OTOY Forums

On the right is my render as it appears in c4d. On the left is the image as loaded into photoshop with a black layer then inserted behind the render.

Is there any way I can get octane to take the brightness of an object into account when generating the alpha channel? It seems as though octane is thinking "that fan blade is only present 50% of the time, so I should make the alpha channel 50% solid" But this doesnt consider how the object visually renders.

This issue causes me problems in many projects where a bright object goes from sitting in front of geometry to being in front of an alpha background. As soon as it moves from being in front of a black model to the black background, the brightness often lowers significantly because of motion blur.

Project attached.

The closest I can get is disabling "partial alpha", but then of course all my alpha channels turn into binary on/off.

Are you using pre-multiplied alphas? If so, give it a shot with straight alphas. Remember, though, that it really would look dimmer when spinning in the real world. The geometry is emitting the same amount of light as when stationary, but when in motion that light is spread out over a larger surface area, reducing the intensity at any given pixel by the difference in surface area. If the blades cover 1/2 the opening, then you should expect it to be around 1/2 as bright in motion, if it is spinning at any significant speed.

That's just the apparent/surface brightness. The actual amount of illumination added to the rest of the scene remains the same.

frankmci wrote:Are you using pre-multiplied alphas? If so, give it a shot with straight alphas. Remember, though, that it really would look dimmer when spinning in the real world. The geometry is emitting the same amount of light as when stationary, but when in motion that light is spread out over a larger surface area, reducing the intensity at any given pixel by the difference in surface area. If the blades cover 1/2 the opening, then you should expect it to be around 1/2 as bright in motion, if it is spinning at any significant speed.

That's just the apparent/surface brightness. The actual amount of illumination added to the rest of the scene remains the same.

a) cant use straight alphas, their colour profile has been broken for years now unless it got fixed recently?, all straight alpha renders come out in linear gamma instead of srgb

b) already tried, its no better.

c) If I had a lightsabre swooshing, and in a single frame it swooshed 90 degrees from upright, down to horizontal. You would expect an incredibly bright swooshing trail behind the object. But because octane says "well, you only took up space along the way for 10% of the time, im going to make the alpha channel 90% transparent." it would look terrible.

In fact here we go, lets try:
Top, lovely swooshing lightsabre exactly as I want it rendered with a black object behind it, or with alpha disabled
Middle, premultiplied alpha on, 90% of my sabre is gone
bottom, straight alpha, screwed up colour profile, as it has been since octane 4.0, useless.

No, I would NOT expect a bright trail behind a light saber in the way you have depicted. That is not how photography works. I would expect something like this, which is exactly what I get at 24 fps with a 180 degree shutter angle.
If you want it to be brighter as it is swooshing, you need to make the emission intensity higher. You'll also need to mess with your before/after sampling, because in the real world, there isn't going to be a bright line anywhere inside the swoosh unless the sabre was motionless for part of the exposure. While motion blur is often depicted that way in comic books and such, that's not what it looks like on film without intentionally making it less realistic in post or by manipulating the exposure.

Sorry but youre really missing the point here. How bright the swoosh is is dependent entirely on how bright the object is. But the brightness isnt the problem, I can crank the brightness of this lightsabre to a million and it will simply show up as a dull washed out grey once the alpha channel kills it.

If I have super bright hot tracer fire coming from a gun leaving a beautiful streak, it will work right up until the tracer fire goes into the sky and the alpha channel of the sky destroys it.

I can have a train swooshing past with a long exposure time, but if the train passes in front of an alpha background, that part of the train will simply vanish like a ghost.

Here, maybe this will demonstrate the problem more. I have my lightsabre fight, its taking place at night against a city scape in the background, all youll see are the sabres and some foreground light they cast, ill be comping the night sky in post.

Image on the right, my render, a nice full swoosh, no problems
Top left image: how it loads into photoshop. the sabre in front of the buildings works fine, as soon as it goes in front of the sky, the motion blur obliterates the object regardless of its brightness
Bottom image: i stick a black layer behind everything, the sabre aint coming back.