The art of color-correction

Introduction

If you ever did color correction with a professional tool you will have noticed that blender missed that completely.

One needs basically two things: color-correction filters and some display to check the result.

This patch is far from making the color-correction professional (especially a plugin for color matching should be added) but it enables you to correct for the most obvious problems.

The waveform display

To judge the quality of the luminance distribution across your video signal, you can view a luma-waveform instead of the usual output display on every control monitor.

The display plots for every scanline the luminance value. The lines are all drawn on top of each other. The points get brighter if the lines cross (which is very likely with several hundred scanlines). You will understand the picture most easily if you plug an oscilloscope to the Luma-video-output of your television set. It will basically look the same.

But what is it good for?

If the waveform does not fill the whole picture you might want to play with the "setup" and "gain" master-sliders in the "gamma"-plugin until it fills the whole picture (contrast autostretch).
With the more advanced gamma-plugin you can decide where you have to desaturate (especially in dark regions).
You can judge if you want to dump the whole thing since it is completely distorted and clips at the top or the bottom.

The U/V scatter plot

To judge the quality of the color-distribution and saturation, you can also view a U/V scatter-plot.

The picture is converted to YUV-format. The U- and V-values represent the angle of the color. For pixel of the picture, one point is plotted in the display at the U and V-value-position. If several pixels happen to have the same U/V-value the pixel in the plot gets brighter. To help you understand what color is ment, a hexagram marking the extreme positions (red, magenta, blue, cyan, green, yellow) is drawn and a red cross to mark the origin.

But what is it good for?

If you picture looks very moody or desaturated you might want to take a look at the U/V-plot. You will most likely see all pixels building a growd at the origin. If you add saturation using the "gamma"-plugin you can see in the U/V-plot if you distort the color.
If you do color-matching on a by hand basis you can match the angle you see of different channels monitors.

Always: remember to activate an additional control monitor of the end result. Color calibration is a matter of taste and depends on what you want.