CinePaint logo
screenshots

Colour Management and
Colour Correction in CinePaint

A final year project at the
University of Edinburgh, logo of school of informatics

  Rays of light pierce the fog that lies over the forest as a lone rider emerges from the trees and into an opening. Two cinemas further, warm evening sunlight bathes the corn field where farmers are taking in the harvest. Light and colours are important means in the creation of a film. They set the atmosphere, and convey artistic ideas. The art of adjusting colours in post-production is called colour correction or colour grading. For his job, the colourist needs a good eye as much as reliable tools. With an increasing number of films edited digitally, this often means software like CinePaint. CinePaint is an image retouching application that was used for films like The Last Samurai and Harry Potter. It is free software developed by volunteers. The goal of this project was to equip CinePaint with new and more reliable means of colour reproduction and manipulation.

The colourist needs to be able to trust the colours on her screen. In the past, computers have been notoriously unreliable for colour reproduction. Colours appear differently on different screens and printers. Often expensive proof prints are the only way to check the final result. The problem originates in colour physics and the fact that different devices reproduce colours differently. It is as if they were speaking different languages, but pretended to magically understand each other, as they exchange colour specifications. The result are misunderstandings and wrong colours. A translator is needed, a software called a colour management system (CMS). The CMS learns from device profiles about the characteristics of different devices and can then translate between their colour representations. This project implemented a CMS for CinePaint which makes CinePaint only the second free software to provide such functionality.

Colours are often difficult to control. To be able to predict the effect of mixing and manipulating them, a colourist needs to know about concepts such as complementary colours, and she needs intuitive tools to support her. In this context, colour wheels and look profiles were developed in this project.

colour wheels

Colour wheels are widely used to adjust the colour balance. An image is unbalanced if it shows a colour cast, i.e. it contains too much of a certain colour. A knob is moved over the wheel to increase the proportion of the colour it is moved towards. Correcting unbalanced images is straight-forward: If you have identified a cast (e.g. blue), increasing the complementary colour which is opposite on the wheel (here: yellow) will remove it. Of course, colour casts can also be created deliberately for their effect.

Look profiles help a colourist to experiment with different manipulations and to make them consistent over a scene. A look profile stores a colour manipulation. A scene can then be watched through it, like looking at the film through coloured glasses. Look profiles are a new concept and only starting to be explored by software developers, as digital editing becomes more common.

Colour Correction in Action
Move the mouse over the left image to see how the red proportion is increased to create an impression of warm evening light. The right image exhibits a blue colour cast. Move the mouse over it to see it removed.
demo 1 demo 2

Copyright (C) 2004 Stefan Klein, kleins at web dot de