NOTE: This website is alive and growing.
Table of Contents
The term colour grading will be used here as equivalent to colour correction.
How to get oriented in the colour wheel
Understanding Cinelerra Color Picker dialog by exploring its parameters is a great way to start understanding the basic concepts of colour correction.
Yu have now opened the Color Picker dialog box.. It’s made of a coloured wheel, a horizontal bar, a vertical bar and 6 sliders.
1. Exploring the dialog
The wheel has three primary colours (Red, Blue and Green) and three secondary colours in between (Magenta, Cyan, Yellow). To understand why the primary colours are Red, Green and Blue instead of Red, Yellow and Blue see PRIMARY COLOURS in the glossary.
In the centre of the wheel there is a tiny black circle. This is the colour selector.
Just below the wheel there is a horizontal bar that shows the selected colour. It should be black.
The vertical bar on the right side of the wheel marks the brightness (or luminance or value) of the colour. It goes from black (at the bottom) to white (at the top).
2. Exploring value
Move the Value slider to maximum. You can see the black selected in the horizontal bar becoming brighter and brighter all the way to white. The vertical bar tracks the change from black to white all through the gray scale.
Note that moving the Value slider other three sliders moved too. They are the Red, Green, Blue levels. They control the amount of primary colour used to get the final selected colour. When all the three primary colours are combined in equal intensity, no colour is produced.
Black, gray and white are not colours. Well… they are in the common sense, but they are not for a colorist. Black, gray and white are the absence of colour with different value, that is different brightness. A black and white picture has plenty of luminance information but no colours.
3. Exploring hue
If when I combine all the three primary colours in equal intensity I get no colours, then to produce a colour I have to combine the three primary colours with different levels. Let’s try.
To know for each selected colour what’s the amount of primary colours, look at the Red, Green and Blue (RGB) levels. Pose the mouse pointer over the knob to get a tooltip with the numerical value.
For istance: the beautiful colour that has hue = 272 is made with a combination of 0.53 parts of red, 0 parts of green and 1 part of blue.
Actually I think it is a little too bright for me. It’s better to lower the value a little. I set the value to 0.80. Haaaa! I like it better!
This new colour has still hue = 272 but the levels of the primary colours are Red = 0.43, Green = 0, Blue = 0.80.
5. Exploring saturation
Let’s start exploring it with a happy colour.
Let’s try again.
Now that the tour has come to an end, I can whisper you a secret: you can select a colour without thinking!
How to correct white balance
Sometimes you might get images with a weird color cast.
Here is an example of salmonish cast but it can be from gold to blue.
The paper in the background was in fact white. My eyes saw it as pure white. Nevertheless the camera recorded the white as salmon, tinting all the other colurs.
Cinelerra is capable of analizing the colour of a single pixel. Let’s do that:
Here are the info for a pixel representing a portion of the white paper.
You have learned from the previous HOWTO that neutral colours like white have the same amount of RGB. Note how in our case on the contrary the RGB levels are uneven. There’s a lot of red and very little blue (and that makes yellow appear).
There was artificial light in the room and some light came from the window. Natural and artificial light usually have different colours because they have different temperature. I forgot to tell the camera which colour was the real white and she got confused.
Now we have a problem. What’s the solution?
The real solution is in the camera (and in the operator). You should tell the camera which is the real white so she can store it as reference. Most videocameras have automatic white balancing that usually works fearly well. But in some conditions (like different light sources or tricking coloured walls or objects) the camera gets fooled. All videocameras have the possibility to have the white balance manually set. Check your camera user guide for instructions.
Cinelerra can help you correcting the colour balance of footage you have already shot, thanks to the Colour balance effect.
After such treatment my image looks like this:
If we analize the pixel on the white paper now we can see very different values:
RGB levels are now nearly even.
The correction is not perfect but the difference is huuuuge!