i need to ask this. is anyone seeing any lines behind the shoulder?behind the shoulder i have a plane. i see strange green lines. what is this?

I can see the lines, but have no idea what it is.

so you see them?  shading rate can not be a problem since the skin looks normal. i use raytraced shadows so that can also not be a problem.

I know exactly what it is. I am preparing a demonstration and a solution for you.

It is a combination of two things:

1) Color banding - RGB values are discrete steps from 0 to 255, not continuously variable smooth values. Thus a very slight gradient across the wall, for example from 190 to 195, has six discrete bands (190, 191, 192, etc.). Within each region the luminance of each RGB value is identical, instead of a continuous change from 190 to 190.02 then 190.05 etc.

2) Optical illusion - human eyes have special circuits to detect and highlight small changes in luminance. Basically, the eye does "sharpening"  - tiny changes at a boundary are magnified by neurons that are looking for these. If I give you two flat colored squares to look at, far apart, and they differ in luminence only slightly, say 190 versus 191, you cannot tell the difference. You won't know which is the brighter one. But if I put them next to each other, you will be able to tell, because of your built-in edge detector. The net effect of your edge detector is that where there is a transition in luminance, you perceive the lighter part near the transition as being more lighter than it is and the darker part more dark than it is. When you have adjacent color bands at the right distance and with the right colors, you perceive a gradient within each constant luminance band, even though there isn't one. Moreover, you will often perceive that within the constant band, the green is changing more than red or blue. This makes it seem like there is extra green near each transition, which tapers off farther from the transition.

i started to notice this when i got a new LCD monitor. first i thought it was the monitor.
strange.

thanks BB for the help.

Yes a good monitor will make you notice it more. My laptop is not very good and I don't notice these gradients so much on it, but my excellent monitor at home makes them very clear.

So here is a big square. I have a gray diffuse color and no specular. I have a spotlight with angular falloff pointing at the square.

You should be able to see the gradient contour lines in the render.

I am uploading PNG files instead of my usual JPG so that the colors are reproduced exactly for you.

Now the problem as I said is that there are curved bands of equal luminance. Our eyes detect and amplify these, as well as creating the illusion of varying luminance within each band.

To stop this, I introduce some random variation in luminance, using a Noise node connected to the Diffuse_Value. This is just one way - you could also accomplish this with some very slight bump noise.

The Noise min is .95 and the max is 1. So it produces luminance varying from 95% to 100% of the value specified in Diffuse_Value. You can adjust this as you like. You are trying to hide the banding without actually introducing a noisy looking texture. (Unless you want that.)

thanks .

Here is the render with the upper half using noise and the lower half not using noise. You should be able to see the bands in the lower half, but not in the upper half. Or perhaps the upper you can still see them but they are weaker. As I said, my laptop screen is not so good, so to me the upper half looks band-free.

A really good monitor produces exact and precise luminance for every pixel. A cheap or old monitor will naturally produce its own "noise" and is unable to make two pixels exactly alike given the same value. Thus, bad monitors introduce the noisy luminance automatically and hide this effect from you.

Because I know of this effect, I always (ALWAYS) use a slight bit of luminance noise on any wall that has a simple constant color on it.

If there is a pattern of any kind (color, luminance, or bump) you don't need to do this.

I've noticed this on renders I've done before, and just blamed it on the difference between my old crt monitor and the lcd monitors I use now. Good to know that I can eliminate it like that. Cheers bb, and ice-boy for bringing it up.

I use a noise-filter in the light-channel(s) since my first render with P5.