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I'm facing an issue with my plastic bottle render. There are some dark shapes at the base that I can't seem to eliminate, except by adding a solidify modifier to the liquid. However, I want to avoid this as it wouldn't be realistic. Do you have any advice on how I can get rid of these shapes or improve the liquid's lighting?

enter image description here

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  • $\begingroup$ How did you make it? Share a .blend file, or screenshots where the model and shaders are visible. blend-exchange.com $\endgroup$ Jan 19 at 8:23
  • $\begingroup$ hi, i just did that. thank you! $\endgroup$
    – Henry
    Jan 19 at 8:38
  • $\begingroup$ The bottle is almost 2 meters in height. You should model to scale. Then you can make the thickness of plastic correct. Also there should be no gap between liquid and plastic. That must be a single surface at the interface between PET plastic and liquid. You have to make sure IOR values are correct in different light interfaces. It's not enough to get IOR value for material, since that is usually for air and that material. You also have to keep normal direction in mind when calculating IOR values. You need air-PET, liquid(water mostly probably)-PET and then air-liquid interfaces. $\endgroup$ Jan 19 at 9:09
  • $\begingroup$ oops @MartynasŽiemys thank you, lok let me try this and get back to you $\endgroup$
    – Henry
    Jan 19 at 9:27
  • $\begingroup$ @Henry I'm working on an answer $\endgroup$ Jan 19 at 9:30

2 Answers 2

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Dark areas are just ok, because light refracted in the liquid and bottle's body stuck with dark cap, label's back and background which strength is 0 even though it's transparent, same as the reference: enter image description here photo by Steve Johnson

Increasing Transmission with Glossy and Total Light Paths Max Bounces could reduce the effect slightly. enter image description here

Or you need to fake some rays with bright background in World node setup. enter image description here

Also, don't use too many subdivisions for surfaces, 'cause smooth shading is still good enough.

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  • $\begingroup$ thank you so much ill look it this aswell! $\endgroup$
    – Henry
    Jan 19 at 11:12
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For a great part @MartynasŽiemys is correct, it is always better to use real-life values if you want to create realistic images etc., although this is not so important for your issue here.

However, of course even with real-life values, realistic bottles and liquids have things like reflections and there are distortions caused by refractions, and I don't think you want to get rid of that for a realistic render.

So inspecting your file I can only tell you the following: the most part of these dark areas are caused by the liquid or to be more precise: from reflections and refractions inside the liquid.

Hiding the bottle for a moment, you can see the dark areas are still there. When I now change the liquid's color to completely white to have no distraction from the tint of the liquid itself, you can see the dark parts are black and green:

dark areas

Obviously the green part is the label or "Sphere.003". If you hide it, only black parts remain:

hiding the label

Looking at your scene I see that apart from the large bright area lights, there is not much environment and the world around is set to a Strength of 0 = black. Setting the strength to 1 and the color to blue instead of the default grey, now the black areas are blue (apart from a very small black part which comes from the "cap"):

showing the background

Now I hide the liquid and bring the bottle and the label back: There is much blue visible in the bottle. So all these reflections were dark before from the black background as well.

background reflections in the bottle

So apart from using real-life values, if you want realistic reflections and refractions, it cannot be avoided to get darak areas from time to time. But what makes glass renders often dark and unrealistic are boring mono-colored or blackground. In reality, glass gets flooded by light and reflections from all around. So try to get some environment, either by modeling it or using HDRIs for the background.

The other thing is, if you are using real values and there are still dark areas that bother you and you are willing to give up some reality, you can try changing IOR values for less refraction, or exclude materials from Ray Visibility to either Glossy or Transmission rays. Like in this example below, sky is still blue and the liquid is back again. On the left as it looks "realistically", on the right I've disabled Ray Visibility > Transmission for the label object "Sphere.003". Now it isn't that dark anymore, but some realism is gone because now the label is no longer visible through the bottle and liquid.

comparison ray visibility

Instead of changing the transmission visibility completely, you could also make a material for the label where you mix the actual material with a Transparent BSDF and for the mix factor you take the Transmission Depth output of a Light Path node to check with a Math > Greater Than node, if the depth is above a certain value. But this depends on the scene, how many transmissive objects therer are and can vary: in this case, the Threshold of 6 worked for me. With that value it was not as dark as before but also not completely invisible.

transmission depth

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  • $\begingroup$ woah thanks so much, this was really insightful and helpful. i learnt a lot. let me see if any options can give me what i'm, looking for $\endgroup$
    – Henry
    Jan 19 at 11:11

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