Sunday, May 31, 2009

How to reveal DSLR dust bunnies

This is a tip you may want to pass on to your customers to help them provide better originals for you to work with.

Many of the original images for print are now created with digital cameras rather than scans of transparencies. Unfortunately, pro DSLR cameras with their interchangeable lenses are susceptible to "dust bunnies" - particles of dust that settle on the glass plate in front of the sensor and show up as shadows in the final digital image. Often, these shadows are not noticed until the press is running. In this sample, a promotional still shot from the upcoming "Transformers: Revenge of the Fallen" movie, several dust bunnies have made their way into the image. Here is the original:
Click on images to enlarge
To reveal the dust bunnies using PhotoShop, create a duplicate layer of the image then go to the "Menu", select "Image"/"Adjustments" and the "Equalize" option.
This exaggerates the visibility of the dust bunnies. Making them more visible makes it easier to clone 'em out of the original image. Click on the image below to see the dust bunnies (where the arrows are pointing) more clearly.The original image was downloaded from HERE

Thursday, May 28, 2009

Printshop Evils

Overheard at various printshops:






Sunday, May 24, 2009

Halftone screen angles


With the exception of FM (stochastic screening), all screens consist of dots arranged in a regular pattern or matrix. The vertical and horizontal distance between successive dot centers is constant and is a function of the screen frequency. When the screen is aligned parallel to the paper edges, the screen angle is said to be 0° or 90°. The rotation angle away from the vertical axis is known as the screen angle. The screen can only be rotated up to 90° before it repeats itself. For example, a screen rotated 15° is at the same angle as 105°, 195°, and 285°.
A black and white halftone image consists of a single screen. The screen pattern is very noticeable when positioned at 0° and is least visible when rotated 45° as illustrated below.For that reason, black and white halftones are usually printed with 45° angled screens – particularly with coarser screens.

When two (or more) screens are printed on top of each another, a visually objectionable pattern known as moiré may occur. The most serious moiré patterns occur at very small angles between screens. Below are two overlaid halftone grids angled at 5 degrees and 10 degrees apart with the resulting moiré pattern:
The best angle between two screens that is least likely to cause moiré, and is most forgiving to small degrees of error, is 45°. However, in four color process printing, four different screens must be superimposed and all four screens must be angled within the 90° limitation.

A set of standard screen angles has been established that is based on a combination of theory and experience. First the least visible color, yellow, is placed at the most visible angle 0° (90°). Then the most visible color, black, is placed at 45°. The cyan and magenta are then placed between these two. Cyan at 15° (105°) and magenta at 75°. These angles represent a best all around compromise for most pictures and represent the standard, most commonly used screen angles. They also form the least objectionable moiré – the rosette pattern (more on rosettes here).
Because the Yellow printer is only 15° from the Cyan printer it produces moiré. The visibility of the moiré can be exacerbated if the Yellow becomes contaminated by ink traveling into it from previous press units. To help reduce the visibility of the Y/C moiré, most screening systems run the Yellow at a slightly higher frequency (lpi) – typically 108% of the frequency of the C, M, and K printers.Left: Yellow at the same frequency as Cyan. Right: Yellow at a higher frequency to help reduce visible moiré.

These standard screen angles are based on analog photomechanical screens and do not work best with electronic screens. At angles other than 0° and 45° a type of moiré patterning within one screen "single channel moiré" may occur. To avoid this problem, some vendors utilize shifted angles of 7.5° to introduce "noise" around the edges of the dots in order to break up and eliminate the visibility of single channel moiré.

Most printers will have a standard screen angle set that is used for all their jobs. However, if certain jobs have images where two of the process colors predominate and where those two colors are less than 30 degrees apart, then that screen set should be avoided and a different one used instead.

The following screen angle sets are all valid and are in common use. The sequence for the screen sets listed below is C, M, Y, K (i.e. the first screen set on the list is: 15C, 45M, 0Y, 75K). Remember that screen angles have quadratic symmetry so 0 degrees is the same as 90, 180, and 270 degrees.

Standard 4/C U.S. screen angle set:
15, 75, 0, 45 (possible moiré in greens since C and Y are only 15º apart)

Standard 4/C European screen angle set:
15, 45, 0, 75 (possible moiré in greens since C and Y are only 15º apart)

Other usable screen angle sets: Keep in mind that when two colors are less than 30º apart there is a risk of moiré
15, 45, 0, 75
15, 75, 0, 45
15, 45, 30, 45
45, 15, 0, 75
45, 75, 0, 15
75, 15, 0, 45
75, 45, 0, 15
75, 15, 60, 45

For 2/C jobs (e.g. duotones): Other angles can be used, however, the guiding principle is that the angles should be 30º or 45º apart and that the darkest color should be at 45º to reduce its visibility and lessen "sawtoothing" (see below)
Dark color at 45
Light color at 75

For 3/C jobs (e.g. tritones):
Darkest color at 45
Medium color at 75
Lightest color at 15

For 5, 6, or 7/C jobs (e.g. Hi-Fi color):
Use the angle of the unused color
Violet/Blue uses Yellow or Black angle
Green uses Magenta angle
Red/Orange uses the Cyan angle
Note that, depending on the original CMYK separation, the Black screen angle may be available to be used for one of the extra colors - V/B, G, or R/O.

Dealing with the Yellow printer moiré issue
Interscreen moiré becomes more visible when the angles of any two screens are less than 30 degrees apart. Yellow is usually allowed to be less than 30 degrees because it is such a light color that the moiré is usually not visible. Also, the frequency of the yellow printer is usually made higher than the other three colors (typically around 108% higher) to further minimize the visibility of the moiré. However, the moiré can become more visible if the yellow printer becomes contaminated/dirtied by the preceding process colors, or if its density is too high.

So, when skin color predominates:
15, 45, 0, 75 (avoids M/Y conflict/moiré but introduces C/Y conflict)

Or when light greens predominate:
45, 75, 0, 15 (avoids C/Y conflict/moiré but introduces M/K conflict)

Some printers use a coarse FM screen instead of a conventional AM screen for the yellow printer.This eliminates the moiré issue completely since FM screens do not have a fixed frequency or angle. For a 175-200 lpi AM screen an FM screen of about 35 microns would be used since that dot size will have a dot gain similar to the AM screened colors.

Other screen angle considerations
In certain circumstances, depending on the size of the graphic and the frequency of the halftone, the selected screen angle can distort the accurate rendering of images.

In the below graphic, the halftone screen angle is the same (45º) but the angle of the gray lines have been changed.Note how the screen has affected the rendering of the gray lines at different angles. The artifact at 1, 2, 3, and 4 is referred to as "ribboning" and is fairly common in automobile images.

In the below graphic, the halftone screen angles have been changed to the standard 4/C process angles (K 45º, C 15º, M 75º, Y 0º) but the angle of the three gray lines have been kept the same (0º).Ribboning has appeared in the Cyan and Magenta angles while the Black and Yellow angles have caused the appearance of different dotted line effects.

In the below graphic, the halftone screen angle is the same (45º) but the angle of the gray box has been changed in 10º increments.Note how the smoothness of the edges of the box changes as its angle relative to the halftone screen angle changes. The ragged appearance of edge of the last box is referred to as "sawtoothing."

Screen angles for more than four color - i.e. "Hi-Fi" printing (5, 6, or 7 colors)

Four color CMYK process printing is a good compromise that achieves a wide enough color gamut for most applications while using the minimum number of inks to achieve it. However, sometimes the printer needs to go beyond 4/C in order to achieve a satisfactory rendition of the image. Typically the gamut deficiency will be in the overprint colors - Red/Orange, Blue/Violet, Green.

Here is an original RGB image:And here is the CMYK version of it:To restore some of the original color impact, the printer may choose to use "bump" or "touch" plates to boost color back into areas where it was lost. However, adding extra colors causes problems since all possible screen angles have already been used by the C, M, Y, and K printers.
In this example, these are the four channels that make up the image:Note that there is virtually no Cyan in the Red/Orange areas, or Yellow in the Blue/Violet areas, or Magenta in the Green areas. Therefore, those screen angles become available for the extra bump inks. So the trick is to use the screen angles of these unused colors.
In this example Violet, Green, and Red:In short, the Violet ink would take the unused Yellow angle, the Green ink would take the unused Magenta angle, and the Red ink would take the unused Cyan angle. Note also that, depending on the original CMYK separation, the Black screen angle may be available to be used for one of the extra colors - V, G, or R.

Wednesday, May 20, 2009

The creative design/production process

The creative design/production process step-by-step:

• Receive the design brief
• Create rough concepts
• Get signed approval to proceed
• Schedule and attend the photo shoot
• Review images and layout with client and various stakeholders
• Select the images and contract any photo retouching
• Check proofs to confirm correctness of retouching
• Create the finished layout
• Preflight the job
• Proof the job – get approvals
• Make revisions as required
• Proof the job and get sign-off
• Go to press
• Go to bindery
• Complete mail drops/distribution• Oooops! notice the leftover layer section clipping path effect
• Hit the bar
• Edit portfolio
• Update résumé
• Pick up a copy of the Atlanta Social Season magazine• Arrgggghh!!!!

(Images courtesy of PhotoShopDisasters.com)

Sunday, May 17, 2009

The Wayback View - Platemaking at the Chicago Tribune - 1937

Preview images from the video

From original art to exposing and processing the plate and finally pulling a proof at the Chicago Tribune in 1937. We also see the use of "Dragon's Blood" in the platemaking process. Interestingly, 76 cases of Dragon's Blood consigned to Brown Brothers & Company to be used in photoengraving went down with the RMS Titanic 15 April 1912. Dragon's Blood is acid resistant, and is used to reinforce the coating that the engraving plates have on them. After the photo sensitive coating is exposed to a bright light, any imperfections in the plate are touched up with Dragon's Blood before they are etched.

video
Please press the play arrow to view the video. Note that it may stop for a moment while the video buffers in the background.

Thursday, May 7, 2009

Grey Balance Unbalanced – An inconvenient truth

Warning: This post may be considered by some, perhaps many, as heresy, sacrilegious, and blasphemous – read it at your own peril.
Achieving grey balance in presswork is touted as the holy grail in today's print production world. It's become the paramount metric - recently defined in unambiguous CIE L*a*b* values - for the G7™ calibration method to align press and proof color as well as for achieving presswork that conforms to GRACoL® 7. Interestingly, despite its promotion by industry pundits, as far as I can determine, there has never been a formal, objective, study of the relationship of grey balance targets measured in press sheet color bars and how they relate to the live image content of the press sheet.

While grey balance certainly has value - particularly in the initial setting up of a print production system - the grey balance targets in color bars, IMHO, have virtually no value as a print reproduction metric in day to day offset printing.

The industry-wide assumption is that the three-color grey balance patch of the color bar comprised of 50C 39M 39Y, performs and shifts identically to the four-color grey balance of a live image comprised of approximately 26C 18M 18Y 32K (at the same tonal density).

Put another way – on press, the assumption is:
I don't think so.

For one thing, the tone values 50C, 39M, 39Y are where dot gains/TVIs are at their greatest and where on-press tonality (and therefore color) is much less stable than at the 26C 18M 18Y tone values – smaller dots are proven to be tonally more stable than larger dots (which is why dot gain is measured at 50% where dots are at their largest).

Also, the 32K in the separation provides a great deal of grey balance color stability in the imagery which doesn't exist in the color bar grey balance patch.

Bottom line:
The grey balance target in the color bar is overly sensitive to minor solid ink density variations in presswork and has no tonal relationship with the screen tint values that are its supposed equivalent in CMYK separations. This is even more true in workflows that use ink optimization heavy GCR separation techniques) on incoming files. As such, the grey balance targets in color bars cannot reflect color shifts that may be happening in live presswork areas.

Put another way – on press, the reality is that:
There are other issues with grey balance being used as a process control metric in production presswork besides the mismatch of screen tint values compared with greys in the live image areas.

The inline ink usage issue

Unlike grey balance in scanning and proofing - grey balance on press is affected by ink usage/coverage. For example, in this press form:The grey patch targets in line with the blueberry image will have a distinct blue/purple cast due to the lack of yellow in the image as well as the need to push cyan and magenta densities to deliver a rich blue in the live image area. On the other hand, the grey patch targets in line with the tomato will have a redish cast due to the lack of cyan in the image and the need to push magenta and yellow densities to deliver a rich red in the live image area.

Bottom line:
The live image areas can match the proof while grey balance on the press sheet do not.

The human visual system issues

C, M, Y grey balance targets are typically located beside a corresponding 50% patch of black in the color bar. The assumption being that the press operator, by comparing the two, can make a quick visual assessment of grey balance in the presswork. There are some significant problems with this assumption.

First of all is that the visual perception of grey is affected by several factors:

1) Unlike instruments, we perceive color in the context of surrounding colors. If you look at the grey bar in this graphic:you'll see that it appears to change its tonality depending on whether it's against the red or green background even though the grey is in fact exactly the same across its width. You may also see a slight shift in color. This optical illusion is called "simultaneous contrast."

Bottom line:
Even if the grey patch in a color bar appears perfectly neural compared with the 50%K patch - the grey within the live image area may not appear neutral due to the effect of surrounding colors.

2) Our perception of color, especially neutral grey, not only varies from individual to individual, but is also affected by age and gender. Older men (e.g. typical press operator) will not see a 3/C grey vs a 50% K grey the same as younger women (e.g. typical print buyer). You can test this yourself with a chart like this one:On the left is the reference 50%K. On the right are a variety of 3/C greys. Try and pick out which one is identical to the reference grey. In fact, if you were to print out the patches, cut them into squares, and asked co-workers to match the 3/C patch to the 50%K patch you'll most likely see the pattern of men/women, younger/older, choosing different 3/C patches that they see as perfectly matching the 50%K one.

Bottom line:
The visual match assessment of 3/C grey and 50% K is inconsistent between people particularly buyers and suppliers.

Lack of instrument agreement

According to a report by Greg Radencic, PIA/GATF, on Spectrophotometer Device Agreement presented at a recent Splash technical color conference, out of seven spectrophotometers tested, four had a DeltaE error over 1.5 when measuring grey balance on 80lpb gloss coated paper. One instrument had a Delta E error of 1.75, another 3, another 4 and the last had a whopping Delta E error of 7 when measuring grey balance.
The seven instruments were factory new and calibrated.

Bottom line:
The measured match assessment of 3/C grey and 50% K is inconsistent/unreliable between different types of spectrophotometers. Even different instruments of the same model may disagree.

The metameric issues

One final issue is that a chromatic 3/C grey (50C, 39M, 39Y) and an achromatic 50% black form a metameric pair. What this means is that they may match color under one lighting condition, however, under a different lighting the 3/C grey will shift hue because it is spectrally different than the 50% black. To illustrate:
This mismatch due to lighting conditions can be exacerbated between inkjet proofs where the 50% black patch may be made up of 4/C and presswork where the 50% K is only made up of black ink.

Bottom line:
Metameric issues can arbitrarily and continuously cause 3/C grey patches to mismatch against the 50%K reference patch as viewing conditions change.