Getting Forward And Backward Backward

You know the difference between forward and backward, right? Guess who doesn't.

Believe it or not, LightWave actually got these directions backward. It sounds like a joke or a prank, but it's not. You can find this yourself when you mess with the Kappa and SSS nodes.

Both of these nodes do similar things: subsurface scattering effects. The first version of the Node Editor included the unintuitively named Kappa node, which is a simple subsurface shader. When its mode is set to Backward, it acts much like any other diffuse shader, with the exception that it can blur shadows that fall on it, and also let light creep across the terminator.

Kappa set to Backward mode, with a 1mm range on the left and a 500mm range on the right.  Notice that the shadows and terminator are softer with the larger range.

When set to Forward, it follows the rules of the translucency shaders, but, again, with the ability to blur shadows and let light creep across the terminator. Aside from its habit of rendering polygon edges if its Falloff was too small, this is a nice little node.

Kappa set to Forward mode and back-lit.  Again, that's a 1mm range on the left and a 500mm range on the right. Notice again, the shadows are softer with the larger range, and the light is creeping around the edges a lot more.

Its big brother, Kappa II, does both forward and backward scattering together, with separate colors and settings for each.

“What are forward and backward scattering?”

They have to do with how light scatters when it hits something. If it's only scattered a small amount, and continues traveling in the same general direction it was going before, that's called forward scattering. It's scattered along its forward motion. Back lit paper, clouds, and dust all exhibit forward scattering. If your windshield is slightly dirty and you're driving toward the sun, that blinding glare you see coming off the dust is forward scattered light. The monitor screen you're reading this on is using a form of forward scattered light because there's a light source behind it that's lighting it up from the inside (unless you're using a plasma or CRT monitor). If that's not clear, here's a pair of photographs showing this in action. This is a tube filled with a slightly cloudy material, and lit from one end.

Backward scattering is what you normally see everywhere, and as its name suggests, refers to light that's scattered more, toward the light source. This is sometimes referred to as diffuse scattering. Skin, plastic, and wax are good examples of how light can enter a substance and bounce back out in the general direction of the source and have different appearances based on the angle the light travels through them. Skin, for example, tends to have the typical flesh color when it's front lit, but takes on a redder glow when it's back lit.

Normally with materials like these, there's a bit of both types of scattering taking place. So, if we take an object that's surfaced with Kappa II (or two mixed Kappa nodes), we can tell it to color the forward scattered light red while the backward scattered light is blue. If we place a light behind it, so the object is between us and the light, we should see the forward scattered color, and the object will appear red.

If we move the light to the same side of the object that we're on, it will look blue because now we're seeing the backward scattered light. And as you can see here, that's exactly what we get.

When the gryphon is back lit, we see the forward scattered color, and it appears red. When the light hits the front, we see the backward scattered color which is blue. When the light comes in from the side, we see a mixture of the two...

Kappa II with a 500mm range on the left and a 5m range on the right

“I thought you said they got it backward?”

They did, but not here. Kappa does it right. SSS2, on the other hand, is wrong.

The two SSS nodes are much like Kappa, but they tend to have better results, though they can also take much longer if radiosity is involved. The SSS node is a single color node, and only does a backward scattering effect (though the developers probably call this backward scattering).

SSS with a range of 500mm on the left and 5m on the right.

SSS2 is much like Kappa II in that it offers forward and backward colors. Only, these are completely backwards as you can see here.  I've set up SSS2 with the same colors as the Kappa II node, with red for the forward scattered color and blue for the backward scattered color. I've bumped the Backward Amount to 200% since it's rendering a bit darker than Kappa did. Here's the object with the light behind it again. This demonstrates forward scattering, but instead of getting our Forward color, which is red, we're seeing blue.

Here's the object demonstrating backward scattering. Again, instead of seeing the blue that was set up for the Backward color, the object appears red.

Here's the object with the side lighting.

So, Backward means forward and Forward means backward. Except when Forward means forward and Backward means backward. Everyone got that? Good. There should be no confusion then.

“Is this a bug?”

Who can tell anymore?

“Will they fix it?”

Possibly, if & when they feel like it, but it will probably be a surprise. As with Sigma or the Gradient node, I wouldn't expect any kind of mention of it either. It's so much more fun for the users to find out their assets have been changed without their knowledge.