PSP Tutorial 2 - Color Correction

Lesson 2 was inspired by Keith Meacham, whose recent posts were excellent candidates for a little color correction to reveal their true potential. In this lesson, we'll explore the use of some of the more basic color correction tools Paint Shop Pro 7 has to offer. Comments and questions, as always, are welcome.

Lesson 2 - COLOR CORRECTION

Does your favorite WC locomotive look a little green behind the gills after scanning your prized shot? Seeing things through a haze of blue? Perhaps that vintage Milwaukee road shot has more orange than the 1871 Chicago fire. What do you do? Unfortunately, incorrect color reproduction is quite common. Fortunately, it is easy to correct with the right tools.

Improper color reproduction is a problem that can effect both film and digital cameras alike. Scanners, although less susceptible, can also succumb to the inaccurate colors under the right conditions. Even the human brain can be tricked to see colors incorrectly (go look through a piece of heavily color piece of plastic for several minutes covering only one eye, then compare the difference between both eyes). Why does this happen? The light source illuminating your subject may not be pure white light (i.e. Fluorescents give a greenish hue, Sodium high pressure lights will produce a very monochromatic yellow). Another cause may be that the colors are being interpreted incorrectly by the camera or by the developing process. Luckily, Paint Shop Pro has several tools to help compensate for this problem.

Automatic Color Balance Tool

Probably the most simple and effective tool that PSP has to offer is the Automatic Color Balance

, located under Effects >> Enhance Photo. This is probably one of the most useful commands for those of us who use PSP for photography. If it's not on your customized toolbar, I recommend adding it as you will probably find yourself using it a lot. Examining the command dialogue, the controls are fairly straightforward:

You have a sample of your original image on the left, along with a preview window on the right. Below, you can set the strength of the tool, and have the option to remove a color cast (more on this in a bit).

Then we get to the primary control. Here we can adjust for the color temperature to that of our desired light source of correction. The illuminating light color is expressed as temperature, in units of Kelvin (Celsius + 273 degrees). This measurement is derived from comparing the color of light emitted from an incandescent light bulb filament, such as tungsten, at various temperatures. The lower the temperature of the filament, the more orange of a light that will be emitted, conversely, the higher the temperature, the more bluish the light will become. A tungsten filament is not capable of emitting much more than yellow light, as tungsten melts at around 3500K. The higher temperature values are usually a projection of the light color v/s temperature correlation from the incandescent filament. Fortunately, we don't need a big drawn out conversion chart as the nice folks at JASC have conveniently marked several points on the slide that correspond to common light sources.

In this first example, our subject was shot in early evening light. Sunlight near dawn and dusk takes on a much stronger yellow to orange-red tone. As a result, contrasting hues such as blue will lose some of their purity, taking on a yellowish, or even orange appearance. To get a healthier looking sky in this case, we will simply set the color temperature to 6500K or sunlight and click okay. The result image has the yellowish cast removed, yielding a much more blue looking sky. Even so, the gold on the locomotive has not lost any noticeable degree of color saturation in the gold.

Figure 1. The uncorrected image exhibits a yellowish cast from the early evening sun.

Figure 2. After correction, the sky takes on a more normal color, yet the gold on the locomotive remains unharmed.

In some more severe conditions, you may find your image looking somewhat devoid of color. The auto balance only compensates for the apparent shift in color, but not the strength. Therefore, some colors may have been more suppressed by the illuminating light temperature. To compensate for this, you can simply increase the image color saturation using a tool that is a close neighbor to the first, the Automatic Saturation Enhancement

Removing Color Cast

When shooting with a light source that is far removed from white, your images will usually end up with much more emphasis on the illuminating light source color, even showing up in areas of color that would normally contrast with that particular color. Shooting under certain types of artificial light (such as Sodium high-pressure, or Mercury Vapor) or even just scattered skylight can cause this problem. In the example below, a simple color balance to sunlight does little to fix to overall blue cast. Why? The auto balance tool already recognizes that most of the scene (the sky) is already inline with where it should be in regard to the color temperature for sunlight. What it is ignoring is that some areas are showing signs of over saturation of the apparent illuminating light temperature. By checking the Remove Color Cast box, the auto balance tool now watches for areas that appear to be weighted heavy towards the overall color makeup of the image and corrects accordingly.

Figure 3. The uncorrected image. The locomotive's shadowed side takes a blue cast from the scattered skylight.

Figure 4. An automatic balance at 6500K does little to fix the problem.

Figure 5. By having the auto balance feature remove the color cast, the blue that was affecting the color of the locomotive is removed. The trade-off is that the sky appears less vibrant, however an appropriate amount of sky color can be recovered using the Automatic Saturation Enhancement tool if desired.

The remove color cast tool can also help reduce blue glare that is sometimes present in photos in direct sunlight. Some types of film, and especially digital camera CCD's are more sensitive to ultraviolet light than visible light. As a result, you may develop a blue cast in particularly reflective areas, such as white. Normally the best fix for this problem is to use a UV filter, but due to many lens designs, installing a filter may not be a viable option. In such cases, a color balance with the color cast removal can lessen the effect of UV exposure.

Manual Color Correction

At times, particularly with older scanned photos, it will be difficult to determine an appropriate light source temperature to correct to, or the colors may just seem to be so far out of whack that you may be unsure where to start with your color adjustments. Here, Manual Color Correction tool

enters the picture. Like the automatic color balance, this tool will shift red, green and blue saturation levels. Unlike the automatic color balance, you provide the tool with a swatch of the image and provide a known color reference for it to use in identifying how to shift the values.

In the dialogue box for the Manual Color correction tool, you have the original image on the left, and the preview on the right. On the left image, you select a portion of the image that you know the proper color value, or can make a reasonable assumption of what the color should be. You then go below and select a category, in this case sky colors, and then pick one of the presets. You can see your immediate results in the preview box so you can experiment with different presets.

Two options available are the preservation of the color lightness and the preservation of the color saturation. Checking preserve lightness tells the tool to ignore the difference in brightness between your selected area and the preset and just correct the red, green, blue color ratio. Checking the preserve saturation will cause the tool to ignore the difference in color saturation between the selected area and the preset.

PSP provides a fairly decent selection of presets in a number of categories that are common to most pictures. Even so, I have over time developed my own presets in the User colors, tailored specifically for use with train pictures. I have created a handful of WC gold and maroon presets under various light conditions and paint conditions. While my digital photos typically never get this bad that I need to resort to manual color correction (I use the automatic balance quite regularly), the references are very handy when attempting to correct scans of pictures.

After color balancing and correction, you will likely want to perform additional image enhancements, such as the Automatic Saturation Enhancement or some light Gamma Correction to get the colors and details to be a little more vivid and revealed. Again, this is normal for virtually any image that requires color adjustment.

So, there you have it. Now you know how to keep your skies looking crystal blue, your golds nice and rich, and your maroons nearly as accurate as the paint formula itself. Color correction is one of the most basic tools that can bring out the best colors your photos have to offer. Until next time, happy experimenting!

-"Evil" Ray