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Introduction

From Imaging Engine (IE) 14.1.1 onwards, a user can specify how screen angles should be interpreted by the RIP: Screen angle direction clockwise (default behavior on FlexRip) or counterclockwise (default behavior on Nexus).

This option is added so output generated by FlexRIP, Nexus PostScript and TotalRIPs can be emulated.

In versions of Imaging Engine prior to V14.1.1, there was an option in the Imaging Engine device's Configure panel to represent screen angles in clockwise or counter clockwise angle direction. However, that option had no effect on screened output: Screen angles in output bitmaps did not change when changing the option.

In Imaging Engine V14.1.1, the angle direction option has been moved to the 'Image to Screened Separations ticket', and now this option has an effect on screened output. When switching from clockwise to counter clockwise, screen angle direction in the output bitmaps will change accordingly.

This document describes clockwise and counterclockwise screen angle direction, it explains how the option can be set and used in IE 14.1.1 and it documents current limitations when using this option on certain screens.

The second part of this document provides some background information about what output to expect when changing screen angle direction for different kinds of screens.
This part also explains in detail why some screens can only be used in clockwise screen angle direction.

For more information on how tickets from previous versions are handled in Automation Engine 14.1.1, see

Interpreting screen angles: What is clockwise or counterclockwise screen angle direction?

Screening angles are measured from the positive X-axis to the positive Y-axis. This is also called the device coordinate system.

Depending on the position of the X-axis and Y-axis, the screen angle direction can be clockwise or counterclockwise.

Counterclockwise screen angle direction

When the positive X-axis is pointing right and the positive Y-axis is pointing up, the screen angle direction is called 'counterclockwise'.

In this case, positive angles are interpreted as counterclockwise (CCW).

This screen angle direction is the default behaviour for screened output generated by Nexus RIP.

Clockwise screen angle direction

When the positive X-axis is pointing right, but the positive Y-axis is pointing down, the screen angle direction is called 'clockwise'.

In this case, positive angles are interpreted as clockwise (CW). 

 

This screen angle direction is the default behaviour for screened output generated by FlexRIP.

Specifying screen angle direction in Automation Engine tickets

Changes in 'Image to Screened Separations' tickets

The Image to Screened Separations ticket in Imaging Engine V14.1.1 contains a new setting, called Screen Angle Direction.

The Screen Angle Direction setting can be found by going to the Separations tab of the Image to Screened Separations ticket
and clicking the Advanced Screening Settings… button.

The Screen Angle Direction can be set by selecting the relevant setting from a popup menu.

The setting allows the user to set the screen angles as used by the ticket to be determined by one of two methods:

  • Clockwise (FlexRip Compatible) This setting places the 0° point at the '3 o'clock' mark, and measures angles clockwise from there. It is the screen angle convention that was used by the FlexRip family of RIPs.
  • Counter Clockwise (Nexus Compatible) This setting places the 0° point at the '3 o'clock' mark and measures angles counter-clockwise from there. It is the screen angle convention that was used by NexusRIP and Nexus TotalRIP.

Using with SmartNames

It is also possible to use SmartNames to control the Screen Angle Direction setting of the ticket.

To enable SmartNames, choose Select SmartName… from the popup menu and select an applicable SmartName. SmartNames must resolve to either ‘CW’, for the Clockwise (FlexRip Compatible) setting, or to ‘CCW’ for the Counter Clockwise (Nexus Compatible) setting.

Limitations for screen angle direction option

Modulo 90 screens that contain surface screening technology cannot be used in CCW angle direction.
That is because surface screening is setup for screens with CW angle direction and mirroring the screen to switch to CCW would change print characteristics.
For more detailed information about this limitation, see Why are some screens restricted from being used in CCW screen angle direction.

Screens that can only be used with clockwise screen angle direction

Screen type Dot Mnemonic Screen Name No counter clockwise screen angle direction support
 Groovy screens GVY1, GVY2, ... Groovy screensAll groovy screens
see Static Groovy screens
 Custom screensSCR01, SCR02, ...  <custom screen name>When surface screening effect is used and base screen is modulo 90
see Customized Screens
 HDFlexo screens HD01, HD02, ... <HDFlexo database> All HDFlexo screens that use surface screening
see HDFlexo screens
FullHDFlexo screensHD01, HD02, ...FullHDFlexo (Inline UV)All FullHDFlexo screens, because all of them use surface screening
see HDFlexo screens

Screens grayed out when counter clockwise direction is selected

Screens that only support clockwise screen angle direction will be grayed out when a user has changed the screen angle direction to counter clockwise direction.

In the below example SCR03, SCR04, SCR05 and SCR06 are enabled while SCR07, SCR09 and SCR10 are grayed out.
All these screens are customized screens, setup and created in ScreenManager.

SCR03, SCR04, SCR05 and SCR06 are enabled because these custom screens support CCW angle direction: They do not use any surface screening effect, or these screens are based on a modulo 180 base dot.

SCR07, SCR09 and SCR10 are grayed out because these screens do not support CCW angle direction: They are based on a modulo 90 screens and they use surface screening effects.

In the below example GVY screens are grayed out. These are static Groovy screens which do not support CCW angle direction.

In the example you can also see that HD01 is enabled, while HD02, HD03 and HD04 are grayed out.

All these screens are HDFlexo screens, verified and created from the HDFlexo application.

HD01 is enabled because it supports CCW: this HDFlexo screen does not use surface screening effects.
HD02, HD03 and HD04 are grayed out because they do not support CCW: these HDFlexo screens have surface screening effects.

Changing to counter clockwise screen angle direction on screens that do not support it

When a user changes the screen angle direction from clockwise to counter clockwise, but when the ticket is based on screens that do not support CCW direction, changing the screen angle direction to CCW will be prevented, a warning message will be displayed.
This is illustrated in the example below:

A user creates a ticket and selects screen HD07 (HD Flexo C76 MC12P – Dupont DPR – Flexibles-MV) as default dot.
This screen uses the surface screening effect MC12P.
When a user wants to change the screen angle direction in this ticket to CCW, the following warning message will be displayed:

Appearance of screen angle direction for different types of screens

The number of different screen angles that a specific screen supports depends on the type of screen: Does the screen use symmetric dot shapes, is it created as a modulo 90 or a modulo 180, does the screen uses orientation-sensitive effects.

Screens with symmetric dot shapes

Screens with symmetric dot shapes only have different screen orientations between 0° and 90°. When the dot shape is symmetric for all tones, there is no difference between for example a 45° and a 135° screen angle.
For these screens, switching from one screen angle direction to another is the same as mirroring the screen over its horizontal axis.
 

Offset screen angle sets:


 Flexo screen angle sets:

Screens with asymmetric dot shapes

When using symmetric dot shapes at gray levels where inked dots are starting to touch each other (e.g. near 50%) irregular dot-bridging can occur. This can result in a graininess appearance or can result in unwanted tone changes.To prevent this, dot shapes are used that have a certain aspect ratio: The dots touch each other first in one direction then in the other. So dot bridging is guided and spread over a larger tone range (e.g. from 40% to 60%).
Some screens use an asymmetric dot shape over the whole tone range (e.g. D (Rugby) screen),other screens only use asymmetric shapes near mid tones (e.g. R (Round-Fogra) screen.

E.g. the D (Rugby) screen uses a dot shape with a certain aspect ratio (the shape is horizontally wider than vertically).




E.g. the R (Round-Fogra) screen uses a dot shape which is round in highlights and shadows, but near mid tones the shape has a slight aspect ratio: in horizontal direction dots will touch each other at lighter tones compared to vertical direction.


Chain direction of a screen

We define the chain direction of a screen as the direction where an asymmetric dot (a dot with a certain aspect ratio) has it largest size. E.g. For an elliptical dot, the chain direction is the direction where the ellipse is widest. In this direction the dots will touch each in the lightest tone, at about 40%. In darker tones, at about 60%, dots will touch in the other direction.


Rosette moiré and optimal chain direction

When printing with three or more inks, it is important to assign the correct screen angle for each of these inks: To print uniform colors and to make printing robust against registration errors, the screen angles of the different separations should be as far as possible from each other.

This can be illustrated with the below images: Each image represents two screens with a specific screen angle difference. From left to right, the screen angle difference is increased.
For screen angles close to each other (left most image) the moiré period is much larger than when using screen angles further away from each other (right most image).
For the left most image the color shift period (= the distance from printing dot-on-dot to dot-nexto-dot) is much larger than the screen period (= determined by screen ruling), and will become visible for human eye.
For the right image, the color shift period is much smaller and close to the screen ruling.

When symmetric dots are used screen angles can be from 0° to 90°, so the largest angle difference for the three most visible inks is 30° (=90°/3).
E.g. C:15°, M:75°, K:45°

When asymmetric dot shapes are used then there are more different screen angles possible: from 0° to 180°.
The largest angle difference for the three most visible inks is then 60° (180°/3). 

It is common practice to always select screen angles that are 60° away from each other, also for screens with symmetric dots.
This is also the case for Nexus RIP where the default angle sets for the most visible inks C,M,K have angles separated by 60°.

Modulo 90 screens

Modulo 90 screens are screens that only have screen angles available from 0° to 90°.
For angles requested above or equal to 90°, modulo 90 of that screen angle is used. E.g. 90° == 0°, 105° == 15°.

Modulo 90 screens can also have asymmetric dot shapes.
As mentioned in previous chapter, to obtain optimal overprint behaviour when using asymmetric dot shapes the screen angles for the basic inks (C,M,K) should be 60° separated from each other. 
To obtain this 60° angle difference for modulo 90 screens, the chain direction of some of the screen angles is internally rotated over 90°.
This is for screen angles 0°, 37.5°, 45° and 52.5°.

Example:
The requested screen angles are C=15°, M=45° and K=75°.
Internally, the chain direction for the 45° screen is 90° rotated.
So the output will be C=15°, M=135° and K=75°.

For modulo 90 screens switching from CW to CCW cannot be done by simply selecting the CW to CCW calculated screen angle.
Therefore switching from one screen angle direction to another is obtained by mirroring the screen tile over its horizontal axis.
This is explained in the following example:
E.g.
The requested screen angle is 15° CCW. If we would select the CCW calculated version of this angle, then we would select the 165° CW angle (180°-15°)
For a modulo 90 screen the 165° CW angle does not exist: screen angles only go from 0° to 90°. So the 165° CW screen angle would actually load a 75° CW screen (165°-90°).
And a 75° CW angle is equal to a 115° CCW angle
=> the resulting angle is different from the original requested screen angle 15°CCW.

Requested screen angle is 15° CCW. We don't translate the screen angle into the new angle direction, but we will mirror the CW version of the screen:
15° CW screen is loaded first, then the screen is mirrored over the horizontal axis.
The output result is 15° CCW which is equal to the requested angle.

Offset modulo 90 screen angle sets: 

Flexo modulo 90 screen angle sets:

Modulo 180 screens

Modulo 180 screens are screens that have screen angles available from 0° to 180°.
These screens have typically asymmetric dot shapes over the complete tonal range. Examples of such screens are X (Eccentric), PARDIA35 (Paragon Diamond35). etc.

For modulo 180 screens, chain direction is not swapped internally. E.g. if a screen angle of 45° is requested, the chain direction will also be 45°.
This is important when selecting an appropriate screen set for modulo 180 screens: make sure the requested angles are always 60° different.

Example:
The requested screen angles for dot ParEll35 are C=15°, M=45° and K=75°.
Since this is a modulo 180 screen, the screen angles in the output will be C=15°, M=45° and K=75°:
Because a modulo 180 screen is used, chain direction is not swapped for the 45° screen angle.
This does not result in an optimal overprint behaviour, because the screen angles are only 30° different.
To correct this, the user should select 135° for M.

For modulo 180 screens, changing angle direction is done by selecting another screen angle, the screen does not need to be mirroring.
E.g. When a screen angle of 15° CCW is requested, and the screen has modulo 180 CW screen angles available, a screen of 165° CW will be selected.
As you can see in the image below, a screen angle of 15° CCW is equal to a screen angle of 165° CW.

Offset modulo 180 screen angle sets: 

Flexo modulo 180 screen angle sets: 

Why are some screens restricted from being used in CCW screen angle direction

What is surface screening?

To improve ink transfer for solid objects, surface screening can be used: The solid object is filled or perforated with a specific pattern so that ink transfer is more efficient, with less pin holing.The below images show the difference between a plain solid object and a solid object painted with Groovy screens

Digital file:

Printed result with white ink:

Surface screening integrated in a screen

The surface screening solutions used by Esko are solutions that integrate a 'surface screening effect' into the screen.

Below is an example of a Groovy screen that is used with the E (Elliptical) dot as it would appear on the digital file.

1st image: Vignette from 30% to 100% and a 100% square, screened with standard E (Elliptical) screen.
2nd image: Vignette from 30% to 100% and a 100% square, screened with E (Elliptical) dot combined with Groovy screens.
 

Surface screening and orientation sensitivity

A screen that uses surface screening effects must be carefully setup:
To have the ink transferred from plate to substrate in the most optimal way, the pattern that perforates solids must match the specific inking system of the printing condition.
For e.g. the pattern frequency and orientation must match the grid and cell volume of the used anilox.
This setup is mostly done by trying several different surface screening patterns (different shapes, different frequencies, different orientations), and from the print results derive the pattern that results in the most optimal solid ink transfer.

The optimal pattern orientation is not always the same as the screen angle direction used for the separation.
In many cases the most optimal orientation for the line pattern of Groovy screens should be about 45°, no matter what ink is used, no matter what screen angle is used.
In the above example, an elliptical screen at 82.5° CW is used, but the line pattern must be kept at 45°.

Surface screening patterns have typically a high frequency (higher than the screen ruling), and are critical to image on plate.
The CW version of a pattern, the mirrored version of the original pattern, can behave different than the original CCW version of the pattern.
E.g. a 45° line pattern might image different than a 135° line pattern due to optical constraints.

Screens containing surface screening effects restricted to be used with CW screen angle direction only

Screens that use surface screening effects are created and setup with the ScreenManager or HDFlexo application.
These tools use CW as screen angle direction.

To guarantee that the surface screening pattern orientation used in custom screens or HDFlexo screens is kept exactly the same as during setup, such screens will be prevented from being used CCW.
That is because Modulo 90 screens that change from CCW to CW angle direction are mirrored by IE, and this would change the surface screening pattern orientation.

On Nexus RIP a user could use any Esko screen in CW and CCW screen angle direction. Also modulo 90 Esko screens that use surface screening effects and HDFlexo screens could be used with CCW screen angle direction. Using these screens with CCW screen angle direction could result in poor output quality.

Static Groovy screens

These screens are available as static dot shapes.
Dot mnemonic GVY1 … GVY5

These screens were introduced with FlexRIP and were created from the CO (Endless) dot.
FlexRIP uses CW screen angle convention, and the CO dot is a modulo 90 screen, so these static Groovy screens only have screen angles from 0° to 90°.

If this screen is to be used with CCW screen angle direction IE would mirror the screen over the horizontal axis.
This would also change the direction of the groovy lines which could result in different printing characteristics.
Therefore static Groovy screens can only be used CW.

Customized Screens

These are screens that have been setup  and created with ScreenManager.

These screens have a dot code that starts with 'SCR'.

Which type of customized screens only supports CW

If the original dot of a custom screen is a modulo 90 screen and if custom screens uses surface screening then the screen can only be used CW.

If the original dot of the custom screen is a modulo 180 screen or the screen does not use surface screening then the screen can be used CCW and CW.

How to know if a customized screen is a modulo 90 screen

In ScreenManager you can easily see if the selected original dot is a modulo 180 screen.
Select the screen and verify the screen details panel on the right.
If a screen is modulo 90 the angles column will only show screen angles from 0 to 90.

How to know if a customized screen uses surface screening effects

It is not always possible to know from the screens name if a custom screen uses surface screening effects.
That is because a user is free to choose the screen name when a customized screen is created.

This can be found out by selecting the screen in ScreenManager:
A customized screen that uses surface screening will have the 'Surface Screening' tab highlighted.

HDFlexo screens

These are screens that are created with the HDFlexo tool.

These screens have a dot code that starts with 'HD'.

Which type of HDFlexo screens only support CW

HDFlexo screens that use surface screening effects can only be used CW.

 

A special type of HDFlexo screens are FullHDFlexo screens.
These screens can be added to the HDFlexo application via the FullHDFlexo FTD Addon package. They are added under the plate type 'FullHDFlexo (Inline UV)'.
These are screens created specially for FullHDFlexo enabled CDI devices and use dedicated surface screening patterns.
e.g. patterns WSI, MG45, MG34 enabling P+ CDI technology.

FullHDFlexo screens can only be used in CW screen angle direction.

 

The same limitation also exist for other 3rd party FTD Addon packages (Dupont Digiflow, Flint, MacDermid LUX).
These screens can also contain surface screening patterns dedicated for CDI P+ hardware, that can only be used in CW screen angle direction

Screens from 3rd party FTD addon packages can only be used in CW screen angle direction.

How to know if a HDFlexo screen uses surface screening effects

A HDFlexo screen that uses surface screening effects can be recognized from the HDFlexo screen name.

e.g. 'HDFlexo C56 MC12P' uses MicroCell of 12 pixels as surface screening pattern.

e.g. 'HDFlexo C25 MCWSI' uses P+ surface screening pattern WSI

e.g. 'HDFlexo C34 MG45' uses P+ surface screening pattern MG45

A user can also see from the HDFlexo application if a HDFlexo screen uses surface screening effects:
Select the screen in HDFlexo and verify the 'Screen Info' information visible on the right panel.
If surface screening is used (e.g. microcells), this 'Screen Info' will list 'Enhanced Shadows and Solids'.

 

Article information
Applies toImaging Engine
Created30-Nov-15
Last revised 
AuthorPEMO
CW Number 
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