White and Black Point Correction

The single most common cause of "muddy" prints is poor selection of the black point and white point in the original scans. The only reason Illustration 1 looks muddier than Illustration 2 is that its highlights never exceed 88% (that's lightness, not ink).


Illustration 1
Illustration 2


If you are using a gamma curve editor, like the one found in Wasatch SoftRIP or Adobe® Photoshop®, the only change you need to make to have Illustration 1 look like Illustration 2 is a simple adjustment to the correction curves (Illustration 3).


Illustration 3


If you were to print the muddy scan in Illustration 1 without this correction, you would get dots in all your highlights and a very muddy looking print when viewed on the reference white of the paper.

This is very hard to see when you look at the muddy scan all by itself on your computer screen. For this reason, people using popular paint and scanning software make this mistake all the time, in spite of the fact that this sort of white point correction is the single most common correction needed for printing "pretty" color pictures from scans that you may receive from someone else.


Correcting White Point and Black Point in SoftRIP

With a scanned image open in Wasatch SoftRIP, move your cursor over one of the lightest highlights. The RGB entry on the information window should show at least one number that is over 90%. If not, you are going to see muddy highlights and colors. Move the cursor over one of the darkest shadows. The RGB entry should show at least one number that is under 10%. If not, your prints are likely to have muddy shadows. The same principle applies to CMYK scans. If you've been given the job of making a nice looking print from a scan with poorly chosen black and white points, there is an easy solution. Make a note of the lowest R, G, or B that you can find in a shadow and of the highest that you can find in a highlight. In a muddy scan, these numbers might be something like 15% and 80%, instead of the 5% and 95% that you would hope to see in a good scan.

On the Curves screen, select the Correction and All Channels radio buttons. We will use the Lightness editing mode in this example.

In the Lightness editing mode, black is in the lower left corner at 0 and white is in the upper right corner at 100 for both RGB and CMYK curves (Illustration 4). Moving a point upward has a lightening effect for both CMYK and RGB images when using the Lightness editing mode.


Illustration 4


Click on the upper right end of the diagonal line and slide it left until the window looks like Illustration 5. Now highlights that had only reached 80% in the original will be stretched to reach 100%, or the density of the base media with no ink applied, in the final print. Next, click on the lower left end of the diagonal line and slide it right until you see a curve such as the one in Illustration 5. Now you've stretched your shadows from 15% down to 0%, which will cause them to reach all the way to the maximum density available with the media and ink that you are using.


Illustration 5


Fixing The Midtones

After this exercise, your shadows should be dark and your highlights should be bright. However, the overall density of your picture (the midtones) will be too dark. Click on a point in the center of the diagonal line and slide it upward until it looks something like Illustration 6.


Illustration 6


You've now lightened the 50% midtones to 75%, while leaving the highlights and shadows where you set them earlier. This is the most common kind of tone correction needed for faulty scans.


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