Half transparent mirror

[Shredd0r]

Hi all,

has someone an idea how to achieve this?

What I want is the possibility to model a surface that reflects half of the rays and lets the other half pass through it by transparency.

Which material would you suggest to use and which further setting would be necessary?

thanks

[Zxx]

but what's the problem?
mia_material with transparency set to 1 and reflectivity to 0.5. BRDF should be 1/1 (0/90 angle reflectivity)

[Shredd0r]

Thanks

That's what I wanted to know. With this material I get this result:

shader "mia_x_inst"  "mia_material_x" (
    "transparency" 1,	
    "refr_ior" 1.0,		
    "reflectivity" 0.6,
	
    "brdf_fresnel" 0,
    "brdf_curve" 1,
    "brdf_0_degree_refl" 1.0,
    "brdf_90_degree_refl" 1.0,
    "brdf_conserve_energy" 1,
	
    "do_refractive_caustics" 1,
    "lights" ["lights-instance"]
)

000105_half.png



Ok now the next problem ist, that I want to have this effect with refraction.
So I modified the material this way:

shader "mia_x_inst"  "mia_material_x" (
	"transparency" 1,	
    "refr_ior" 1.5,		
	"reflectivity" 0,
	
	"brdf_fresnel" 0,
	"brdf_curve" 1,
	"brdf_0_degree_refl" 1.0,
	"brdf_90_degree_refl" 1.0,
	"brdf_conserve_energy" 1,
	
    "do_refractive_caustics" 1,
	"lights" ["lights-instance"]
)

000105_ior.png

Now you see, thou I set the reflectivity to 0 the yellow cube is reflected with 100%.

[bart]

Please leave the reflectivity at where you had it, ie 0.6. Reflectivity is considered first, then transparency is taken from diffuse.

So set your transparency to 1.0

That will give you 60% reflection and 40% refraction.

[Shredd0r]

Unfortunately not

"transparency" 1,	
"refr_ior" 1.33,		
"reflectivity" 0.6,
	
"brdf_fresnel" 0,
"brdf_curve" 1,
"brdf_0_degree_refl" 1.0,
"brdf_90_degree_refl" 1.0,
"brdf_conserve_energy" 1,

gives:
000105_0.6.png

How can that be? Maybe my mental ray version is somehow broken in that case? It's 3.9.4.13
It's terrible

[maxt]

you have both "brdf_0_degree_refl" and "brdf_90_degree_refl" set to 1.0

[Zxx]

Shredd0r, on last image I can see both reflection (yellow box) and refraction (red boxes). 60% reflectivity and 40% transparency. set "reflectivity" to 0.5 if you want 50/50 ratio.
what's wrong?

[Zxx]

you have both "brdf_0_degree_refl" and "brdf_90_degree_refl" set to 1.0

it's correct for perfect mirror. this brdf multiplied by reflectivity value.

[Shredd0r]

The ratio and how the brdf curve goes doesn't matter at the moment.

What I want to have is the reflections in the image with the refractions more transparent.
But the reflections created by the IOR are never transparent at my rendering.

On the following images I chose a very low IOR:

"refr_ior" 1.05,		
"reflectivity" 0.0,

000105_000.png
As you can see at the yellow cube there is a part of the reflection which is transparent as it is desired. I added a red arc to this area.
When I change the reflectivity this area also changes its transparency:

"refr_ior" 1.05,		
"reflectivity" 0.7,

000105_07.png


But the area depicted by the yellow arc never changes. And when I increase the IOR this area grows.

"refr_ior" 1.03,
"reflectivity" 0.69,

000105_003.png

"refr_ior" 1.07,
"reflectivity" 0.69,

000105_007.png

Why is there this intransparent area that grows with IOR?
What I need in the end is an IOR of 1.3, at this value the transparent area is not there anymore.

[Zxx]

Why is there this intransparent area that grows with IOR?

well, the path of the refraction rays changes. it just returns other color.

are you aware about reflection/refraction trace depths and cutoff threshold? this can greatly decrease the amount of reflection/refraction rays resulting in loss of energy (i presume).
check this options:
refl_depth
refl_cutoff
skip_inside_refl
refr_depth
refr_cutoff

and trace depths in rendering options. it require a big depth for highly transparent and reflective materials.