Digital Artform

This is what many people do. This resembles a bunch of "normal" layers in Photoshop with the "opacity" set to a midrange percentage. People may call this transparency, but that's a lazy use of the word.

The main thing you get by doing this is a muddy color that doesn't match your swatch - even over a white background - and nothing more interesting than a progressive drop in luminosity of the same hue with each passing layer.

Here's an alternative approach. Notice how the color in the top layer matches the color swatch in the shader, and how successive "multiplies" of the pale, desaturated color cause it to darken and saturate, just like multiple layers of colored cellophane do.

This techique works best over bright, less saturated backgrounds. It's hard to see colored cellophane, digital or real, over a black background. A little bit of color in the diffuse channel will help visibility, and is often warranted. Too much color in the diffuse channel coupled with a lot of color in the transparency channel will produce an odd "negative" effect. If you are going for realism, you probably shouldn't have pure (0 0 0) black in your image anyway.

Here's a somewhat related entry on

Transparency Mapping and Matte Lines

UPDATE 4/16/2006

Thanks to all the interest in this entry generated by mentions on web sites such as Death Fall, I am expanding this entry to include an example of colored rgb transparency in a more typical setting.

Let's look again at a simple blinn shader with color set to a low value and transparency set to a color.

Here's a single-sided sphere under a white point light. Notice that the sphere (as seen against the bright checkerboard) appears to be the same orangey-red as the transparency swatch. Notice also that the specular highlight is white.

Here's the same sphere and the same shader, only this time the normals are reversed so as to make the sphere a hollow bowl facing the camera.

Finally, here is the shader on a double-sided sphere. Notice what happens:

1) We are seeing through two layers of red filtration, so the red is richer. How much richer? It has the same color filtration value as you'd get by multiplying the color transparency swatch by itself.

2) We are seeing the inner specular highlight trough one layer of red filtration, so it has become an obvious red color.

To see this phenomenon in real life, take a look at these amazing new colored bubbles!