Last Updated March 6, 2012

Transparency

Screen door Transparency

A simple and quick form of transparency is "Screen door" transparency. There the face is drawn through a "Mask". The mask is defined as a square pattern of bits. The mask is placed over polygon to be drawn, and the refresh buffer is only updated if the mask over a pixel contains a 1. The proportion of 1's in the mask defines the opacity of the mask.

Front cube rendered with a screen door mask (50% opacity)

Close up of previous image

Front cube rendered with a screen door mask (50% & 25% opacity)

Both transparent objects rendered with the same mask)

Both transparent objects rendered with complementary masks)

Screendoor Transparency in OpenGL

Alpha Channel

In addition to R,G & B channels, most graphics systems support a 4th colour channel, called alpha. The alpha value is normally interpreted as at measure of the pixel's opacity (a value of one means it is totally opaque, zero is totally transparent).

Alpha Mask

The simplest use of the alpha channel is the alpha mask. An alpha mask technique draws a pixel only if it's alpha value is greater than zero. This is very useful for billboarding effects where a image is transparent in parts. With alpha masks, either a pixel is drawn or not drawn, there is no blending.

Alpha function used to overlay an image with transparent parts.

Alpha Mask in OpenGL

Alpha Blending

Alpha blending is used when it is necessary to render a translucent (partially transparent) object. In this case a source pixel is mixed with a destination pixel. How the pixels are mixed depends on the Blending function.

Pixel blending.

The blending function is usually in the form;

\[ pixel= src_{factor}.src\_pixel+dest_{factor}.dest\_pixel\]

For transparency, the best effect is achieved with the following blending function; \[ pixel= src_{\alpha}.src\_pixel+(1-src_{\alpha})dest\_pixel$\] where $src_{\alpha}$ is the alpha value of the source pixel.

When alpha bending is in use, pixels in the frame buffer are written using this equation, therefore this is much slower than drawing opaque objects.

Important notes about drawing translucent objects ;

If the depth buffer is enabled, and an object is positioned behind a translucent object, its pixels will not get drawn as they will be culled by depth buffer HSR.
Translucent objects should be drawn after all opaque objects are drawn.
Disable writing to the depth buffer before drawing translucent objects.
For best results, sort the translucent objects and draw them in back to front order.

Alpha Blending in OpenGL

Alpha Blending in XNA