31.10.2023 – Premult and Colour grading

Week 5 – Premultipication and Colour grading

Premultiplication: Multiplies the RGB values by the alpha.

Nuke operates differently from After Effects in one notable way – it does not automatically premultiply images. Here is what you need to know:

• Use the ‘Premult’ node before any Merge operations if your input images are unpremultiplied. This crucial step prevents unwanted artifacts, such as fringing around masked objects.
• When color correcting a premultiplied image, begin by connecting an ‘Unpremult’ node to the image. This transforms the image into an unpremultiplied state for accurate color correction. After adjustments, add a ‘Premult’ node to restore the image to its original premultiplied condition for Merge operations.

As a general guideline, most 3D rendered images come premultiplied. A handy rule of thumb is that if the background is black or very dark, the image is likely premultiplied.

To manage premultiplication in Nuke, you can employ the ‘Unpremult’ node to divide the input by alpha and the ‘Premult’ node to multiply the input by alpha. You can apply these nodes efficiently within your node tree. If you need to add or adjust the original input, simply place a node at the top of your existing nodes. While it’s possible to adjust the premult option in other nodes such as grade, it is recommended to place nodes between ‘Unpremult’ and ‘Premult’ for better efficiency and ease of management, especially when you need to change values.

For keeping tabs on the size of your working file, check the information tab above the timeline. The corner figure represents the size of the bounding box.

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Colorspace and Linearization

Understanding the colorspace and linearization of your footage is crucial when working in Nuke. Here are some key points to remember:

• Colorspace: This term defines how your footage was originally captured or exported. Most files are in a non-linear colorspace, and correctly identifying the colorspace is essential for accurate linearization. In the world of digital imaging, real life is linear, and this understanding is fundamental.
• Linearization: This is the process of converting your footage into a linear colorspace. Nuke’s tools are all built around linear math, enabling the seamless integration of various media types. This process is particularly valuable for ensuring consistent, accurate results.

• LUTs: Look-Up Tables (LUTs) can be used for creative or technical purposes, such as converting from a log to a linear colorspace or adding a specific “look” to your footage.
• CDLs: Color Decision Lists (CDLs) are primarily creative tools that allow you to apply a desired “look” to a clip, enhancing its visual impact.

Graded footage refers to content that has been color-corrected to achieve its “final” look. This step is vital in creating the desired atmosphere and visual style for your project.

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Color Grading

Nuke operates in a 32-bit linear float color space, providing you with a powerful canvas to work on. Several nodes allow you to adjust the size and color of your input.

• Grade: In color correction, you should always think in terms of Highlights, Midtones and Shadows. Two commonly used nodes are ‘Grade’ and ‘ColorCorrect’. Both let you grade H, M, S of a shot. To grade Highlights you can utilize GAIN or MULTIPLY, to grade Shadows LIFT allows you to control it, and to grade Midtones you can use GAMMA(in the ColorCorrect the LIFT is called OFFSET.)
• Reformat: It lets you resize and reposition your image sequences to a different format (width and height). However, beware that changing the format to a larger or smaller one without the right filter (default: cubic) can result in blurriness or degradation. Use filter options to enhance the clarity and quality of your adjusted input.
• Copy: This node lets you copy selected channels, streamlining your workflow.
• Toe: Think of this node as a more organic version of the ‘Lift’ knob in the Grade node, perfect for adding that artistic touch to your compositions.

White-Balancing: It is a valuable technique to ensure your footage achieves visual consistency.
Start by pushing the gain in the viewer all the way up and use the eyedropper tool (Press Ctrl+Shift and drag over the area of interest in the input) within the Grade node to select the darkest point in the images. This step sets the baseline for dark areas in your footage. Now, push the gain in the viewer to its lowest setting and use the eyedropper tool within the Grade node to select the bright or white point. This step establishes the baseline for bright areas in your footage.

To fine-tune the luminosity and match it across different clips, manipulate the ‘multiply’ function in the Grade node. This allows you to create a consistent visual tone throughout your project. White-balancing your assets before applying grade adjustments is a useful technique, especially when you need to ensure that your content seamlessly blends with the background and achieves a harmonious overall look.

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Shortcuts

• PgUp/PgDn: Quickly switch between channels for your input.
• D: It makes disable a node temporarily.
• Ctrl+Click on a Pixel: It provides you with instant access to pixel information.