Curve Creation

To create a custom ink curve, open the Curve Creator window by clicking on the Curve Creation under the Tools menu item. Open an existing Quadtone Ink Descriptor File (qidf) from the File/Open menu or select File/New to start with a blank qidf. When you have filled in the details of the curve (see below) save the qidf (File/Save) and press the Create Curve button.

Ink Descriptor Details

Printing Model: Select a printing model name from the drop-down list. This will typically be the same printing model as you selected on the main QTRgui window.

The Ink Setup tab

Default Ink Limit: This percentage will be used for all inks, unless a specific limit value is specified in the ink's box.

Black Boost: The limit value from the Black ink box is used in all gray tones. To get the darkest dMax possible, increase Black Boost. This puts a steep curve in the black ink right at the end pushing black all the way to Black Boost percentage. On some printers even 100% black isn't quite enough, so there's a gray overlap percent which puts out more ink overall and finishes with this percentage of the darkest gray overlapping with the maximum amount of black. See Gray Parts, below.

Individual ink boxes: There is a box for each ink that your printer uses, Cyan, Magenta, Yellow, and Black are available on all printers supported. Some printers add in addition, Light Cyan, Light Magenta, and even Light Black. If your printer does not have an ink, the ink box will not be enabled. For each ink, select the ink usage (Gray Ink, Toner, Not Used, etc.) from the drop-down box. If you wish to use a percentage of ink different from the default ink limit, fill in the percentage in the appropriate limit box. Also enter the relative density for the ink. The density is relative to the darkest ink used, normally black, see Ink Characterization.

> Ink Usages

> Not Used: As the name implies, inks with this selection are not used in printing the image.

> Gray Ink: Each ink that has Gray Ink selected as its ink usage will be the primary inks used in printing the image. Gray Ink is controlled by the Gray Curve specified on the Gray Curve tab (see below.)

> Toner and Toner 2: Inks with these selections are used for fine tuning the image densities. See Toner Curve tabs below.

> Copy curve from: The inks for the 2200, 7600, and 9600 have a light-black ink which is quite warm. These can be neutralized by adding Light Cyan and Light Magenta to balance the tones. You can do this by selecting the Copy curve from ink usage and selecting Light Black (LK) as the ink to copy. Using Black, Light Cyan and Light Magenta limits you can balance the color for an exactly neutral tone. This does take some trial and error because the relatively large amount of Light Cyan and Light Magenta introduced increases the density quite a bit. So to compensate Black must be reduced—the idea is to get a combination to produce the same density as a full Black amount.

> For some usages, the black ink is also too warm. To compensate for this, use the Copy curve from for Cyan and Magenta copying the Black (K) curve. Sepia tones can be created by setting copying the black (or light black) curve to the Yellow ink.

> Load Curves: This ink usage gives you ultimate control over how an ink is used in printing the image. There are two options for specifying the ink curve. You can create a Curve using Photoshop or you can specify individual coordinates of your curve. In Photoshop, using a grayscale file you run the Curves command to create whatever curve you like. Save the Curve as a ".acv" file for re-editing. and specify this file name from the File Name tab. Select the Point List tab if you wish to provide individual curve coordinates. You may enter up to 50 points in <x> <y> pairs. The point coordinate values may be any number between 0 and 100.

The Gray Curve tab

Due to a phenomena called Dot Gain, just mixing inks gives a very dark gradient, so there are several ways to shape the gradient. The next four variables control the shape of the final grayscale gradient.

Highlight and Shadow: These are a special control that can fine tune the highlights and shadows with separate values. The larger the value the lighter that portion of the curve. The values that seem to be a very good starting point are both equal to 10

Overlap puts out more ink overall and finishes with this percentage of the darkest gray overlapping with the maximum amount of black.

Gamma is the traditional gamma correction. In Photoshop the middle value in the Levels Command is a gamma correction value. 1 gives no change, >1 lightens and <1 darkens. This correction is very strong and in general is probably not needed, so use gamma = 1 unless you absolutely need a lot lighter gradient.

Gray Curve is the most flexible, allows you to create a Curve using Photoshop. This is the filename you assign to Gray Curve.

If you have a densitometer, the above can be just approximated—see Linearization.

With the Linearization feature the densities will be corrected later and a simple (default) straight line works quite well. Just adjust the Ink Limits to how much toner you'd like. It's still possible to use the Highlight value to fine tune the highlights.

The Toner Curve tabs

This section is similar to the Gray Parts section. The variables have all the same meaning except that there is no Overlap variable. You can setup two different toner curve for toner ink control. The toner curves are applied to the inks by using the Toner and Toner 2 ink use selections.

The toner has so little effect in the shadows that Shadow values are often very low. With heavily toned curves the added toner ink increases the density throughout the whole grayscale, so in toned curves increasing the Gray Parts Highlight to 18 and Gray Parts Shadow to 12 help compensate for this.

The Linearize tab

The basic calibration procedure of QuadToneRIP partitions the gray inks and creates smooth transitions between all the inks. Once there is a smooth curve the objective is to shape the final densities to an ideal shape—one where the steps are separated equally for human vision. Our vision is better able to see differences in the highlights rather than the shadows so the slope of the density curve must increase towards the shadows. QuadToneRIP can take a list of densities from an evenly spaced stepwedge and create a correction curve that will then produce the ideal densities for the final curve set. This is called the Linearization process.

Any densitometer can be used for this process.

To use this feature, simply create the ink description file, doing the partitioning. The Gray Parts Highlight and Gray Parts Shadow fields can be approximated at 4 and 8 respectively. Create the curve, print out the 21.step.tif stepwedge using this curve, and measure the densities. Fill in the linearization values measured above. The densities can be absolute or relative to dMin. Just list them in order from step-0 (dmin) to step-100 (dmax). Note: you may use LAB values instead of dMax See the Ideal Density chart for all the ideal density values. (If you have measured the densities in Lab color space, then enter them in order from the high to the low value.)

If you have created these values in a spreadsheet or other application with clipboard capabilities, you can copy the numbers and paste them into the linearization values with the paste values button.

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