Howto render transparent glass sphere

[ToyBuilder]

Hi,

I am wondering what I am doing wrong when trying to render a solid glass sphere with Indigo. My goal is to model a plane intersecting the sphere. However, the part of the plane which is in the inner of the sphere is not visible, it seems that no light ray refracts at the sphere surface, reflects at the inner solid plane and then refracts again on the sphere surface.

Here is an image of my example scene:

Sample Scene with a Sphere

It consists of a spherical light behind the camera, a solid plane (yellow) and a sphere with transparent material:

Code: Select all

	<material>
		<uid>6</uid>
		<name>sphereMaterial</name>
		<glossy_transparent>
			<exponent>
					<constant>3000</constant>
			</exponent>
			<internal_medium_uid>8</internal_medium_uid>
			<absorption_layer_transmittance>
				<constant>
					<rgb>
						<rgb>1 1 1</rgb>
						<gamma>2.2</gamma>
					</rgb>
				</constant>
			</absorption_layer_transmittance>
			<layer>0</layer>
		</glossy_transparent>
	</material>

		<medium>
			<uid>8</uid>
			<name>sphereMaterialMedium</name>
			<precedence>10</precedence>
			<basic>
				<ior>1.1</ior>
				<cauchy_b_coeff>-0</cauchy_b_coeff>
				<step_size>0.1</step_size>
				<absorption_coefficient>
					<constant>
						<uniform>
							<value>-0</value>
						</uniform>
					</constant>
				</absorption_coefficient>
			</basic>
		</medium>

My first guess was that it might have to do with the sphere model itself. I have produced it as a triangle mesh by myself and maybe I did something wrong with computing the normals, etc. Therefore, I tried to model a sphere with Google SketchUp and use Skindigo to render it. It renders fine if I apply any kind of opaque material, but as soon as I use one of the transparent materials from the Indigo material database, it is completely black:

So it seems that thereis something wrong with my handling of transparency. Are there any ideas?

I use Indigo 3.4.18, 64-bit on Windows 7. For rendering, I tried with/without GPU support, I tried Path tracing and bidirectional path tracing. The renders took 10 minutes (around 3000 spp).

Thank you for any comments.

[ToyBuilder]

Update:

I found a bug in my material properties: the absorption value should be 0, not 1. Also, I have removed the <absorption_layer> section in the material. Now, some light rays from the interiour of the sphere hit the camera, after 10mins of rendering, I have the following image:

My question now changes a bit: It seems that the rendering of a transparent sphere with refraction takes really much time (something like 5h ? while the rest of the scene is finished after less than 10mins). Is there a way to speed this up? Should I use bidirectional path tracing, MLT or GPU support?
Are there any modeling tricks to speed this up?

[StompinTom]

Best to use BiDir + MLT in that kind of situation. Good luck.

[cotty]

And make shure that the sphere has real scale and is not too big...
(and you can increase the subdivision of the mesh to get a more smooth surface)

[ToyBuilder]

Ok, I tried path tracing with GPU and bidirectional+MLT (only supported on CPU), and it seems that the GPU variant converges faster.

Thanks for the replies.