Setting the Black and White points

Setting the Black point seems to come up quite a bit , so I thought I should post about Black and White points.

RGB Black Point
The Black point is the Darkest set of pixels in an image, while the White point is the Brightest. By setting the darkest and lightest pixels to 100% Black and White, respectively, we can increase the tonal range as well as the contrast in an RGB image. This step also ensures that any solid areas of Black or White are set to print with 100% coverage. Increasing the tonal range and contrast in the RGB mode will always improve the results from the YRGBK script.

The Levels dialog. To change the black point move the Black arrow.
To change the black point move the Black arrow.

Levels or Curves
There are several ways to approach setting the Black points, but for the most part in Adobe Photoshop we can use either Levels or Curves. Both tools can do the trick, but for the most control over the results I always go for the Curves. In Photoshop’s menu go to:

Image/Adjustments/Levels
or:
Image/Adjustments/Curves

The Histogram
A histogram is a graphic representation of the pixels in an image. The Levels dialog is a Histogram with sliders, where the left side represents the dark values and the right side is the light values. The number of pixels in a value range is represented by a line going up from the bottom, so the taller the line in the graph the more pixels in that range. The Arrow on the left sets the Black point, while the arrow on the right sets the White point. The Curves dialog can also display a Histogram if you check the Histogram checkbox.

Convert to Adobe RGB to expand the color gamut.
Convert to Adobe RGB to expand the color gamut.

The Techniques
In this tutorial I am actually going to describe two techniques for setting the Black and White points in Adobe Photoshop’s RGB mode. Which approach you use depends on the image and your goals. The best place to start is by converting an image to Adobe RGB, if it is not already.

•  Edit / Convert to Profile..
Destination Profile : AdobeRGB (1998)

 The Droppers
In both the Levels and curves dialogs, there are three eyedropper icons on the right or along the bottom; the left one sets the black point,  the right one sets the white point, while the middle dropper sets the neutrals.

The droppers set the Black and White points.
The droppers set the Black and White points.

Open the Levels or Curves dialogs:
• Double click on the Black eyedropper to open its color dialog. Set the color to 100% Black. Do the same for the White, setting the value to 100%.

Now, when you select an eye dropper and click on the image, that value will be set to 100% White or Black.

• Select the Black eyedropper, click on the darkest area in the image to set it to Black. Do the same with the White eyedropper, setting the lightest areas to white.

The Middle dropper can be used to set the neutral values in an image. This can be helpful with photographs taken with the wrong lighting settings, or if you have a gray card in the image. This dropper can cause a large color shift in the image, and is not needed to set the Black and White.

The Dropper approach is good for photographs, especially if they lack contrast or had poor lighting. It can also be a quick way to set a Black or White point on simple separation.

Using Curves
The Droppers may cause a noticeable shift in color, and in some cases we just want the light and dark areas adjusted. We can manually set the points using the Curves dialog, while keeping the middle values in place, to avoid color shifts.

Setting the Black and White using Curves.
Setting the Black and White using Curves.

Single Step
Open the Curves dialog:
In the curves dialog we can also set the Black point by moving the 100% point on the curve to the darkest pixels in the image.

• Start by setting the Show Amount to Pigment/Ink %
• Check the Histogram check box.
• Click and drag the 100% point to the first set of pixels in the histogram. Do the same for the 0% point to set the white.

If you click and hold on the image with the curves dialog open, you will get a reading in the dialog showing the original and new values. This can be used to measure the values in the image. If you Cmnd click in the image, or click along the curve line, you can set a point on the curve.

To avoid making color shifts we can break this adjustment into two steps and set values along the curve.

Two Steps
Open the curves dialog:
• Set points along the curve by clicking in the image or along the curve line. Set points below 50%.
• Slide the 100% point to the darkest pixels in the histogram. This value can be determined by measuring in the image.
• Adjust the curve if needed for a good tone transition.

Setting the Black point manually using curves.

Click OK and repeat the process for the White. This time setting points above 50% and moving the 0% point to the right.

This is a good approach when we need to set the Black and white, but do not want to shift the colors in an image. It also gives good control over the resulting value transition. In this example it shows in the white where it fades into an off white shade, and I could soften my curve to smooth that transition out.

Setting the white point manually using Curves.

 

Setting the Black and White points of a color channel in a separation
We can also look at the Black point of each channel in a color separation to be sure they have good contrast and range. The techniques are the same as above, with a single color channel selected.

Color channels generated by the YRGBK script have their black points set based on the saturation and brightness of the ink colors. We can customize these colors by changing the black point of a channel, and adjusting the channels ink color to match.

It is also helpful to adjust the Black point when a channels darkest values hover over 90%. We can shift the value to 100%, while setting points on the curve, and improve both the coverage and range of the color.

Applying a 2% clean up curve to all channels is a good idea.

Finally, the white point of a color channel should be inspected to avoid random dots and 1% areas, which can be a problem when making screens.

Setting the Black and White points is a crucial step that is often overlooked. Ensuring  good value ranges and solids in an image from the start will always improve the results.