Resizing an object causing render artifacts

[scooternva]

I've run into a strange issue and I'm trying to understand what's happening and how to deal with it.

This is a simple asteroid model I created in LightWave out of a tessellated sphere with jittered points. It has a spherically-wrapped trio of images attached to its material definition (I've included all the source files in an attached zipfile) on the diffuse, bump, and normal channels. In this image, the object is 100 meters in diameter:

Asteroid diameter: 100 meters

This is the same model, scaled 10x so that it's 1000 meters in diameter. I rescaled the projection to correctly wrap the larger object, moved the camera further away to compensate for the larger object size (so that to the camera it has the same apparent visual size), and increased the size (and intensity) of the scene lights to approximate the lighting conditions used to image the 100 meter asteroid. But as you can see, this time you can see the tessellated triangles underlying the kilometer asteroid:

Asteroid diameter: 1000 meters

I've tried maxing out the depth of diffuse, specular and glossy channels, upping the minimum and maximum samples, increased the sample rate on the lights, cast a couple of voodoo spells... nothing works. I realize I'm using the same image files on objects that are an order of magnitude different in size, but the camera was moved back on the larger object so the level of apparent detail is the same.

So why does the second image look like garbage, and what do I need to do in order to fix it? I suppose I could render my asteroids at the small scale of the first image, but that will make scene setup more tricky with the spaceship that's supposed to be flying by these objects. The asteroid is supposed to be twenty times larger than the ship, not twice as big.

Thanks in advance for any suggestions!

Scaling Issues.zip

LightWave objects, scene files, and images

(20.15 MiB) Downloaded 145 times

[mikefrisk]

Increase the Ray Epsilon in the Kernel node.

[frankmci]

It looks like a Ray Epsilon issue. Different scales can require different levels of precision for calculating self-shadowing of a surface. Try dropping the Ray Epsilon precision by a decimal place or two in your Kernel settings. That's the number that, among other things, controls how close "behind" a surface a shadow falls, and as the scale of a scene changes by orders of magnitude, that calculation accuracy can fall outside the floating point precision of the render.

[scooternva]

ACK! I knew I forgot to check something. Yep, that was totally the problem. Thanks guys!