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Week 24 – Treating Smoke Footage

In matte painting, you often work with various stock footage elements like fire, smoke, dust, and more. To seamlessly composite them onto different backgrounds, it’s crucial to treat them properly. When dealing with smoke footage, simply premultiplying it and merging it over the background can result in an unnatural and artificial look.

Here are some techniques to improve smoke compositing:

  • Haze and Depth: While not strictly smoke-related, this technique can create a dusty atmosphere. Using the Ramp node, generate a gradient alpha and blur it. Then, use this alpha to adjust the scene with ColorCorrect or Grade nodes. By focusing on specific areas and rotoscoping the foreground, you can achieve a dusty appearance with depth.
  • LumaKey: If the footage lacks an alpha channel, you can extract one using LumaKey. Avoid merging the footage with the Screen operation, as smoke thickness won’t be preserved, resulting in a flat and artificial look. Ensure the operation is set to Over. After extracting the smoke’s alpha, merging it over other footage may still appear unnatural. You can improve the edge quality with the EdgeExtend node (untick the ‘Premultiply’ knob).
  • LumaKey with Log: Log2Lin (operation: Lin2Log) enhances the alpha quality when using Keyer or Keylight. Before connecting Keyer, convert the footage to Log, extract the alpha, then unpremultiply it and apply Log2Lin again. Premultiply it and merge it over the original footage, which should also be in Log format. Finally, convert it back to Linear.
  • Relight the Smoke: Relighting smoke can be achieved by merging (using the Average operation) with the blurred background plate and the edge-extended smoke footage. If necessary, use the Colorspace node with YCbCr. Unpremultiply the smoke, convert it from Linear to YCbCr, and do the same for the blurred background plate. Then, copy the green and blue channels from the background to the smoke. Finally, convert it back to Linear and premultiply it. This results in smoke that’s influenced by the background, appearing more saturated.