Difference between revisions of "Shaders in Oolite"
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+ | <small>'''''See also:''' [[Materials in Oolite]]''</small> |
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'''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 '''[http://en.wikipedia.org/wiki/GLSL OpenGL Shading Language]''', also known as '''GLslang''' or '''GLSL'''. |
'''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 '''[http://en.wikipedia.org/wiki/GLSL OpenGL Shading Language]''', also known as '''GLslang''' or '''GLSL'''. |
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− | Shaders are supported in Oolite 1.67 for Mac OS X and later |
+ | Shaders are supported in Oolite 1.67 for Mac OS X and later, and from 1.68 on other platforms. However, the shader support is still evolving. This page currently documents the state in Oolite 1.72. |
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| ''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.) |
| ''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.) |
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− | | ''glsl-vertex'' || string || GLslang code to use as a vertex shader. This is ignored if ''vertex_shader'' is specified. ('''Deprecated''' – may be removed in Oolite 1.69.) |
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| ''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.) |
| ''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.) |
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− | | ''glsl-fragment'' || string || GLslang code to use as a fragment shader. This is ignored if ''fragment_shader'' is specified. ('''Deprecated''' – may be removed in Oolite 1.69.) |
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− | | ''glsl'' || string || Synonym for ''glsl-fragment''. ('''Deprecated''' – may be removed in Oolite 1.69.) |
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| ''uniforms'' || dictionary || Uniforms to pass to the shaders. |
| ''uniforms'' || dictionary || Uniforms to pass to the shaders. |
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| <code>OO_LIGHT_0_FIX</code>||In current versions of Oolite, light number 0 is effectively always attached to the camera, giving the effect of the player shining a light on anything they’re looking at. When this is fixed, <code>OO_LIGHT_0_FIX</code> will be defined. (Not yet defined in any version of Oolite.) |
| <code>OO_LIGHT_0_FIX</code>||In current versions of Oolite, light number 0 is effectively always attached to the camera, giving the effect of the player shining a light on anything they’re looking at. When this is fixed, <code>OO_LIGHT_0_FIX</code> will be defined. (Not yet defined in any version of Oolite.) |
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− | | <code>OO_REDUCED_COMPLEXITY</code>||Defined if user has selected lower-complexity shaders. |
+ | | <code>OO_REDUCED_COMPLEXITY</code>||Defined if user has selected lower-complexity shaders. |
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| <code>OO_TEXTURE_UNIT_COUNT</code>||An integer specifying the number of texture units supported by hardware, which is the maximum number of ''tex'''N''''' uniforms that will be meaningful. This should be the same as ''gl_MaxTextureUnits'', but a macro has the advantage that it can be used to completely exclude parts of the shader. (Defined in Oolite 1.69 and later.) |
| <code>OO_TEXTURE_UNIT_COUNT</code>||An integer specifying the number of texture units supported by hardware, which is the maximum number of ''tex'''N''''' uniforms that will be meaningful. This should be the same as ''gl_MaxTextureUnits'', but a macro has the advantage that it can be used to completely exclude parts of the shader. (Defined in Oolite 1.69 and later.) |
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| <code>OO_USER_DEFINED_BINDINGS</code>||Defined to indicate that [[Shaders in Oolite: uniforms|arbitrary uniform bindings]] are available. (Defined in Oolite 1.69 and later.) |
| <code>OO_USER_DEFINED_BINDINGS</code>||Defined to indicate that [[Shaders in Oolite: uniforms|arbitrary uniform bindings]] are available. (Defined in Oolite 1.69 and later.) |
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+ | | <code>OO_TANGENT_ATTR</code>||Defined to indicate that the <code>tangent</code> vertex attribute is available. (Defined in Oolite 1.73 and later.) |
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[[Category:Oolite]] |
[[Category:Oolite]] |
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+ | [[Category:Oolite scripting]] |
Latest revision as of 19:07, 28 February 2010
See also: Materials 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, and from 1.68 on other platforms. However, the shader support is still evolving. This page currently documents the state in Oolite 1.72.
Contents
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:
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.) |
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.) |
uniforms | dictionary | Uniforms to pass to the shaders. |
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.
Uniforms
Uniforms are variables whose value is set by the host application, i.e. Oolite. Oolite automatically supplies one uniform of type sampler2D
for each texture, named tex0
, tex1
etc. It is also possible to bind uniforms to certain properties of the entity the shader is being applied to, for instance, its engine level. Additionally, constant values can be specified for uniforms, allowing a single shader to be used on different entities with differences in configuration.
For more information on uniforms, see Shaders in Oolite: uniforms.
Macros
These preprocessor macros are defined by Oolite and may be used with the #if
/#endif
directives to selectively remove sections of code.
Name | Description |
---|---|
OO_LIGHT_0_FIX |
In current versions of Oolite, light number 0 is effectively always attached to the camera, giving the effect of the player shining a light on anything they’re looking at. When this is fixed, OO_LIGHT_0_FIX will be defined. (Not yet defined in any version of Oolite.)
|
OO_REDUCED_COMPLEXITY |
Defined if user has selected lower-complexity shaders. |
OO_TEXTURE_UNIT_COUNT |
An integer specifying the number of texture units supported by hardware, which is the maximum number of texN uniforms that will be meaningful. This should be the same as gl_MaxTextureUnits, but a macro has the advantage that it can be used to completely exclude parts of the shader. (Defined in Oolite 1.69 and later.) |
OO_USER_DEFINED_BINDINGS |
Defined to indicate that arbitrary uniform bindings are available. (Defined in Oolite 1.69 and later.) |
OO_TANGENT_ATTR |
Defined to indicate that the tangent vertex attribute is available. (Defined in Oolite 1.73 and later.)
|
Limitations
The current implementation does not provide the information necessary to implement the most common form of normal mapping.