Color Adjustments by the Numbers

This image has a green tint distorting the sky and making the trees, shrubs and grass look too green. How much of an adjustment to make can be determined through repeated trial and error using the the Color Balance and Color Curves filters. However by analyzing the values of the Red, Green and Blue channels and applying their complement values as a overlay filter frequently leads to a faster and better adjustment. This is not the type of adjustment you would use every day, but it may be the fastest adjustment for special situations.

Begin by adding a overlay track. Select the black solid color from the text/background tab and align over your video track as you did in the previous tutorial. To begin with set the opacity on the overlay track to roughly 40-50%.

The default settings for the color picker work area accessible when you drag a sold color to the timeline is zero for each of the color channels. Either type in 127 for the Alpha channel (A) or move the slider on the vertical color slider. Move the cursor over a representative frame on the timeline, click so it is visible in the preview window. Now click on the eye dropper tool and move it over the preview window. Click over a representative area of green. You do not have to be too precise however you may wish to sample several spots in your video to find the best range. The goal should be to find a typical pixel to key off of, so try to avoid hitting any pixel that is either too light or dark, leaning more towards the mid range of greens in your image.

Once you click in the preview window, you'll see the values in each of the channels change as shown in the next snapshot of the Color Picker work area. Your entire image with have a very green tint to it at this point. Depending on your source material you may need to change the opacity value on the main timeline to avoid having the image levels too light or dark.

Don't freak, this is just the first step. I selected the area in the grass just to the right the shadows cast by the smaller tree on the left just under the larger shrub.

Notice how the values under each color channel changes once you click the eye dropper tool. The next step is to calculate the complement value for each of the channels. This will determine the offset values on the color scale for each channel, which will cancel out the color cast you're attempting to eliminate. The range for each channel ranges from 000 at the extreme left meaning none of the primary color is present in the RGB mix up to 255 or maximum saturation for the channel if the slider is pushed all the way to the right. So by subtracting the values in each channel that make up the unwanted color shading you arrive at the complement value, or what is needed to balance it off.

The last step only requires plugging in the complement values for each of the channels. You'll see the image change in hue as you enter the offset value for each channel. In this example you would enter 155 in the Red channel, a value of 96 for Green and a value of 174 for Blue. After you do, you'll see the values change as reflected in the following snapshot of the Color Picker. The image in the preview window should now be well on its way to being color corrected.

In effect what we've done in these steps is create a semitransparent mask or filter that offsets the unwanted tint. In this example green Of course you can do the same thing with any color impurity, just use this tutorial as a guideline. Now that you have a color corrected image, adjust the color slider to the left of the RGB sliders to effect the overall brightness and contrast. Lower values in the Alpha channel will darken the image, higher values will lighten. Adjust the Opacity setting on the timeline also to get the best combination.

You now have the basic knowledge to create any semitransparent filter that works similar to the principles of a camera filter.

Once you've made the major correction you can apply Color Curves and other filters to further enhance the image if needed. Note that the final values in effect have created a semitransparent purplish filter, exactly what we need to offset the greenish tint. If necessary, minor adjustments of the color sliders plus minor adjusting of the Opacity value on the timeline, tweaking with Color Curves can further improve the result.

Now the final image reflects a blue sky, the grass, and shrubs are restored to their true color range. As an added bonus the overlay layer allows you to more precisely fine tune levels by adjusting the Alpha channel and opacity strength, thus allowing more exact control of brightness and contrast not possible with the more simplistic brightness/contrast filter alone.

To further tweak the image you can switch to HSL mode by clicking on the button above the eye dropper. This method adjusts hue, saturation and luminance more than the HSL filter alone provides. My experience with adjusting Hue using the regular HSL filter is that the range is very limited and the image becomes badly distorted for hues if anything much either side of either extreme of the range of the slider is attempted. This method allows you the full range of hue adjustments in very small steps without any distortion.

While it is difficult to see the subtle differences in these small images when viewing off a web page I'm illustrating how you can apply very fine adjustments not as easily accomplished using FX filters alone. The difference are more obvious when viewed full size and more pure when not further filtered by your browser's limited palette which tends to muddy some colors.

This type of adjustment is mostly for the knob twisters that demand very subtle changes. By adding a Alpha channel and dropping on a solid black color then dropping down the opacity to 5-10% you've given yourself a new dimension in HSL control.

By varying the values for the Alpha channel (under color) along with varying Hue, Saturation and Luminance value combined with tweaking the transparency of the overlay layer you can further fine tune results. For the more daring you can trying a using a different solid color as the overlay. Try orange to improve on sunsets for example.

H-000 S-000 L-000 A-240	H-000 S-025 L-030 A-006
H-000 S-025 L-030 A-240	H-225 S-233 L-034 A-112