Transparent materials in AI photos: glass, water and smoke

Transparent materials are hard for AI image models because light doesn't bounce off them but bends through them, so glass, water and smoke often show up as a vague, solid blob instead of a genuinely see-through material. The model works with flat pixels, not physics, so without direction it guesses at how light should refract through your subject. Here is why that goes wrong and how to render glass, water and smoke convincingly anyway.

Why transparent materials are so hard for AI

Transparent material bends the light passing through it (refraction) instead of reflecting it back, and image models struggle to learn that behaviour from flat training images. The model recognises pixels, not optics, so it quickly reads a bent background or a bright patch as a solid object. Because models lean on hard edges and contrast, a semi-transparent glass also tends to blend into the background.

How much light bends depends on a material's refractive index. According to physics, air has a refractive index of about 1.0, water 1.33 and ordinary window glass around 1.5 (source: Wikipedia, "Refractive index"). The higher that index, the more the light changes direction. Glass therefore bends light more than water, and that is exactly the effect a model has to imitate believably.

The difficulty isn't imagined. Recent research (the paper "Refracting Reality", accepted to CVPR 2026) shows that generative models still don't follow the laws of optics well for transparent objects, and proposes a method to make the light bend properly according to Snell's law.

Refraction and reflection: what's the difference?

Refraction is the bending of light as it passes through a transparent material; it is why you see the background behind a glass distorted, shifted or upside down. Reflection is light bouncing off a surface; it produces the bright highlights on a glass and the mirror image in a window.

Glass and water do both at once: you look partly through them (refraction) and partly see something mirrored in them (reflection). So for a believable image your model has to get two things right. Pure reflections are a topic of their own; here it is mainly about what you see through the material.

How to describe glass and water in your prompt

The key trick: give the model something to bend. A glass in empty space becomes a grey shape; a glass in front of a recognisable background becomes real glass.

• Name the material and the behaviour explicitly: "clear glass, background visible and slightly distorted by refraction".
• Describe what sits behind it, so there is something to refract: "a lit window behind the wine glass, distorted through the stem".
• Ask for highlights and caustics. Caustics are the bright, curved light patterns a glass or water surface casts on a table or floor: "bright rim highlights on the glass, caustic light patterns on the table below".
• For water: "water surface with refraction, objects below appear shifted and magnified, caustic light on the floor".
• Don't overload the background. A calm, high-contrast backdrop makes the distortion read as intentional rather than as a mistake.

Smoke, mist and steam: a different kind of transparency

Smoke, steam and mist aren't transparent but translucent: light doesn't pass cleanly through them, it scatters inside. That is why you steer them differently from glass. The key is backlight, because a cloud of smoke or steam only becomes visible when light shines through or behind it.

• Put a light source behind it: "thin wisps of smoke, backlit, with soft edges".
• Describe density and direction: "light steam rising from the coffee cup, against a dark background".
• Keep it subtle. "Thick smoke everywhere" muddies your image; thin, directed wisps read far more believably.

Check the background behind your glass

After the render, always look at what you see through the material. That is where it most often fails: a distorted, duplicated or illogical background behind a glass or bottle gives the image away instantly.

If only that patch is wrong, don't regenerate the whole photo. Repair the background or the edge specifically with inpainting in the photo editor, or make a new version with image-to-image in the photo generator. If you can't find the right wording, the prompt generator helps you capture material, light and background in a single prompt.

Frequently asked questions

Why does my glass look like a grey, solid blob?

Usually because there is nothing behind it to refract and you didn't name the refraction. Give the glass a recognisable background and explicitly ask for a visible, slightly distorted background plus bright rim highlights.

How do I get believable steam or smoke?

Backlight it and keep it thin. Describe direction and density ("light steam rising, soft edges, against a dark background") instead of a thick cloud across the whole image.

What is the difference between refraction and reflection in my image?

Refraction is light bending through the material, so you see the background distorted. Reflection is light bouncing back, which gives you the highlights and mirror images. Glass and water show both at once.

Transparent materials often take a few attempts, but with a background to bend and the right words you get there fast. Create an account and test how a glass or water surface changes the moment you name the refraction.