Hi elele - thanks for the glasses.

An opacity map could be used, but I built the gradient controls right into my glass shader so it isn't strictly needed.

The built-in gradient is adjustable as well. The nodes in the second column control the gradient. This kind of control would be harder to do with a map.

Gradient Min is the V coordinate where the gradient is black.

Gradient Max is the V coordinate where the gradient is white.

For these glasses, I set Min=.9 and Max = .7.

If you scroll down in the shader, you can see the actual gradient.

Then to apply the gradient, there are two other nodes of interest.

Transmission Gradient: Applies the gradient to the amount of light transmitted. It modulates the Transmission value. If this is set to 0, there is no gradient used on the transmission. If you set it to any value +0 to 1, then the gradient effect becomes progressively stronger on the transmission. In a couple of the shaders I set this to 1. This makes the top darker than the bottom.

You can also set this to a negative number form -0 to -1. In that case, the inverse of the gradient is used. This would make the bottom darker than the top.

The maximum transmission is still controlled by the Transmission node. So if you set it to .6, with a Transmission Gradient set to .5 then the effective transmission would be .3 to .6. If Transmission Gradient is set to 1, then the effective transmission would be 0 to .6.

Diffusity Gradient: Works the same but modulates the Diffusity value. Diffusity simulates some opaque material just under the surface of the glass - like tiny opaque particles suspended in the glass. It blocks some of the transmission and produces a diffuse reflection instead of refraction or transparency. If Diffusity is 1, then the material is completely opaque and transmits no light.

When Diffusity = 1, the material becomes more like an opaque glass. If you increase the Metallic value, it becomes more like a shiny metal.

When Metallic = 1, then the shader is 100% a shiny metal surface, with no ability to see any part of the interior.

If you change the IOR to 1.33, this material becomes water.

If you change the IOR to other numbers, it can variously become car paint, plastic, etc.

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