Shaders in Oolite

Shaders are programs which run on a graphics processing unit. They provide a more flexible alternative or supplement to textures for specifying objects’ appearance. There are two widely-used types of shader, vertex shaders and fragment shaders (also known, less accurately, as pixel shaders), which are generally used in combination. Shaders can be implemented in a number of special-purpose programming languages; Oolite uses the OpenGL Shading Language, also known as GLslang or GLSL.

Shaders are supported in Oolite 1.67 for Mac OS X and later. At the time of writing, no released version of Oolite for other platforms supports shaders, but version 1.68 for Windows will.

Specifying shaders

Shaders are specified in a dictionary named shaders in the ship’s definition in shipdata.plist. The keys of this dictionary are names of textures used in the ship’s or entity’s .dat file, and the values are dictionaries specifying shaders to use instead. The elements are: The uniform variables currently provided by Oolite are:

shaders dictionary values

Name	Type	Description
textures	array of strings	A list of textures used by the shader.
vertex_shader	string	The name of a vertex shader file to use. Oolite will search the Shaders folder of all installed OXPs for a shader of the appropriate name. (Requires Oolite 1.68 or later.)
glsl-vertex	string	GLslang code to use as a vertex shader. This is ignored if vertex_shader is specified.
fragment_shader	string	The name of a fragment shader file to use. Oolite will search the Shaders folder of all installed OXPs for a shader of the appropriate name. (Requires Oolite 1.68 or later.)
glsl-fragment	string	GLslang code to use as a fragment shader. This is ignored if fragment_shader is specified.
glsl	string	Synonym for glsl-fragment.

An example

A full explanation of GLslang is beyond the scope of this article, but for illustrative purposes, here is an overview of the fragment shader code in the Freaky Thargoids example OXP.

Vertex shader

This vertex shader, like many vertex shaders, exists primarily to prepare information for the fragment shader.

varying vec3            v_normal;

void main()
{
    v_normal = normalize(gl_NormalMatrix * gl_Normal);
    
    gl_TexCoord[0] = gl_TextureMatrix[0] * gl_MultiTexCoord0;
    gl_Position = ftransform();
}

The first line declares a varying variable. This may sound like an oxymoron, but varying variables have the special property that they are interpolated across geometry and passed to the fragment shader. For instance, if you set a varying variable to red at one corner of a triangle, green at a second corner, and blue at the third, the colour as seen by the fragment shader will vary smoothly across the triangle:

This is followed by the function main(), which is the function that will be called by the GPU for each vertex. This sets the varying normal (direction facing out from the surface), and performs additional set-up required for positions and texture co-ordinates to make sense in the fragment shader.

Fragment shader

A fragment shader is called for each fragment of a generated polygon. A fragment is a pixel, in screen space, on which the polygon is potentially visible. (It is potentially visible because the shader may discard the fragment, and later polygons which are closer to the camera may draw over it.)

// Information from Oolite.
uniform sampler2D   tex0;
uniform sampler2D   tex1;
uniform float       time;

// Information from shipdata.plist.
uniform float       reciprocalFrequency;

// Information from vertex shader.
varying vec3        v_normal;


float wave(float t)
{
    return sin(t * 6.28318530718) * 0.5 + 0.25;
}


#define LIGHT(idx) \
    { \
        vec3 lightVector = normalize(gl_LightSource[idx].position.xyz); \
        color += gl_FrontMaterial.diffuse * gl_LightSource[idx].diffuse * max(dot(v_normal, lightVector), 0.0); \
    }


void main(void)
{
    // Calculate illumination.
    vec4 color = gl_FrontMaterial.ambient * gl_LightSource[0].ambient;
//  LIGHT(0);
    LIGHT(1);
    
    // Load texture data
    vec2 texCoord = gl_TexCoord[0].st;
    vec4 colorMap = texture2D(tex0, texCoord);
    vec4 glowMap = texture2D(tex1, texCoord);
    
    // Multiply illumination by base colour
    color *= colorMap;
    
    // Calculate glow intensity
    float t1 = reciprocalFrequency * time + glowMap.a;
    float lightLevel = wave(t1);
    
    // Add glow.
    color += lightLevel * glowMap;
    
    gl_FragColor = vec4(color.rgb, 1.0);
}

The first section declares several uniform variables, which are used to pass information from Oolite to the shader. The two sampler2D variables are used to read from the two textures used by the shader. The float variable timer is a number which increases by 1.0 each second. The float reciprocalFrequency is set in shipData.plist; this allows a shader to be used for different ships (or different shaders on the same ship) with small variations. Uniform variables can only be read, not written to.

The varying declaration is the same as in the vertex shader, but fragment shaders can only read varying variables, not write to them.

The custom function wave() is used to smoothly vary the glow map from on to off.

This is followed by a macro definition, LIGHT. A macro is substituted into the code each time it occurs; in main(), LIGHT(0) and LIGHT(1) will be replaced by the text of the macro definition, with idx in the macro being replaced with 0 or 1 respectively. Ideally, this would be a function, like wave(), but for technical reasons this causes performance problems. Specifically, under Apple’s implementation of GLSL (and probably others), this causes a set of uniforms for every light the graphics card supports to be pulled into the shader.

The last part is the function main(), which is the function executed by the GPU. It first calculates the contribution of light source 1, using the information prepared in the vertex shader and interpolated by the GPU. (Light 0 is used for the demo screen and shipyard; light 1 is the sun, or an arbitrary light source when in witchspace. Future versions of Oolite may change this, though. Ideally, the shader would check light 0, but in current versions of Oolite this causes problems.) It then loads values from the two textures. The first, the colour map value, is simply multiplied by the incoming light. For the second, the glow map, an intensity value is calculated based on the time, the reciprocalFrequency uniform and the alpha channel of the glow map, which specifies animation phase. The glow map is likewise multiplied by its intensity value, and added to the total light. Finally, the combined alpha channel is forced to 1.0, and the result assigned to the special variable gl_FragColor, which determines the colour of the generated fragment.

Uniform reference

The uniform variables currently provided by Oolite are:

Uniform variables

Name	Type	Description
tex0	sampler2D	Sampler for the first texture.
tex1	sampler2D	Sampler for the second texture.
…
texN	sampler2D	Sampler for the N+1th texture.
engine_level	float	Engine thrust. 0.0 for no movement, 1.0 for full thrust. Greater than 1.0 for injectors or hyperspeed.
entity_personality	float	A randomly-generated value in the range 0.0 to 1.0 that stays with the entity for its lifetime. Useful for adding random variations. (Requires Oolite 1.68 or later.)
entity_personality_int	int	Same as entity_personality, scaled to the range 0 to 32767. (Requires Oolite 1.68 or later.)
hull_heat_level	float	Hull temperature. 1.0 is damage level. (Requires Oolite 1.68 or later.)
laser_heat_level	float	Laser temperature, ranging from 0.0 to 1.0.
time	float	Uniformly increasing timer, in seconds.

In order to enable shaders to use new features while remaining backwards compatible, each available uniform (except texN) has an associated macro (starting with Oolite 1.68). For instance, to test whether hull_heat_level is available, use:

#ifdef OO_HULL_HEAT_LEVEL
uniform float hull_heat_level;
#endif

Limitations

The current implementation does not provide the information necessary to implement the most common form of normal mapping.