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Translucent material reflections? (UE4.10)

Translucent materials don't receive reflections, e.g.

(1) Open the LightRoom marketplace demo.

(2) Add a 'Basic sphere' about a metre away from the glass vase in the back of the room (in the darkest part of the room).

(3) Add the glass material used on the vase to the sphere.

(4) Search the content browser for 'Blueprint_Effect_Fire' and drag it in between the vase and the sphere.

The sphere object and vase do not receive reflections.

However, if you make the sphere glass material 'Opaque' then you get the reflection.

Is this this the old 'need forward rendering' limitation, a bug or something else?alt text

Product Version: UE 4.10
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asked Nov 26 '15 at 06:52 PM in Rendering

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2 answers: sort voted first

Ok, I found it. What you need to do is enable Screen Space Reflections on the Translucency tab in your materials properties. I don't believe this is on by default within the material.

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answered Dec 04 '15 at 02:32 PM

avatar image ash22 Dec 04 '15 at 03:53 PM

Excellent!. Thanks Logan!

Btw, is there a reason why the reflection only appears on the very edge of a sphere?

You can't navigate to a better view of the reflection because when the fire is off screen it is no longer in screen space and if the fire is in between the camera and the sphere it will block the reflection.

It looks ok on planar surfaces, so perhaps perfectly spherical objects are an extreme case and the reflection can only appear on the outer edge?

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ssr.jpg (544.2 kB)
avatar image Logan Bailey ♦♦ STAFF Dec 04 '15 at 05:14 PM

What you are seeing is the refraction of light on the sphere. I did some tweaking with the material, made it two sided, eliminated a few calculations I didn't need and I was able to achieve a larger area where the light is drawn. However, this is not an issue with Material or SSR but is expected behavior of how light works with translucent, reflective objects.

alt text

Have a look at this video. It's a little crude but the point I'm trying to make is the look of how the light is pushed outward towards the edges of the sphere. This holds true if you rotate around your sphere and you see the sun being reflected on the sphere. The sun is actually a light source. A particle does not emit it's own light. This is why you don't see a glowing yellow orb in the center of the particle. So, what you see is actually what you would expect when you have light reflected off of your glass sphere.



refraction.jpg (96.8 kB)
avatar image ash22 Dec 04 '15 at 05:45 PM

Aaahh, I see what you mean.

Nonetheless, I do like your 'tweaked' version much more than the original method because (albeit not correct) it does show the effect a little clearer.

The original calc may be more 'accurate' but it shows so little SSR on the edge of a sphere that it's hardly worth enabling for that 8% of an effect whereas your method is five times more visible! I'll try your method and hack around the material a little.

In case you discard your test project later, perhaps you could post a screenshot of your tweaked material in case anyone else wishes to reproduce the exaggerated SSR effect. I know I'll be using it a lot.

Thanks for all your help!

avatar image Logan Bailey ♦♦ STAFF Dec 04 '15 at 05:51 PM

Absolutely, I'll post a screenshot here.

alt text


glassreflection.jpg (197.0 kB)
avatar image ash22 Dec 04 '15 at 06:35 PM

Perfect! Thanks Logan!

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Hello Ash22,

It is possible to get reflections. They are not going to be as vivid because of how translucent materials are designed. Translucent materials are designed to allow for a certain amount of light to pass through the surface as opposed to bouncing light off like opaque materials.

Here is an image to better reflect, no pun intended, what I mean when I say light passes through these objects.

alt text

A more scientific diagram showing how light reflects off of more opaque surfaces.

alt text

This is a test I made in UE4 4.10 showing that translucent objects do reflect the world around them just not to the degree that solid/metallic/smooth objects do.

alt text

This is why you see less reflections in translucent objects. Certain things can affect this such as roughness, specularity, metallic properties, and refraction. Try looking at this documentation that could help with further questions you may have about translucent materials.



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answered Nov 30 '15 at 09:11 PM

avatar image ash22 Dec 01 '15 at 10:02 AM

Hi Logan, I see what you're saying, translucent object do reflect the world around them. However, my issue is not with world reflections but with 'particle system reflections'. Translucent objects don't appear to reflect particles systems (see my screenshot).

avatar image Logan Bailey ♦♦ STAFF Dec 01 '15 at 06:32 PM

I just tested this with my same scene. It looks like the fire is reflecting but it is dependent on the angle at which you are looking at the glass.

alt text

avatar image ash22 Dec 04 '15 at 11:53 AM

Just as a double-check, I recreated your scene using the glass assets from the starter content (M_Glass.uasset M_StatueGlass.uasset:


There are still no particle reflections visible, so I'm assuming your glass material is somehow different?

Let me know how my test_project/glass material is different to yours, or alternatively, post a link to your glass material so I can test/analyse it in my project.


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