Poser SkyTexture node

  • At Caisson's request I'll open this thread on using the SkyTexture node in Poser. Please note I am not an expert on this, but I'll share what little I know.

    First, the node only seems to work connected to the background root node.

    Secondly, there is no way to control brightness. In Blender the skytexture node connects to a specific background node which does have a brightness slider. The easiest way (for me) to get around that is to run the node through an HSV node and control brightness using values input.

    Third, it won't add light to the scene unless the Cast_Light check box is ticked on the background node.

    0_1501891017480_background 4.JPG

    Once it's set up, the only parameters worth looking at are Turbidity, Ground Albedo and Sun direction.
    Turbidity control scatter (Mie scatter mainly by the looks). Default of 2.2 is a really clear sky. 8 is smog. or fog. Under 1 is approaching open space (ie, NO atmosphere... it's black).
    Ground Albedo is the light bouncing back into the sky and adding light from below. 0.3 is average, 0.6 or 0.7 is for snowfields.
    Sun direction is pretty obvious, and is the 'pig' I was mentioning. It uses XYZ coordinates (sun position uses 2 coordinates in the real world, azimuth and elevation. When using an infinite as a sun I use 2 coordinates, -x for elevation and y for azimuth. Z is always 0).

    With sun direction the elevation is controlled by the middle number (Y being up). It runs from 0 to 1 (so first problem is calculating a percentage to match degrees rotation on the -x for the infinite light). This bit is actually not that hard, just clunkier than needed. It's the azimuth calcs that are painful.
    The first number is the X parameter and it controls whether the sun is to the east or west. If you come into the scene and zero the rotations on your camera let's assume that's north. For the sun to be due east, the first number is -1. For due west, it's 1. Here's where it gets stupid; 0.5 is still due west. You need to add z values to move it around the horizon. Sort of.

  • So here's some examples:
    If north is 0 degrees rotation and west is 90 degrees, here are the settings that will get you other angles:

    60 degrees: -1, X, 0.5 (X is elevation, so I left it as X)
    45 degrees: -0.5, X, 0.5
    30 degrees: -.25, X, 0.5

    These aren't the only ways to get these angles, but you have to know what the inputs represent and how they relate to the physical space to use them.

    I requested the sun position parameters be changed to azimuth and elevation angles so you can just match it to the infinite.

  • I said the sun was a figure. It's a sky dome. To get the sun to behave like the infinite light and be positionable in azimuth and elevation, it needed to be attached to a sphere.

    The set up is this;
    0_1501892291935_skydome 1.JPG

    Basically what I did was take advantage of the fact that point lights flare when close to surface. The sun in my image is a point light positioned very close to the sky dome. The materials are mostly (97%) transparent, the rest is translucent. That provides sufficient scatter for the light without really impacting the rest of the sky.

    The point light was set to inverse square with a strength of 100%. The apparent size of the sun disc is controlled by how close to the dome it is. Given the distance, at 100% and inverse square attenuation it won't add light to your subject. I still use an infinite for that.

    If you position the light at 0,0,Z (so only the distance needed for the sun disc in Z, others to zero) and parent the light to the dome, then the dome obeys the same azimuth and elevation inputs as the infinite does.

    You can also attach a moon image for animatable glowing moons, and separate clouds that can be lit from behind and below... the potential is quite good.

  • Final comments for now;
    the node is very good but stops at 0 degrees elevation (sun on the horizon). To be truly representative it should go to -18 degrees, because that's how far the sun needs to sink below the horizon before it stops adding ambient light to the sky.

    The difference between global illumination and an infinite is that global is GLOBAL and infinites are DIRECTIONAL. The skydomes provide global illumination, the infinite provides the sun rays which are almost parallel by the time they get to us. If you don't have both, you are not getting realistic results. Some very high quality HDR's provide both, but HDR's are useless to me because I do sequential images of the same locations, day and night. I have yet to see a high quality HDR set that comes in hourly increments. Probably couldn't afford it if I did...

  • Oh, one thing I should clarify... the point light needs to be just outside the dome to work like a sun. For some reason I couldn't get it to flare on the inside. So the dome needs to be between the point light and the camera.

  • Oh, just saw this thread and --- it needs more eyeballs ! Thanks for your lessons on the SkyTexture node.