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Post processing uniwb files

Monday, August 8, 2011
Since uniwb (uni white balance) is a rather interesting topic, I think someone who is going to use it needs to know some advance levels of post processing. More specifically: color correction. Uniwb makes everything green in your photo. Why? Green is the luminance color channel. If you look at the separate RGB channels of a histogram the luminosity channel will be a duplicate of the Green channel.

Another matter to understand is that your camera will be using a linear curve when you shoot in uni white balance. Since uniwb uses RAW files, post processing is easy and nondestructive to the file. But, you have to know how to get from point A to point B with some sort of artistic vision.

I am going to use Nikon Capture NX2 in this tutorial. I like Capture NX and think it does a better job at rendering the colors of Nikon Raw files than Photoshop. Just my opinion. The first photo below is a uniwb raw file right out of my Nikon D700. Linear curve, no sharpening, no added saturation, contrast or D-lighting. Pure binary code from the camera.

Uniwb Raw file out of camera

This photo is rather easy to color correct in post processing. That is why I chose it as an example It has neutral colors (white and black) and greenery to get a basic color reading. With the "set gray point" white balance tool, I chose the front of the middle musicians shirt. It looks like a plain white t-shirt. I chose the front of the shirt because if I choose the top, there might be some green color from the trees reflecting on that area. When choosing white balance, you must be concerned about reflective color from surrounding objects. It can throw off the white balance of the whole image.

Next I like to find a dark, neutral black (if possible) and use the "black control point" tool in conjunction with a watch point measuring that point. I like to find this black point around RGB 8-12. I set the black point on part of the black bag hanging on the fence to the left of the center musician. Set the RGB channels so they all record at RGB 12. That should mean the dark ends of our photo are correct and give some detail in a print or on a computer screen. It also means it is neutral black.

You can do the same to a white point in the photo. Since I used the white t-shirt for the white balance I give it a cursory check and see it is near neutral so there is no need to set a white point. Below is a photo of the color changes after setting white balance and a black point. A slight improvement but still, there is a Green color cast.

Uniwb Raw file with white balance and black point applied
At this point I like to check "known colors" such as foliage, skies, skin tones, things that I think should be neutral. I also like to add a little Nikon Capture NX2 d-lighting. When shooting uniwb with a linear curve, there is no pop, or mid-tone contrast in the photo and the blacks could be blocked up or show no detail. I put the black point in before the d-lighting so the black point can be an anchor and the rest of the darks can be moved to the right so there is the desired amount of detail showing. I call this setting the slope of the darks on the histogram with d-lighting. If you have used d-lighting in Capture NX2 you will know what I am talking about.

Now I check the greenery color ranges in the background. I am a purist and normally check all my colors with the LAB and CMYK color spaces. But Nikon Capture NX2 does not have these color spaces. This makes me have to open the file as a TIFF in Photoshop and check the colors in more detail. For this tutorial I will stay in Capture NX2 and do all my readings in the less accurate RGB color space.

The general color rule in RGB for plants is that the Blue channel should be the lowest number, Green highest, Red closer to Green. In some parts of the photo Red number is higher than the Green value in the plants, so I make a slight change to a curve reducing Red in the color range of the greenery and especially in the area of the curve where the greenery has too much red. Additionally, I want to hold my white and black points to near neutral. So I put a point on the top part of the curve and the bottom part of the curve so they won't move. See photo below.

Make sure all of this is done in color mode so it doesn't affect the luminance (tone) of the photo. In Capture NX2 you want to open the opacity section of the curve at the bottom, choose Luminance and Chrominance. This splits the adjustment into two channels, color and tone. Turn off the luminance and the adjustment will only apply to the color. In Photoshop this is represented as blending modes - color, luminosity. See photo below.

Red Channel Curve

After a change to the Red channel I also see that the flesh tones are way off. There is too much green and yellow in the photo. Separating flesh tones and greenery can be difficult because they usually lay in the same tonal range (between RGB 140-175). The general rule for flesh tones in RGB are Blue should be the lowest number, Red Highest and Green closer to Blue. In this photo I see flesh tones numbers that are RGB 121, 92, 50. The Green is right in between the two other channels and I need it to be closer to Blue.

I can also visually see that there is too much Yellow in the musicians faces so I add some blue in that color range, bringing it closer to the Green Channel number and reducing the yellow, along with a slight bit of Green. I check my greenery numbers in various places making sure I haven't knocked any of them out of the color range rules and, presto. A finished uniwb color correction.

My final color correcting numbers after applying these curves are in RGB values - Black bag hanging 27, 28, 29 - Front of White t-shirt 231, 230, 231 - Greenery point - 79, 87, 48 and forearm of middle musician - 128, 81, 55. These numbers make me happy before I apply any luminosity, contrast or style changes. See photo below.

Uniwb Raw file after color correcting curves applied
Most everything is in a good color range. When color correcting know elements such as neutral colors, greenery, flesh tones, skies, you are looking for those things to be in a range. By doing cursory checks around all of the different tones of the plant life I can make sure the plants stay in the Greenish-Yellow range and not become Reddish-Yellow. Same with flesh tones. There is no exact number, just ranges, and these ranges can be wide. But you want them to be in the Yellowish-Red color spectrum.

As I start to stylize the Raw file, the colors will change, but if you start in a good place then you always have a baseline to create from. Now you can work on luminosity, contrast and personal style. Below is a finalized photo with some luminosity curves, saturation and sharpening applied to make the photo - POP - a little.

Sample finalized photo from uniwb Raw file

When a person decides to use the method of uniwb to bypass the in-camera white balance multipliers, it definitely adds to the digital photography work flow and is not for the light at heart. Uniwb is just a method to help a person get a more accurate exposure. Post processing the files means you really have to know and understand color correction. I hope this post processing tutorial of unwb raw files helped you understand what it means to color correct a photo, some guidelines to follow and how to accomplish it.

You may also want to read the blog tutorial on color correcting skin tones in digital photography.


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