Subject: Those Darn Lens Burns
From: [email-masked] (Sean Moyer)
To: [email-masked]
Date: Sun, 30 Jul 1995 13:20:32 -0400

With all the mailer problems at webcom,  I'm not sure this one got through
So here we go again - A Lens Burn Solution

(Also liked the Blurred Partcle/Lensflare trail solution)

>On Mon, 24 Jul 1995, Mads Storm Andersen wrote:

>>Lens burns, it's the effect you get if a light source is VERY bright and
>>you move the camera(Or light) you will get a streak of "burned" light on
>>the iris of the camera...

>That makes sense... I've no idea how that would be created tho... I don't
>even think it's posible to do in lightwave... But I hope I'm wrong on this
>one.

*******

Alright already. Lets get this sorted out once and for all. This may be a
little sketchy, but the proccess should be sound.

1) Render the scene as normal, with your point source moving across the screen

2) save the path of the light (this is assuming one source for simplicity)
and save the path of the camera

3) In modeler, use path extrude to make a tube along the path, the diameter
of which will be the width of your burn.

4) in layout (in a new scene), give the tube the color you want the burn to
be, make it somewhat transparent (50% ?) and turn on transparent edges.

5) Use your initial render as an Bg Image sequence.

6) Assign a point light to the original light path and offset it so that it
moves parallel to the tube, just in front of it (no lens flare). Give it a
little bit of falloff so there's a definite hotspot on the tube as the
light moves

7) Here's the tricky part :-)
You need to make a poloygon that will sit in front of the camera and will
cover an area bout twice the size of the screen. This polygon will be
between the camera and the tube. Apply a front projection map of your BG
image seqence to the color, make it 100% luminous, 0% diffuse.

8) To make this whole crazy thing work, the big polygon needs a
transparency map on it, a vertical stripe of transparency that is as wide
as the lens burn is long. You'll need to make it in a paint program, and it
needs to be ramped as such:

assume your light is moving across the screen right to left and assume the
line below is the length of the burn

____________________________________________________________
 0                   10
100

black at zero            white at 10                         black at 100

depending on how long you want the burn, the stripe should probably not be
more than 5-6% of the width of your transparency map.

To make life a little easier, load the transparency map as a modeler BG
Image, import the Big polygon & size the image to fit the polygon, then on
each edge of the white stripe, make a row of points to act as a marker in
layout. Export it back layout, and assign the image as a planar map in
layout.

9) Almost there! The 10 mark on your transparency map needs to follow the
light so that it is always sitting over the light exactly. You can probably
load the lights motion path and tweak it to work.

10) Remember the camera motion path you saved from the original scene? Load
it back in for the camera here. In a sense, you are doing a
motion-controlled beauty pass for the lens burn. The front projection map
mattes out the part of the burn object (tube) you don't want to see, and
viola! lens burns.

A couple of disclaimers:

This is a thought experiment, I have not tried this. Proceed at your own
risk! All numbers are theorectical

I sit in front of Alias machines all day (still waiting for LW SGI, Xaos!)
so if some of the terminology is incorrect, please forgive me, no machine
here for refernce (my Amiga's are at home)

Sorry this turned into LW: The Novel. Remember this next time you think
about saying..."I don't think that's possible on Lightwave." (!!)

Sean Moyer
Lightwave Knight Errant
Chedd-Angier Prod. Co,
Boston MA

See you all at SIGGRAPH!

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