Keying

Definitions
  • Matte - grayscale image used to identify specific pixels in an image. Mattes are used to composite only the relevant pixels of a foreground image over a background image. Pure white pixels indicate areas where the foreground image will completely obscure the background image, and black pixels indicate areas where only the background will be visible. Intermediate gray pixels indicate semitransparent areas where the foreground blends with the background.
  • Key - matte that has been procedurally generated by the computer. An image is analyzed and - based on certain criteria such as color or luminance - a grayscale matte image is created.
  • Mask (aka rotoshape)
    • Rotoscoping - process of generating a matte by manual articulation. Instead of using procedural techniques, an artist uses points, curves, or paint to specify the opacity of the pixels in the foreground image of a composite.
  • Alpha Channel - matte that has been incorporated into the image data of an image file. Not generally applicable to video/film footage.
 
Types of Keying Processes
  • Luma-Key - uses luminance values to derive a matte image. Useful when trying to key an image that is very high contrast
  • Chroma-Key - uses a range of hues to create a matte image. Better chroma-keyers allow the user to specify hue, saturation, and luminance.
  • Difference Matte - uses two images to define a matte. An image with the subject and its background and an image with the background only. The two images are subtracted from each other to produce a difference matte.
  • Color Difference Key - Defined as a process of matte extraction, color correction, and image combination.
    • Not limited to color ranges. Uses channel substitution and subtraction techniques to remove key color.
  • Optical Flow
 
Rotoscoping Techniques and Keying
  • Garbage Matte - Removes problem areas from around the subject
  • Hold-out Matte - Removes problem areas inside the subject
 
Dealing with 4:1:1 Footage
  • Chroma Blurring
  • See 4:1:1 smoothing in Final Cut Pro
 
Advanced Matte Techniques
  • Core Matte
    • Main matte extracted using chroma-keyer or color difference keyer or Primatte
    • Dilate/Erode/shrink matte to get inside of the details
    • blur matte to get rid of edge problems
  • Edge Matte or Detail matte
    • Detail can be extracted from the luma channel (often the green channel)
      • luma based matte can be ADDED to core matte
    • Edge matte - useful for detail, light-wrap, and gmattes. May not be available in many programs
 
The Steps
  1. Setup shooting environment
  2. Acquire Footage
  3. Correct for uneven lighting in the backing screen if needed
  4. Remove grain/noise
  5. Compensate for 4:1:1 sampling if needed
  6. Draw a Garbage mask
  7. Generate a key (difference keyer, Primatte, Chroma Keyer, etc.)
    1. May take several passes to get all of the color out
    2. Consider an edge matte using the luma or green channel and adding it to the main key
  8. Apply matte to source footage
  9. DeSpill (spill suppression)
  10. Color match background
  11. Background wrap (don't overdue it!)
  12. Add grain/noise to match background
 
Shooting for Keying
  • Lighting
    • Light the screen as evenly as possible
    • Consider gelling lights to same color as background
    • Allow enough space between background and foreground to avoid shadows and be able to light the foreground to match the directionality of the background plate
  • Screen color
    • Green
      • Pro
        • Easier to light
        • Cleaner edge to green channel in video footage
        • Less noise in green channel
        • Less likely to have similar color in foreground objects
      • Cons
        • More possibility for spill due to light reflection
        • Ugly color if some is left in the edge of object
    • Blue
      • Pro
        • Reasonable color to have around object (easier to suppress)
        • Considerably less spill problem
      • Cons
        • Requires more light
        • Blue channel has noise in film and sampling issues in video footage