On printers with multiple dot sizes and/or multiple shades of color, which dot size gets printed and where one size transitions is an important part of the rastering process. Using small dot sizes in light tones ensures a smooth appearance in printouts, while using large dots in full tone areas ensures good color coverage of the material. This ensures that the accurate tone of color is printed and that it is done with the most efficient amount of ink. So the dithering has an influence on both the look of the printout and how much ink is needed. Ideally, good dithering settings will make your printouts look better and use less ink!
A typical scenario is a printer with 3 dot sizes (Small, Medium, Large) printing a gradient from white to color. The white gives way to light tones printed with small dots that smoothly get denser as the color gets darker. At a certain point, going into the mid tones, coverage gets dense enough that printing with small dots is no longer efficient. At this point, the dithering switches over to the medium dot size to cover more material with the same number of dots printed. Towards the end of the gradient, where the darkest shades of the color are found, another switch to the large dots happens to ensure full coverage of the material.
As an example, let's have a look at how the dot distribution for black affects a gradient. On the left, you see the dot distribution setting, and on the right is a visualization of the resulting gradient. The different dot sizes are visualized by shades of gray. Light gray is the small dot, medium gray is the medium dot, and black is the large dot.
This process is controlled by the dithering settings. Ergosoft's default dithering settings are designed to provide the best dot distribution and dot size usage on the widest range of printers. However, for some applications like specific ink/media combinations or a special output goal, you might want to customize these dithering settings. Read on to find out how!
Ergosoft RIP features two main types of Dithering Methods.
Digital Printing Applications is the default dithering and is suitable for all regular printing applications. It is an evolution of the smooth diffusion raster pioneered by Ergosoft RIP 15 and offers a smooth and efficient raster for nearly all printer types.
Screen Printing Applications is a dithering method created specifically to print rastered images for use in traditional printing applications (e.g., printing onto screens for use in screen printing). As such, it has a completely different set of parameters that we won’t be covering in this article. For more information on this, see the article Printing Separations and Screens.
Legacy Dithering Methods Dithering Methods from older RIP Versions, such as Stochastic3e, Stochastic3s, and Smooth Diffusion, are still available for legacy purposes but not recommended for use anymore. To use one of the legacy methods, check the box «Show All» and select it from the drop-down menu.
To access the dithering settings, open your Print Environment Settings and go to the Quality tab. Make sure that «Digital Printing Applications» is selected under Dithering, then click the Settings button.
This will open the dithering settings window. To show all available options, click the Advanced button. This will enable the Combination of light inks and dot size segments and the dot/ink distribution graph.
With all the options before us, let's have a look at what they are for.
Ergosoft comes with a range of predefined settings for dithering. This setting determines whether the dithering method should attempt to use more of a specific dot type or ink.
You can apply these ratios either to all colors equally or specifically to one color. If you want to apply the setting to all colors, make sure the box Same Settings for all Channels is checked. If you want to apply a setting to only one color, uncheck Same Settings for all Channels and select the color you want to use the setting with from the dropdown menu (Note that when using Light Inks for any color, channel settings have to be done individually for all color channels).
Now you can select your preferred setting in the drop-down menu Mode:
Auto will determine the ratio of dot types by itself. This is usually a value that works for the widest range of applications on this printer.
Prefer Small Dots will attempt to use more small dots, so e.g., you’ll find that small dots are used up to the mid tones.
When used in combination with Light Inks, this mode gives decent output quality while only moderately increasing ink application/cost (With Light Inks, the progression goes Light small dot > Fulltone small dot > Fulltone medium dot > Fulltone large dot).
Prefer Medium Dots will attempt to use medium dots in more situations, so e.g., you’ll find that medium dots are used up to the full tones.
When used with light inks, this mode will use only light inks until the midtones, where the fulltone medium dot comes into play (Light small dot > Light medium dot > Fulltone medium dot > Cyan large dot).
Prefer Light Inks will attempt to use more light inks whenever possible, e.g., using light inks in the mid tones. This mode generally provides the best output quality when using light inks, as it has the smoothest linear progression using different dot sizes in light inks (Light small dot > Light medium dot > Light large dot > Fulltone large dot). It does, however, use a lot of light inks, which in turn increases ink costs.
User-defined is one of the modes that lets you configure the dot size usage yourself. It lets you alter existing dot sizes and remove dot sizes from use, but it does not let you add dot sizes back in or add dot sizes twice.
Advanced is one of the modes that lets you configure the dot size usage yourself. It lets you alter existing dot sizes, remove dot sizes from use, and add dot sizes back in, but it does not let you add dot sizes twice.
Expert is the most flexible of the modes that lets you configure the dot size usage yourself. It lets you alter existing dot sizes, remove dot sizes from use, add dot sizes back in, and even lets you add dot sizes twice.
Additionally, the progression of dot sizes with Expert does not have to rise from lightest to darkest, so theoretically you could configure it to start with large dots for light tones, etc.
Print test chart with Ink shades: Allows you to print patches in the tones of each channel and each dot size side by side. This makes it easier to visually select which dot size of the full and light inks to combine for advanced custom halftoning.
Import/Export: These buttons allow you to import and export custom light/full tone ink and dot size settings (called ink level settings).
This setting can be useful when noticing printing artefacts caused by the printer. Moving the slider to the right towards 100 will make the dithering method attempt to counteract artefacts in the printout by adding noise to the raster. This type of generated noise can help hide artefacts that appear because of certain printer behaviors.
It’s important to note that while this can help to get a cleaner printout initially, using higher values can lead to a noticeable loss in quality when the noise becomes visible and creates an uneasy/blurred type of printout.
For certain applications or printers with unique behaviors, you might want to create custom dithering settings to control dot size usage and optimize the distribution of dots and light colors. Ergosoft's flexible interface makes it easy to adapt the dithering to your requirements and get the best out of your printer.
To begin customizing your dithering settings, switch the Mode in Combinations of Light Inks and Variable Dot Sizes to be used to either User Defined, Advanced, or Expert (See the previous chapter for the differences between these modes).
Now we can alter the dot size and light ink usage directly in the graph to the right by clicking and dragging the pillars. Each pillar represents a dot size (Or a shade of an ink if there are multiple, such as a light cyan). So one pillar represents small dot sizes, one medium, etc. Hover your mouse over one of the pillars to see which dot sizes and/or colors they represent.
The horizontal axis of the graph represents optical density/ink application (When the dot size is used). All the way to the left is the material color/no coverage, while all the way to the right is full ink coverage/maximum optical density. You can adjust the position of the pillars along the horizontal axis to make dot sizes come into play at earlier or later stages. Right click on the position where you want it and go to Add > then select the respective dot size or ink type.
The vertical axis of the graph is the maximum amount of ink applied (How much of this dot size is used at most?). You can now increase or decrease the maximum usage of a dot size by dragging the pillar for that dot size up or down. Notice how this affects the area that this dot size covers (I.e., when this dot size is used) and how it overlaps with previous dot sizes.
In most cases, you’ll want the optical density to rise continuously so each step is darker than the one before. Therefore, it's best to have the maximum ink application get higher with rising optical density.
For example, if you notice in your printouts that your mid tones appear too light and don’t have enough coverage, you might want to click-drag the pillar for the medium dots higher so more of them can be applied, increasing density in that area. Or if your light tones get too dark too quickly («Jumps» within a gradient), you might want to click and drag the small dot pillar lower, or move the medium dot pillar further to the right so mid tones are applied later.
In some cases, you might want to remove certain dot sizes from the dithering entirely (Small dots causing ink spray, large dots bleeding on the specific material you are calibrating, etc.). To remove a dot size from the dithering, set the Mode to User Defined, Advanced, or Expert. Select the pillar for the dot size you want to remove by clicking on it. Click the Delete button to remove the dot size.
If you want to add a dot size again later on, click the Add button and select the dot size/ink type you want to add to the graph. If the dot size you want to add is the largest one, you’ll find that you can’t directly add it to the right position. Instead, simply add the dot size, then right-click on the pillar and select Modify> Dot Size: Large.
When using the Expert mode, you can even add the same dot type multiple times. This can be useful for applications such as fine art printing where minute control over dot sizes is needed.
Ink spray and stray dots in light areas
In some cases, fast head speeds can cause printed small dots to fly and spray as they are not heavy enough to fall down in a straight line. This can cause visible dots to appear in unintended areas and cause a generally fuzzy appearance of the printouts.
When this problem occurs, it can help to either slow down carriage speed or remove the small dot of either that specific ink or all inks altogether to get rid of the spraying.
To remove the small dot size, open the dithering setting and activate the advanced menu. Set the Mode to Expert and click on the pillar for the dot you would like to remove. Click the Delete button to remove the selected dot size.
Confirm the dialog with Ok to save your changes.
On some printer/ink/material combinations, it's possible that too much ink is applied when using all dot sizes, especially in areas where multiple dot sizes overlap. In such cases, it can be helpful to either space out the different dot sizes so there is as little overlap as possible by dragging the individual pillars away from each other, or to downright remove one of the dot sizes altogether so it is not used in printing. If the problem is specific to one color/color mix, it might be enough to do this for one color tone, or it might be necessary to do it for all colors. The most common dot size to remove in such cases is the medium one, as it tends to overlap with both the small and large dot.
To remove the medium dot size, open the dithering setting and activate the advanced menu. Set the Mode to Expert and click on the pillar for the dot you would like to remove. Click the Delete button to remove the selected dot size.
Confirm the dialog with Ok to save your changes.
Most of the custom dithering setting modes feature some sort of restriction to ensure that gradients remain a transition from lighter to darker values. However, in some applications like fine art printing, it might be necessary to circumvent these restrictions to do special fine-tuning of different inks, use the same ink type multiple times, etc. For these applications, you may set the Mode to Expert to remove all restrictions. Expert mode allows you to set the same dot type multiple times, set smaller dot sizes after larger ones (ignoring the optical density/size progression), etc.
It’s important to note that this mode is only intended for expert users, and changes should regularly be tested and verified to ensure the results are as intended.