Friday, August 27, 2010

Halftones as you've never seen them before

I use a variety of image analysis tools when investigating how different halftone screening solutions perform. These tools are normally used in the medical field to do image analysis of microscope acquired imagery. However I press them into service to analyze various aspects of halftone dot structures.

Here is a microscope view (200x) of a conventional AM/XM printed halftone dot (175 lpi elliptical):
And here's a microscope view (also 200x) of a 20 micron FM screen:
One of my favorite tools is to use image analysis software to project the pixel density values in the images into height - creating a 3D image that shows the relative ink density (ink film thickness) differences between the two screens. The thicker AM/XM:vs the thinner FM:Using color mapping instead of the actual ink color makes the difference in ink film thickness even clearer (yellow = greatest - blue= lowest ink film density):Lowering the viewpoint and warping the perspective of the 175 lpi AM/XM screen begins to turn the image into a kind of landscape: However, using terrain mapping software on those original microscope images of the AM/XM and FM screens really makes the transformation of the images into proper landscape views a reality.

175 lpi elliptical dots:
Sunlight across a deep FM canyon:
A low flight over a a barren land where FM and AM screens meet.
Sunrise over an AM screen mesa.
Moonrise over an FM peak.
FM screen hits the wall.

Planet Round Dot.

And if you have a pair of these:
You can add a bit of dimension to your halftones:
Of course, this is all very serious work - not fun at all. Really. ;-)

Wednesday, August 25, 2010

Every letter tells a story - print problems revealed in type reproduction.

Spend even a small amount of time in a printshop and you'll quickly realize how much effort press operators put into scrutinizing their presswork through a loupe. The two things they're typically looking for are clear centered rosettes - this tells them that the presswork is in register – and print problems revealed by how well individual letters of type are reproduced in print.

Here's how type printed offset reveals problems:

Normal ink transfer - no apparent problems.

A ghost image appears just behind the primary graphic causing an increase in dot gain. More information about doubling HERE

A smeared ghost image appears just behind the primary graphic. More information about slur HERE

Over emulsified (a.k.a. emulsified, water logged)
Ink is designed to accept a small amount of water (fountain solution) on press. Too much water appears as tiny droplets in solid areas and reduces gamut, image sharpness, contrast, and increases dot gain.

Tailing (a.k.a. misting, slinging)
Too high a solid ink density or ink that's too "long" can cause thin "tails" of ink strands to appear extending behind the primary image causing a loss of sharpness, contrast and an increase in dot gain.

Picking (a.k.a. pinholing)
Small white specks appear in solids. This usually results because the ink tack is too high or there is not enough impression cylinder pressure ("squeeze").

Ink breakdown
On press, fountain solution acts like a solvent. If it is too aggressive or acidic it can break down the ink that forms the image resulting in presswork that appears muddy and/or "soft." It also reduces total gamut because it reduces overprint trapping efficiency.

Wednesday, August 18, 2010

How was it printed? Simple ways to determine the printing method used.

The proliferation of new printing technologies like commercial production toner and ink jet presses as well as the rapid increase in quality of some traditional systems like flexography, has made it a bit difficult to determine exactly which printing method was used for a given sample. Was it offset or digital ink jet? Flexo or offset?

Here is a guide to help you identify which printing method has been used. The best method is to look very closely with a loupe at the letter forms and then confirm by looking at the halftone dots themselves. Although these images were taken at about 200x to show as much detail as possible, they are not that different than what would be seen by the human eye under a loupe at 10-20x.

Offset lithography

Type edges are sharp and well defined. The ink density is similar across the letter. The paper around the printed ink is usually clear/unprinted. The sharpness and consistent ink density is reflected in the halftone dots. Note that this same sharp appearance occurs if the halftone screen used is FM (the third image) rather than AM (second image).

All graphic elements appear halftone screened so the edges of text show a rough halftone appearance. Although presswork color may appear rich, viewed under a loupe, the halftone dots may have a watery appearance. Individual dots may appear to have a hole in their centers.

Flexography uses a rubber-like plate that, under pressure, transfers ink to the substrate. This results in type that often appear to be surrounded by a sharp-edged "halo." The same effect, but in opposite, occurs in the halftone dots which may have dark edges and light centers. Both type and halftone dots usually have very sharp and well defined edges. Note that this dot sharpness also occurs with an FM screen printed with flexo (third image).
Electrophotographic (a.k.a. laser toner, Xerographic)

With electrophotographic systems the image is created by depositing a powder toner. This results in a "sprayed on" appearance. Type edges are soft and there are often specks of wayward toner on what should be unprinted paper. On close examination you'll likely see apparently random Yellow dots. Those are anti-counterfeiting tags. Halftone dots have a similar fuzzy appearance.

Under a loupe, ink jet type has a "blocky" "chunky" edge appearance. This is caused by the low resolution/large droplet size typical of these devices (despite the claims of high dpi). Sometimes there will be the occasional "satellite" ink droplet near the letter (two are shown in the image below). Because of the way the ink is sprayed onto the substrate, halftoned areas appear to be done with an FM screen. However, unlike the FM screening used in offset lithography, there is usually no apparent pattern or organization to the dots.

Engraving is typically used for currency/bank note printing, stamps, and sometimes very exclusive stationary. Type is very crisp and sharp. The process is capable of extremely fine detail and as a result is used to print security microtype. Halftones are usually created with line effects rather than a conventional halftone screen. Softly passing one's fingers over the printed work will reveal a tangible texture due to the raised ink.

Thermography is commonly used on wedding invitations, letterheads, business cards, and greetings cards as an attractive alternative to the more expensive engraving process. Type has a raised surface texture similar to engraving however, with thermography type appears to be printed under a transparent coating. The low resolution of this process makes it unsuitable for quality halftone screening.
LCD display (print to screen - just for fun)

The letter "e" as displayed black against a white background on my laptop's display. Since the LCD display produces no light of its own, it requires a cold cathode fluorescent lamp situated behind the LCD panel. Electronically-modulated pixels filled with liquid crystals are arrayed in front of the light source to produce images in color. Passing the backlight through the red, green, and blue liquid crystals recreates the backlight white, while blocking the backlight creates black. Anti-aliasing of the letter helps to smooth the shape of the letter form and compensates for the relatively low resolution of the screen itself. Note that the pixels forming the image are not square but rectangles.

Saturday, August 14, 2010

Optimizing images in 8 easy steps - the real basic basics

Who this post is for
Designers and people who supply images to printers and want to provide the best image possible without a lot of technical complication. Also, printers who receive poor quality images who want to help their customers tweak images to get a better result.

The software
I'm using PhotoShop CS3, but any image editing software that is capable of doing these basic manipulations will work.

The image
I'll be using this image:because it has, I hope you can see, some obvious color problems. The image is from the daughter of make-up man Harry Buchman who worked on the movie "Jaws."

The 8 basic steps
There are many ways and tools that achieve the same or similar results - most are very simple, but like anything they can be made more complicated. This following is a basic, pragmatic, method to getting better images for reproduction.

These are the 8 steps:
1) Check the computer monitor display to make sure that it is representing the tone scale correctly.
2) Check the computer monitor display to make sure that it is representing neutrals correctly.
3) Check the image resolution.
4) Set the color target.
5) Adjust color
- with an "auto-correction".
- with a manual correction.
6) Set Black and white points.
7) Adjust contrast.
8) Sharpen image and save (under a new name).

Step One - the tone scale
Download this graphic and open it in your image editing application:You should see a 21 step ramp going from 0% on the left to 100% black on the right. The black should look black and the white look white. You should be able to distinguish every patch of grey separate from each other however it is acceptable if the last two patches (95% and 100%) blend together.

Step Two - neutrality
The patches of grey in the graphic should all look neutral.

If your display passes #1 and #2 then you're good to go. If not, you may need more sophisticated help.

Step Three - image resolution
Check the image resolution. Typically the image size is based on a resolution of 72 pixels per inch (a.k.a. 72 dpi):This image is sized at 8.333" x 5.625" at 72 pixels per inch. 72 pixels per inch is pretty standard for web pages. For printing, a higher resolution is required. Changing the resolution of this image to 300 pixels per inch with "resample" not selected: shows that this image is only suitable to be reproduced in print at a size of 2" x 1.35". An image resolution of 300 pixels per inch (a.k.a. 300 dpi) at 100% reproduction size is quite standard for print application.

Step Four - set the color target
This tells the image editing application what the intent, or final destination, of the image is and helps it to display the image correctly. The color target is embodied in a color profile.
The two most common color profiles used are:
1) sRGB - which is used for images destined for the web or printed on desktop inkjet and photofinishing printers.
2) Adobe 1998 - which is used for images destined for print reproduction.By default, most digital cameras embed the sRGB profile. It can be reassigned to the Adobe 1998 profile if the destination is a printing press.

Step Five - adjust color
Many image editing applications have an auto-correction capability:Sometimes that will give an acceptable result or one that can be a base for further work:Auto-correction tends to work best with images that contain a great variety of colors.


Manual color correction takes four steps:
1) identify neutrals
2) neutralize mid-tones
3) neutralize shadow-tones
4) neutralize highlight-tones

This image contains several likely neutrals:The highlights on the water, the midtones of the beret, the shadow-tones in the darkest part of the vest.

Use the eyedropper tool to measure the RGB values of the mid tones in the beret:The eyedropper indicates that the RGB values in this part of the image is:
R 131
G 71
B 60

The large number for RGB relative to the G and B values indicates that the image has a red cast (too much red). To be neutral grey the RGB values should be similar. By adjusting the RGB color values so that they are similar makes the mid-tones represented by this area of the image neutral:It's not important that the RGB values are absolutely identical - close enough is good enough.
In this case the final RGB values are:
R 81
G 87
B 85
Note that the color adjustment is balanced in that Red is lowered while Green and Blue are increased.

Repeat the color adjustment, using the same procedure for the highlights and the shadows.

The result:
Step Six - adjust black and white points
Use the "eye dropper" tool in concert with the "Show Info" dialog box to get the RGB values of the specular highlights. Highlights that should be absolutely white with no detail:Pure, no detail white should have RGB values of 255 each. This image's values are:
R 242
G 235
B 235
Which means that although they may appear white - the highlights are actually grey. Adjusting the highlights using the "level adjustment" option sets the highlights to their appropriate values:The same thing would then be done to the darkest black in the image which should be close to:
R 0
G 0
B 0

The result:
Step Seven - adjust contrast
Adjusting contrast changes the relationship of the tones of the image making it appear "harsher" or "softer. The "Curves" menu option is used to alter contrast:
Step Eight - sharpen image and save (under a new name)
The final steps are to "sharpen" the image (of which there are many different methods):In general, images destined for the web should be sharpened enough so that they look "natural". Images destined for print should appear slightly over-sharpened, a bit harsh looking because halftoning has a bit of a softening effect on images.

So, following these "simple" 8 steps, here's where we started:And here's where we ended:
Obviously more work could be done on the image. Other, more sophisticated, methods could be used. However following these simple 8 steps should provide much better results for those people who don't want to get too technically involved.