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Effect fun with brake discs heating up, using Cg shaders.

INTRODUCTION

This document describes how to add glowing brakediscs to your car in Racer (possible since v0.6.2, 8-8-08). It uses a Cg fragment shader so your graphics card must support this (most modern cards do).

THE IDEA

Racer keeps track of the temperature of the brake disc in each wheel. Braking increases heat stored in the disc, and letting go of the brakes, plus car & spin velocity make it cooler again. The variables in car.ini involved are:

• wheel<x>.brake_heating; speed of temperature increase with brake force. Example: 0.003
• wheel<x>.brake_dissipation; speed of dissipation (cooling down). Example: 0.0005
• wheel<x>.brake_max_temp; maximum temperature to avoid excessive values. Example: 1000

The temperature is passed to the rendering system as the variable 'heat', which can be used in your Cg fragment shader. This 'heat' parameter is normalized to a value between 0 and 1, where heat=currentTemp/brakeMaxTemp.

Note that currently (v0.8.3) the brake temperature is NOT used in the physics engine, and so has no influence on driving behavior yet.

THE STEPS

Take the following steps to get glowing brake discs:

• Make sure you have a (Racer) shader for the brake discs in your car.shd (in your car directory), i.e. shader_discs.
• Add a fragment shader to shader_discs (outside of any layers):
  fragment_shader
  {
  file=cg/disc_heat.cg
  param=heat
  }
• Make a Cg shader by editing a text file as data/cars/<yourcar>/cg/disc_heat.cg and copy the code at the end of this page.
• In the car.ini, add the brake_* parameters from above (in the 'Idea' section) to each wheel (note that ~ in ini files can save you from copying the same parameters for each wheel). To test if it works quickly, use brake_heating=0.03 for example; it's a value that is much too fast for real cars, but it helps to see it work.
• Try the car!

NOTES

Note that this simple example skips lighting that you normally have when using the fixed OpenGL pipeline. However, if you start out with dark brake discs it might be fine as it is (they're always hidden behind the wheel anyway).

DISC_HEAT.CG FILE

Here is the disc_heat.cg shader file:

struct FragIn
{
       float2 tc0 : TEXCOORD0;
};       

struct FragOut
{
         float4 col : COLOR;
};

FragOut main
(
         FragIn fin,
         uniform sampler2D tex0 : TEXUNIT0,
         uniform float heat
)
{
  FragOut fout;
  float4 red=float4(1.0,0.25,0.1,1); 
  float4 cColor=tex2D(tex0,fin.tc0);

  cColor=(1-heat)*cColor+heat*3.0*red;

 fout.col=cColor;

 return fout;
} 

(last updated November 13, 2012 )