all the data you need) and file size.

Another factor that affects file size is file format--due to varying compression methods used by GIF, JPEG, and PNG file formats, file sizes can vary considerably for the same pixel dimensions. Similarly, color bit-depth and the number of layers and channels in an image affect file size.

Photoshop supports a maximum file size of 2 GB and maximum pixel dimensions of 30,000 by 30,000 pixels per image. This restriction places limits on the print size and resolution available to an image.

Now here's the short answer for Edge printing:

Shoot for pixel dimensions of 100 ppi per inch at the size you plan to output the image when you process the image in your image editor.

Experiment with the different halftone types, LPI settings and color profiles available to you in Omega.

The two best halftone types in Omega IMHO are Classical Dot and STC Photo (Omega 2 or later).

The rule with LPI is the higher the number, the smaller the halftone dots, and the lesser the graininess .... but at a sacrifice of tonal quality. The lower the number the larger the dots and better tonal quality .... but more graininess. For work with gradients or photographs, LPI seems best to me in the range of 42 to 53 LPI depending on typical viewing distance. For non-photo work without gradients, I prefer LPI settings in the range of 60 to 90.

The best general purpose color profile of the lot is "Gerber Edge II 300 DPI CMYK". To access it, you must turn off the Automatic Profile selection in GSPPlot and manually select if from the drop down menu.