142 lines
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5.9 KiB
Markdown
142 lines
No EOL
5.9 KiB
Markdown
# Materials: Commands {#mat_commands}
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## blendfunc
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### Syntax
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**blendFunc <simplefunc>**
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**blendFunc <srcBlend> <destBlend>**
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### Overview
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Blend functions are the keyword commands that tell the renderer how
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graphic layers are to be mixed together.
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### Usage
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#### Simplified blend functions {#simplified_blend_functions}
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The most common blend functions are set up here as simple commands, and
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should be used unless you really know what you are doing.
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##### add {#add}
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This is a shorthand command for `blendFunc GL_ONE GL_ONE`. Effects like
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fire and energy are additive.
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##### filter {#filter}
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This is a shorthand command that can be substituted for either
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`blendFunc GL_DST_COLOR GL_ZERO` or `blendFunc GL_ZERO GL_SRC_COLOR`. A
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filter will always result in darker pixels than what is behind it, but
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it can also remove color selectively. Lightmaps are filters.
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##### blend {#blend}
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Shorthand for `blendFunc GL_SRC_ALPHA GL_ONE_MINUS_SRC_ALPHA`. This is
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conventional transparency, where part of the background is mixed with
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part of the texture.
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#### Explicit blend functions {#explicit_blend_functions}
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Getting a handle on this concept is absolutely key to understanding all
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shader manipulation of graphics.
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BlendFunc or "Blend Function" is the equation at the core of processing
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shader graphics. The formula reads as follows:
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` [Source *`<srcBlend>`] + [Destination * `<dstBlend>`]`
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**Source** is usually the RGB color data in a texture file.
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**Destination** is the color data currently existing in the frame
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buffer.
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Rather than think of the entire texture as a whole, it maybe easier to
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think of the number values that correspond to a single pixel, because
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that is essentially what the computer is processing: One pixel of the
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bitmap at a time.
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The process for calculating the final look of a texture in place in the
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game world begins with the pre-calculated lightmap for the area where
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the texture will be located. This data is in the frame buffer. That is
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to say, it is the initial data in the Destination. In an unmanipulated
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texture (i.e. one without a special shader script), color information
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from the texture is combined with the lightmap. In a shader-modified
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texture, the $lightmap stage must be present for the lightmap to be
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included in the calculation of the final texture appearance.
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Each pass or "stage" of blending is combined (in a cumulative manner)
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with the color data passed onto it by the previous stage. How that data
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combines together depends on the values chosen for the Source Blends and
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Destination Blends at each stage. Remember it's numbers that are being
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mathematically combined together that are ultimately interpreted as
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colors.
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A general rule is that any Source Blend other than **GL_ONE** (or
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**GL_SRC_ALPHA** where the alpha channel is entirely white) will cause
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the Source to become darker.
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##### Source Blend <srcBlend> {#source_blend}
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The following values are valid for the Source Blend part of the
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equation.
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- **GL_ONE** This is the value 1. When multiplied by the Source, the
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value stays the same the value of the color information does not
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change.
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- **GL_ZERO** This is the value 0. When multiplied by the Source, all
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RGB data in the Source becomes Zero (essentially black).
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- **GL_DST_COLOR** This is the value of color data currently in the
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Destination (frame buffer). The value of that information depends on
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the information supplied by previous stages.
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- **GL_ONE_MINUS_DST_COLOR** This is nearly the same as
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**GL_DST_COLOR** except that the value for each component color is
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inverted by subtracting it from one. (,i.e. R = 1.0 - DST.R, G =
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1.0 - DST.G, B = 1.0 - DST.B, etc.)
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- **GL_SRC_ALPHA** The TGA file being used for the Source data must
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have an alpha channel in addition to its RGB channels (for a total
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of four channels). The alpha channel is an 8-bit black and white
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only channel. An entirely white alpha channel will not darken the
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Source.
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- **GL_ONE_MINUS_SRC_ALPHA** This is the same as **GL_SRC_ALPHA**
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except that the value in the alpha channel is inverted by
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subtracting it from one.(i.e. A=1.0 - SRC.A)
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##### Destination Blend <dstBlend> {#destination_blend}
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The following values are valid for the Destination Blend part of the
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equation.
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- **GL_ONE** This is the value 1. When multiplied by the Destination,
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the value stays the same the value of the color information does not
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change.
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- **GL_ZERO** This is the value 0. When multiplied by the Destination,
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all RGB data in the Destination becomes Zero (essentially black).
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- **GL_SRC_COLOR** This is the value of color data currently in the
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Source (which is the texture being manipulated here).
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- **GL_ONE_MINUS_SRC_COLOR** This is the value of color data currently
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in Source, but subtracted from one(i.e. inverted).
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- **GL_SRC_ALPHA The TGA** file being used for the Source data must
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have an alpha channel in addition to its RGB channels (four a total
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of four channels). The alpha channel is an 8-bit black and white
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only channel. An entirely white alpha channel will not darken the
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Source.
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- **GL_ONE_MINUS_SRC_ALPHA** This is the same as **GL_SRC_ALPHA**
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except that the value in the alpha channel is inverted by
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subtracting it from one. (i.e. A=1.0 - SRC.A).
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##### Doing the Math: The Final Result {#doing_the_math_the_final_result}
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The product of the Source side of the equation is added to the product
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of the Destination side of the equation. The sum is then placed into the
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frame buffer to become the Destination information for the next stage.
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Ultimately, the equation creates a modified color value that is used by
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other functions to define what happens in the texture when it is
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displayed in the game world.
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#### Default Blend Function {#default_blend_function}
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If no blendFunc is specified then no blending will take place. That's
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just a fact of life. |