Materials: Commands

blendfunc

Syntax

blendFunc

Overview

Blend functions are the keyword commands that tell the renderer how graphic layers are to be mixed together.

Usage

Simplified blend functions

The most common blend functions are set up here as simple commands, and should be used unless you really know what you are doing.

add

This is a shorthand command for blendFunc GL_ONE GL_ONE. Effects like fire and energy are additive.

filter

This is a shorthand command that can be substituted for either blendFunc GL_DST_COLOR GL_ZERO or blendFunc GL_ZERO GL_SRC_COLOR. A filter will always result in darker pixels than what is behind it, but it can also remove color selectively. Lightmaps are filters.

blend

Shorthand for blendFunc GL_SRC_ALPHA GL_ONE_MINUS_SRC_ALPHA. This is conventional transparency, where part of the background is mixed with part of the texture.

Explicit blend functions

Getting a handle on this concept is absolutely key to understanding all shader manipulation of graphics.

BlendFunc or "Blend Function" is the equation at the core of processing shader graphics. The formula reads as follows:

[Source *] + [Destination * ]

Source is usually the RGB color data in a texture file.

Destination is the color data currently existing in the frame buffer.

Rather than think of the entire texture as a whole, it maybe easier to think of the number values that correspond to a single pixel, because that is essentially what the computer is processing: One pixel of the bitmap at a time.

The process for calculating the final look of a texture in place in the game world begins with the pre-calculated lightmap for the area where the texture will be located. This data is in the frame buffer. That is to say, it is the initial data in the Destination. In an unmanipulated texture (i.e. one without a special shader script), color information from the texture is combined with the lightmap. In a shader-modified texture, the $lightmap stage must be present for the lightmap to be included in the calculation of the final texture appearance.

Each pass or "stage" of blending is combined (in a cumulative manner) with the color data passed onto it by the previous stage. How that data combines together depends on the values chosen for the Source Blends and Destination Blends at each stage. Remember it's numbers that are being mathematically combined together that are ultimately interpreted as colors.

A general rule is that any Source Blend other than GL_ONE (or GL_SRC_ALPHA where the alpha channel is entirely white) will cause the Source to become darker.

Source Blend

The following values are valid for the Source Blend part of the equation.

GL_ONE This is the value 1. When multiplied by the Source, the value stays the same the value of the color information does not change.
GL_ZERO This is the value 0. When multiplied by the Source, all RGB data in the Source becomes Zero (essentially black).
GL_DST_COLOR This is the value of color data currently in the Destination (frame buffer). The value of that information depends on the information supplied by previous stages.
GL_ONE_MINUS_DST_COLOR This is nearly the same as GL_DST_COLOR except that the value for each component color is inverted by subtracting it from one. (,i.e. R = 1.0 - DST.R, G = 1.0 - DST.G, B = 1.0 - DST.B, etc.)
GL_SRC_ALPHA The TGA file being used for the Source data must have an alpha channel in addition to its RGB channels (for a total of four channels). The alpha channel is an 8-bit black and white only channel. An entirely white alpha channel will not darken the Source.
GL_ONE_MINUS_SRC_ALPHA This is the same as GL_SRC_ALPHA except that the value in the alpha channel is inverted by subtracting it from one.(i.e. A=1.0 - SRC.A)

Destination Blend

The following values are valid for the Destination Blend part of the equation.

GL_ONE This is the value 1. When multiplied by the Destination, the value stays the same the value of the color information does not change.
GL_ZERO This is the value 0. When multiplied by the Destination, all RGB data in the Destination becomes Zero (essentially black).
GL_SRC_COLOR This is the value of color data currently in the Source (which is the texture being manipulated here).
GL_ONE_MINUS_SRC_COLOR This is the value of color data currently in Source, but subtracted from one(i.e. inverted).
GL_SRC_ALPHA The TGA file being used for the Source data must have an alpha channel in addition to its RGB channels (four a total of four channels). The alpha channel is an 8-bit black and white only channel. An entirely white alpha channel will not darken the Source.
GL_ONE_MINUS_SRC_ALPHA This is the same as GL_SRC_ALPHA except that the value in the alpha channel is inverted by subtracting it from one. (i.e. A=1.0 - SRC.A).

Doing the Math: The Final Result

The product of the Source side of the equation is added to the product of the Destination side of the equation. The sum is then placed into the frame buffer to become the Destination information for the next stage. Ultimately, the equation creates a modified color value that is used by other functions to define what happens in the texture when it is displayed in the game world.

Default Blend Function

If no blendFunc is specified then no blending will take place. That's just a fact of life.

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