Linear Workflow and Gamma Correction

In part one of four articles, Elliott takes a look at linear workflows and gamma correction, and explains why both are so important.

What is it and why do I need to care?

Light intensity works linearly, whereas electronic displays do not. In the real world, two lights of the same intensity directed on the same spot will illuminate the area with twice the intensity of a single light. This can be expressed as a simple graph, as shown in figure 1 (below left).

This is perfectly sensible and is to be expected but a problem arises when light is displayed on electronic equipment. If you double the voltage running over a liquid crystal (Liquid Crystal Display), the intensity of light emitting from that crystal doesn't double, as it does in nature, and therefore isn't linear. Because of this, computer monitors cannot display images and video without a certain amount of pre-processing.

Both hardware and software typically apply what is known as a gamma correction curve to images and video so that they can be displayed on monitors within a sensible colour range. This gamma correction curve typically has a value of 2.2 and can be seen in figure 2 (above right).

So, if the manufacturers of hardware already correct the deficiencies of the monitor, why do I need to care?

Because any manipulation of any image or colour you make on a computer is simply a change to an already gamma corrected image, which is itself only a best guess, resulting in images that aren't physically accurate and are generally of poorer quality.

The linear workflow method tries to address this issue. The process involves applying an inverse gamma curve (de-gamma) to all input images and video so that the footage is converted back into a linear format and is then ready to be worked on and manipulated. This is shown in figure 3 (below left). The value of this curve is obtained by dividing the target gamma value of 1 by the current gamma value of 2.2, therefore the value of the inverse gamma curve is 1 / 2.2 = 0.4545.

The linear workflow for the 3D industry boils down to image input and output. As you now know, images will almost always need an inverse gamma curve applied to them when they are brought into a 3D application. This will ensure that you are working in a linear workspace. When the image has been rendered, post-processed and finalised, a gamma correction curve needs to be applied so that it can be displayed on computer monitors. This process is demonstrated in figure 4 (above right).

I'm actually quite satisfied with the images that I'm producing. Is it really worth all the trouble?

Yes! Below are just some of the benefits of working in a linear workspace.

• You will spend less time tinkering your images to get realistic results as working in a linear workspace yields physically accurate results.
• There should be no need to render out different channels and composite them later in post, as again the rendered image will be physically accurate. (It is also worth pointing out that any blending of layers together in post, such as Add or Multiply, is completely inaccurate and mathematically insane, when not working in a linear workspace. This is because you are blending together layers that themselves have been 'corrected'.)
• Effects, like fog, lights and motion blur, work better in a linear workspace.
• Eliminates unrealistically strong reflections.
• Smoother gradients in darker areas.
• Less artifacting around specular highlights.
• Fewer blown out or overexposed areas.
• You can use lower intensity lights in your scene and push them further.
• Smaller file sizes, as there is no need to add all the individual render channels.

In parts two and three of the series, I will look at 3ds Max's built-in gamma correction, 3ds Max and VRay, and 3ds Max and Mental Ray. In the meantime, if you want to find out more, give the team a call on 03332 409 309 or email 3D@Jigsaw24.com.