PreTreating Plates

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Intro

  • Preprocessing the plate can help pull a key.
  • This processed FG plate is only used to create a matte and not used as the RGB element that is combined with the background plate
  • The danger with too much pre-treatment is that it becomes mismatched from the plate and dosent line up. For more info read Steve Wrights 'D.C.f.F.&V.' chapter 2

There are two opposing schools of thought.
  • Do not pre-treat. Key off the raw scan to keep all the detail and only process within the keyer or after with the matte.
  • Pre-treat the plate to pull the best key

Degrain

De-graining the plate minimizes noisy chattering keys, but can reduce detail by making the plate soft.

Colour Corrections

  • Using the neutral grade can be preferable if the raw scans colour balance is biased towards the screens colour.
  • Often keying in Log colourspace is preferable than Linear.
  • Increasing contrast is often implemented
  • If using a colour difference keyer these methods can be effective as well as solving the edge shortfall problem that occurs with this type of keyer.

Colour Clamping

If areas of the fg subject has higher channel levels than the screen colour due to spill or transparency, the key will have holes in the fg subject. By subtly decreasing the maximum value of the screen colour channel and/or increasing a fg minimum colour channel value these problem areas can be removed. However in doing so the relationship between the channels in other areas changes and a before/after check of the output of the key should be used to check for nasty side effects elsewhere in the image.

Colour Shifting

Use when separation between the fg subject and fg screen is not enough. By raising the values of the screen colour and possibly decreasing the other channels the separation between the channels is increased creating more contrast in the matte. Make sure that the screen values are not higher than the values in fg subject or you will get holes in your matte. Again a before/after check of the key is required, if problems do occur these might be minimized by using a curve rather than an simple add/subtract.

Example

  • The plate is an interior of a car with unwanted strong spill on the inside edge of the window.
  • The keyer is a colour difference keyer against green identical in both before & after.
  • A slice tool for shake(external link) (cache) / for nuke(external link) (cache) or plot scanline cropped to the edge examines the rgb channels over the edge area.
  • The red channel had an add applied erasing the red value on the window edge so it is greater than the green value.
platebeforeafter
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beforeafter
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Alternatively if hole in the fg matte is in a bright area decreasing the green channel rather than raising the red or blue would fill the hole.

Screen Correction

An un-uniformly lit greenscreen makes it difficult to get a consistent keyed edge round the whole FG. There are a few methods to correct irregularities in the screen, to create a nice even screen surface, making it easy to key.

Manual Screen levelling

  • Using roto's and gradients to add to the darker areas of the screen so it is all even.
  • The correction is done on the both the FG subject & screen of the plate so edge detail is levelled.
  • This has to be separate adjustments to RGB not just the green channel, so all channels in the dark area match the bright area.
  • A slice tool that generates a curve is a accurate and fast tool for doing thos


Procedural screen levelling using an empty screen.

  • This method uses a fg/subject less version of the screen (clean plate), It is commonly called the Ultimatte method of screen correction.
  • A clean plate without the fg subject is required for this method - if this has not been shot then it needs to be made. This can be done with patches from other frames and is far easier if the camera is locked off or has minimal moment
  • If the shot is not locked off then you have to make a clean plate by painting/patching one together on an oversized tracked clean plate (time-consuming). Alternatively the edges can be painted 'in' per frame. It is only edge detail that is difficult to key, so only this area needs to be painted. This last method easily maintains the animation of the grain.
  • The same method (i.e divide op) could be used to clean up the matte rather than the plate if you weren't pre treating the plate, but just cleaning a matte produced from the key

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Procedural screen levelling with time offset.

Another method to do this is with a time offset input/output loop: see http://www.fxshare.com/shake/tutorials/keying/201-1.html(external link)
It works best with locked off shots where the fg subject moves around to reveal the background. If you get trail edges use a dilateErode to adjust it.
  • build script - adjust dilate erode where there is the most movement. Two FileIn's of the same plate are required due to timeX bugginess. Set keyers to only affect RGB
  • render out frame '2-5' using a shake FileOut
  • load render and set first frame to '1' to lastFrame of scanLength
  • connect render to Input 1
  • render '3-' lastFrame of scanLength (make sure file out has not got 2-5 frames in filename output)
  • last frame is the cleanplate
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Procedural screen levelling with pixel fill

  • This is of limited use as it is only a blurred average of the area
  • A previous hard key is required to do it
  • Not as accurate as the above methods but quick to set up if you have a hard key
  • Use a pixel fill and a expanded key to fill in the inside.

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Chromatic aberration

By removing the chromatic aberration (i.e. offset of colour channels at the side of the lens due to lens distortion) a sharper edge can be achieved if required. (useful to minimize fringing when luma keying against a sky)

Links

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Created by ome. Last Modification: Thursday 26 of March, 2009 11:11:30 GMT by ome.
List of attached files
ID Name desc uploaded Size Downloads Actions
107 default screen_correction.shk shake Mon 30 of Apr., 2007 17:26 GMT by ome 19.06 Kb 2287 View Download