Source SDK 2013: Your Second Shader

Introduction

This page will teach you the basics of passing parameters to your shaders.

Prerequisites

This tutorial assumes the following:

1. You've followed all of the instructions on the Source SDK 2013: Your First Shader page.
   • And you've been able to successfully build the shader source code on that page.

Overview

What are shader parameters?

Shader parameters are bits of data that you can feed into a shader from a Valve Material File. It turns out that VMT files are actually KeyValues which you can add additional data to. The following snippet is from materials/dev/lumcompare.vmt:

"screenspace_general"
{
    "$PIXSHADER"            "luminance_compare_ps20"
    "$BASETEXTURE"          "_rt_FullFrameFB"
    "$ALPHATESTED"          "1"
    "$DISABLE_COLOR_WRITES" "1"
}

Recall back to the first tutorial. The first line in a VMT file is the name of the shader to draw that material with. The subsequent lines that begin with $ however, are known as material parameters. In this example, there are four parameters, each of which are associated with a value.

You might be asking yourself how we know what the parameters of a given shader are. Unfortunately, there isn't a simple in-game command (as far as the author is aware of) that lists them for you. You'll need to take a look at the source code of the shader. Luckily, we have the source code for screenspace_general. The parameters it can handle are listed in its BEGIN_SHADER_PARAMS block:

Note:For Source SDK 2013, the screenspace_general code lives under src/materialsystem/stdshaders/screenspace_general.cpp.

SHADER_PARAM( C0_X,SHADER_PARAM_TYPE_FLOAT,"0","")
SHADER_PARAM( C0_Y,SHADER_PARAM_TYPE_FLOAT,"0","")
SHADER_PARAM( C0_Z,SHADER_PARAM_TYPE_FLOAT,"0","")
SHADER_PARAM( C0_W,SHADER_PARAM_TYPE_FLOAT,"0","")
SHADER_PARAM( C1_X,SHADER_PARAM_TYPE_FLOAT,"0","")
SHADER_PARAM( C1_Y,SHADER_PARAM_TYPE_FLOAT,"0","")
SHADER_PARAM( C1_Z,SHADER_PARAM_TYPE_FLOAT,"0","")
SHADER_PARAM( C1_W,SHADER_PARAM_TYPE_FLOAT,"0","")
SHADER_PARAM( C2_X,SHADER_PARAM_TYPE_FLOAT,"0","")
SHADER_PARAM( C2_Y,SHADER_PARAM_TYPE_FLOAT,"0","")
SHADER_PARAM( C2_Z,SHADER_PARAM_TYPE_FLOAT,"0","")
SHADER_PARAM( C2_W,SHADER_PARAM_TYPE_FLOAT,"0","")
SHADER_PARAM( C3_X,SHADER_PARAM_TYPE_FLOAT,"0","")
SHADER_PARAM( C3_Y,SHADER_PARAM_TYPE_FLOAT,"0","")
SHADER_PARAM( C3_Z,SHADER_PARAM_TYPE_FLOAT,"0","")
SHADER_PARAM( C3_W,SHADER_PARAM_TYPE_FLOAT,"0","")
SHADER_PARAM( PIXSHADER, SHADER_PARAM_TYPE_STRING, "", "Name of the pixel shader to use" )
SHADER_PARAM( DISABLE_COLOR_WRITES,SHADER_PARAM_TYPE_INTEGER,"0","")
SHADER_PARAM( ALPHATESTED,SHADER_PARAM_TYPE_FLOAT,"0","")
SHADER_PARAM( TEXTURE1, SHADER_PARAM_TYPE_TEXTURE, "", "" )
SHADER_PARAM( TEXTURE2, SHADER_PARAM_TYPE_TEXTURE, "", "" )
SHADER_PARAM( TEXTURE3, SHADER_PARAM_TYPE_TEXTURE, "", "" )
SHADER_PARAM( LINEARREAD_BASETEXTURE, SHADER_PARAM_TYPE_INTEGER, "0", "" )
SHADER_PARAM( LINEARREAD_TEXTURE1, SHADER_PARAM_TYPE_INTEGER, "0", "" )
SHADER_PARAM( LINEARREAD_TEXTURE2, SHADER_PARAM_TYPE_INTEGER, "0", "" )
SHADER_PARAM( LINEARREAD_TEXTURE3, SHADER_PARAM_TYPE_INTEGER, "0", "" )
SHADER_PARAM( LINEARWRITE,SHADER_PARAM_TYPE_INTEGER,"0","")
SHADER_PARAM( X360APPCHOOSER, SHADER_PARAM_TYPE_INTEGER, "0", "Needed for movies in 360 launcher" )

The next section will cover each piece of the SHADER_PARAM macro.

Understanding SHADER_PARAM

The SHADER_PARAM macro has the following definition:

#define SHADER_PARAM( param, paramtype, paramdefault, paramhelp ) \
	static CShaderParam param( "$" #param, paramtype, paramdefault, paramhelp, 0 );

Note:Essentially, every shader parameter you create with this macro gets written as a static variable in memory. When your shader is loaded into the game, each shader parameter adds itself to a static vector called s_ShaderParams, which was created in the BEGIN_SHADER macro. The materialsystem uses this vector to load in data from VMT files.