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Friday, October 08, 2010

Rotoscoping hair

I forgot that Focal Press (the publishers of the VES Handbook) asked me to write a short article for them a few months ago.  It's online at one of their sites.

Rotoscoping Hair Article  <- currently invalid link but leaving it for the moment.

[So it seems that Focal Press has dropped the ball and allowed their website to disappear so I'm posting it below]


Rotoscoping Hair

One issue that comes up when people start learning rotoscoping is how to rotoscope hair.

Simple haircut with a few loose strands:
Roto a head of hair just like you do any other shape on a person.  You want to capture the shape of the hair.  If there are a few loose strands of hair don’t worry if these get cutoff.  It’s unlikely the audience will notice as long as you follow the other key rotoscoping rule – make the animation smooth.  To make the animation smooth try to keep the number of key frames and number of spline points to a minimum.  Without a reference to the version before roto, the audience is unlikely to notice the roto unless it jumps around.

Wind blown loose hair:
You don’t roto this type of hair if at all possible. Trying to follow hundreds of hairs moving around and hand cutting them out is a losing task.  The hair will likely be a pixel or two wide with sections appearing and disappearing depending on the lighting and the background.

If it’s at a distance or fast moving you can roto for the shape of the haircut.  If there’s a full sequence of blowing or frizzy hair in close-ups then you should seriously consider the possibility of shooting green or blue screen.  If that’s not possible (or the footage is already shot) then you have a few alternatives.  In any case, roto the main head and hair.

Options:
1. For the strands of hair create a garbage matte that contains them and then try to pull a key off the hair or background.  This is by far the most common method of dealing with hair that has super fine detail or requires strands to show up. If you have blond hair over a dark background or dark hair over a light background try a lumikey (brightness key).  Whatever makes the hair standout against the background is what you want to leverage to form a keyed matte.  You may have to separate the hair with multiple mattes in case the lighting or the background changes radically during the shot or across the frame.

2. If pulling a matte is difficult or there are only some strands (enough to truly be noticed if they were clipped) then it’s possible to create a single spline for each hair.  For traditional roto you’re creating an enclosed shape with the spline to use as a matte.  With this technique you just have individual spleens literally following a single hair.  Once these are in they should be rendered as paint strokes in the alpha channel or whatever other method your composting software provides for turning the splines into straight mattes.  And yes, this takes time.

3. Same as above put paint in the single hair strand mattes using a tiny paintbrush tool in the alpha channel.

4. In some case companies roto the haircut and in order to replace the clipped hair they build a digital double of the actress/actor, match-a-mate (key frame digital double to match the live action images) the digital double, simulate the hair motion and then render the hair.  This can be reasonable if you already have a haired digital double and the hairstyle lends itself to hair simulation.

In all of these cases you don’t need to capture every single strand of hair.  You only need to approximate the feel of the hair enough for the scene so it works when cut with other shots and played back at full speed.  That may mean dozen hairs instead of the hundreds you see in the original.

[Update: I have a Rotoscoping Basics post that links to the various roto posts, covers some basic notes and provides a link to the fxGuide  article on roto that discusses the history of rotoscoping and some of the currently available tools. It also includes an interview with me talking about Commotion, which is the package I did the video demos with.]

7 comments:

  1. Hi, the link is broken. Any chance it could be fixed?

    JOhn

    ReplyDelete
  2. Hi, the link is broken. Any chance this could be fixed?

    JOhn

    ReplyDelete
  3. I've pasted in a version of the document. I don't control the website that this was linked to.

    ReplyDelete
  4. Thanks, Scott. My VFX guy bailed, so now I get to roto some ECU hair. Hoping your alpha paint idea will work with few/no jitters...

    ReplyDelete
  5. If your paint can use splines then try to use those to minimize jitter.

    ReplyDelete
  6. thanks for the tutorials sir, keep it up

    ReplyDelete
  7. Very nice information. I just love multimedia animation, rotoscoping, visual effects and vfx information. I am looking this kind of blog. Please keep updating..I am going to bookmark your blog.

    ReplyDelete

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