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 the next Windows release 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. (Not implemented in 1.67.1 or earlier.)
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. (Not implemented in 1.67.1 or earlier.)
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;
varying vec4            v_ambient;

// Light 0 information
varying vec4            v_diffuse0;
varying vec3            v_direction0;

// Light 1 information
varying vec4            v_diffuse1;
varying vec3            v_direction1;

void main()
{
    v_normal = normalize(gl_NormalMatrix * gl_Normal);
    
    v_ambient = gl_LightModel.ambient * gl_FrontMaterial.ambient
            + gl_FrontMaterial.ambient * gl_LightSource[0].ambient
            + gl_FrontMaterial.ambient * gl_LightSource[1].ambient;
    
    // Set up light 0 information
    v_direction0 = normalize(gl_LightSource[0].position.xyz);
    v_diffuse0 = gl_FrontMaterial.diffuse * gl_LightSource[0].diffuse;
    
    // Set up light 1 information
    v_direction1 = normalize(gl_LightSource[1].position.xyz);
    v_diffuse1 = gl_FrontMaterial.diffuse * gl_LightSource[1].diffuse;
    
    gl_TexCoord[0] = gl_TextureMatrix[0] * gl_MultiTexCoord0;
    gl_Position = ftransform();
}

The first section declares several varying variables. 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:

These declarations are followed by the function main(), which is the function that will be called by the GPU for each vertex. This prepares state required to perform diffuse Gouraud shading with two lights, and 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.)

/*  Freaky Thargoid: shader example for Oolite.
    This shader performs per-vertex lighting (with two lights) and
    glow mapping. The glow map varies over time, with the alpha
    channel specifying the phase of the glow effect.
*/
uniform sampler2D tex0;
uniform sampler2D tex1;

uniform float time;


#define RECIPROCAL_FREQUENCY    1.5


varying vec3            v_normal;
varying vec4            v_ambient;

// Light 0 information
varying vec4            v_diffuse0;
varying vec3            v_direction0;

// Light 1 information
varying vec4            v_diffuse1;
varying vec3            v_direction1;


float wave(float t)
{
    // approximates a sine waveform by summing 4 triangular waveforms
    float s0 = t;
    s0 -= floor(s0);
    float sum = abs( s0 - 0.5);
    
    float s1 = t - 0.125;
    s1 -= floor(s1);
    sum += abs( s1 - 0.5) - 0.25;
    
    float s2 = t - 0.250;
    s2 -= floor(s2);
    sum += abs( s2 - 0.5) - 0.25;
    
    float s3 = t - 0.375;
    s3 -= floor(s3);
    sum += abs( s3 - 0.5) - 0.25;

    return sum;
}


void main(void)
{
    vec4 color = v_ambient;
    vec3 normal = normalize(v_normal);
    
    // Calculate contribution from two lights
    float NdotL;
    NdotL = max(dot(normal, v_direction0), 0.0);
    color += NdotL * v_diffuse0;
    
    NdotL = max(dot(normal, v_direction1), 0.0);
    color += NdotL * v_diffuse1;
    
    // 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 = RECIPROCAL_FREQUENCY * time + glowMap.a;
    float lightLevel = wave(t1);
    
    // Add glow.
    color += lightLevel * glowMap;
    
    gl_FragColor = color;
}

The first section declares three 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. Uniform variables can only be read, not written to.

The #define line defines a macro, which is a value substituted into the code. In this case, wherever RECIPROCAL_FREQUENCY is encountered, it will be replaced with the value 1.5. This allows “tweak factors” to be grouped together at the top of a file for easy identification and editing. (The value of RECIPROCAL_FREQUENCY is the only difference between the thargoid and tharglet shaders in Freaky Thargoids. Ideally, Oolite would provide a way to specify uniform variables in shipdata.plist so that the same shader could be used, with minor tweaks, on different models, but this is not currently the case.)

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

The custom function wave() is an approximation of the trigonometric sine function. It is used to smoothly vary the glow map from on to off.

The last part is the function main(), which is the function executed by the GPU. It first calculates the contribution of light sources 0 and 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.) 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 (multiplying by 1.5 gives a frequency of 1.5 pulsations per second, or 1/1.5 = 0.666… Hz) 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 (colour) of the 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.
time	float	Uniformly increasing timer, in seconds.
engine_level	float	Engine thrust. 0.0 for no movement, 1.0 for full thrust. Greater than 1.0 for injectors or hyperspeed.
laser_heat_level	float	Laser temperature, ranging from 0.0 to 1.0.
hull_heat_level	float	Hull temperature. 1.0 is damage level. (Not implemented in 1.67.1 or earlier.)

In order to enable shaders to use new features while remaining backwards compatible, each available uniform (except texN) has an associated macro. 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.