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# Materials: Commands {#mat_commands}
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## alphagen
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### Syntax
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**alphaGen <func>**
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### Overview
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The alpha channel can be specified like the [rgb channels](rgbgen.md). If not specified, it
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defaults to 1.0.
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### Functions {#functions}
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#### portal {#portal}
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This rendering stage keyword is used in conjunction with the
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[surfaceparm](surfaceparm.md) keyword
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portal. The function accomplishes the "fade" that causes the scene in
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the portal to fade from view. Specifically, it means "Generate alpha
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values based on the distance from the viewer to the portal."
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Use `alphaGen portal` on the last rendering pass.
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