Sky reflection and artifacts on transparent object


ib4: I also postet this on the blender forum, since I did not know this forum 10 minutes ago :slight_smile:

I followed a Tutorial of the german video-training of Helge Maus. In the tutorial he showed how to set suzanne transparent.

The Material settings on suzanne are:
Diffuse Intensity: 0
Transparency: checked & Raytrace
Alpha: 0.1
Fresnel: 2.4
IOR: 1.8
Depth: 4
the rest is set to the default values

In the Scene is a tin, suzanne, a floor and walls. All are set up to recieve transparent shadows. And the one lamp in the scene is set to ray Shadow.

There is a blue sky in the scene too.

Now, when i render that scene on my PC (win 7, ATI HD 5850 (drivers are up to date)) i have a strange reflection of the sky at suzanne. I know that it is the sky because when i change the color of the sky, the color of that “reflection” changes too.

There is no Mirror set up @ suzanne.

When Helge Maus renders that scene in the video everything looks great and there are no black or blue artifacts.

Iam using Blender 2.57b

Thanks for any help,

EDIT: I have the same broblems with this .blend-File: . When I render it, there are black artifacts at the upper border of the glasses

EDIT2: I used the Blender Renderer

It is the IOR value. The sky isn’t mirrored, the rays are bent.
You’re obviously german, the IOR is the “Brechkraft” of the material. The higher it is, the more the ray is bent on the surface.

Imagine a ray coming from the camera. If the IOR is “1” the ray wanders straight trough the object.
If the IOR has a higher value, the ray is bent when it hits the material and leaves in another angle. In your case the ray is bent multiple times in the object and leaves it in the direction of the sky and that’s why you see it.

For the black stuff inside Suzanne, are you sure you set the glass material too to receive transparent shadows? It has to cast a transparent shadow on itself.

On suzanne herself … Iam not 100% sure, but I will check it when Iam back home at the ATI-PC!

Thanks for the info about the IOR! I think Helge Maus did not had the Problem, because he was using the 2.5 alpha 2 … maybe it was not implemented at this moment!

If suzanne herself is not set to receive a transparent shadow, the side of the transperant object receiving light, will cast a shadow on the backface the other side of the object. If the material is not set to receive transparent shadows, the shadow will be cast solid.

Helge Maus might have had the very same settings, but it is a lot about the geometry and scene setup.
I don’t know your level of school education and if you have already lerned (or ever learned) “Wellenbrechung an Oberflächen”.
It really depends on how many surfaces the ray passes, how long the ray stays inside the object due to refraction and how the object is placed in the scene. The rays might all hit another object or might for the most part hit the sky, just depending on the scene setup, object placement or camera position.

For my attached rendered image i just opened the .bend-file Helge Maus provided on his DVD and pressed F12 for render! So there should be no difference between my scene and his! But it looks much different, as i explained.

When I am at home today i will attach a screenshot of his rendered scene. And I have another example, where I have modeled a plastic water bottle, and I get strange black artifacts as well.

btw. I am study Business Informatics, and I understand things like “Wellenbrechung an Oberflächen” or “Brechkraft” at least I have a idea of it :). But the effects my rendered image show do not look realistic to me. I will check the recieve transparent shadow-setting, but as i said the same settings work at the DVD-Training.

Thanks for your help

EDIT: the black stuff in my rendered Image may be caused by a too low rate of Raytrace Depth …