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Dynamic shader assembly overview

ACG supports shader generation at runtime and offers a modular shader assembly method. The main class of interest is ShaderProgGenerator, which generates a complete GLSL program composed of vertex, fragment and optionally geometry and tessellation shaders. This page documents the assembly process of generated shaders and how ShaderModifers and templates will affect the process. GLSL versions 1.3 and higher are supported.

# Fixed shader code

Some parts of shader code are fixed and will be available in any generated shader. These include, but are not limited to:

- essential input uniforms (model-view-projection matrix, global lights..)
- essential input/output between two shader stages (ie. normal, texcoord)
- vertex transformation
- default lighting functions
- default texturing

# Naming convention

The following naming conventions for uniforms, stage input/output and local variables are used:

- Defines: prefix "SG_", all uppercase
- Uniforms: prefix "g_"
- Local variables: prefix "sg_"
- vertex shader input: prefix "in"
- vertex shader output: prefix "outVertex"
- tessellation-control shader output: prefix "outTc"
- tessellation-evaluation shader output: prefix "outTe"
- geometry shader output: prefix "outGeometry"

- fragment shader output: prefix "outFragment"

  Modifiers and templates can not actually control any shader code provided by the ShaderGenerator, but they can add additional code lines that may overwrite or extend the results of the generated code.

# Default defines

The ShaderGenerator writes the current configuration given in ShaderGenDesc to a shader with the help of defines. These defines in the generated shader reflect the state of the active ShaderGenDesc.

Shading mode: defines in which stage lighting should be performed. One of the following defines is generated based on the state of ShaderGenDesc::shadeMode.

#define SG_GOURAUD 1 // only added iff shadeMode == SG_SHADE_GOURAUD
#define SG_FLAT 1 // only added iff shadeMode == SG_SHADE_FLAT
#define SG_UNLIT 1 // only added iff shadeMode == SG_SHADE_UNLIT
#define SG_PHONG 1 // only added iff shadeMode == SG_SHADE_PHONG

If textures, vertex colors and or lighting is enabled in the descriptor, these defines are specified in the shader.

#define SG_TEXTURED 1 // only added iff textured
#define SG_VERTEX_COLOR 1 // only added iff vertex colors are used
#define SG_NORMALS 1 // only added iff lighting is enabled ( shadeMode != SG_SHADE_UNLIT )

Additionally the light setup is passed to the shader with the following defines.

#define SG_NUM_LIGHTS N // N = numLights
#define SG_LIGHT_TYPE_X Y // type of light X where X = 0,...,N-1 and Y = SG_LIGHT_DIRECTIONAL,SG_LIGHT_POINT,SG_LIGHT_SPOT

Furthermore, there are stage-independent IO defines for default vertex attribute IO. It is recommended to make use of these, instead of using the direct names.

SG_INPUT_POSOS // input object space position
SG_INPUT_POSVS // input view space position
SG_INPUT_POSCS // input clip space position
SG_INPUT_NORMALOS // input object space normal
SG_INPUT_NORMALVS // input view space normal
SG_INPUT_TEXCOORD // input texcoord
SG_INPUT_VERTEXCOLOR // input vertex color
SG_OUTPUT_X // output corresponding to SG_INPUT_X, ie. SG_OUTPUT_POSOS ..

Request defines provided by shader modifiers or templates. Add these somewhere at the beginning of the shader.

SG_REQUEST_POSOS // shader requires object space position
SG_REQUEST_POSVS // shader requires view space position
SG_REQUEST_NORMALOS // shader requires object space normal
SG_REQUEST_NORMALVS // shader requires view space normal
SG_REQUEST_TEXCOORD // shader requires texcoord
SG_REQUEST_VERTEXCOLOR // shader requires vertex color

# Default uniforms

The following default uniforms are guaranteed to be added to any generated shader:

mat4 g_mWVP; // proj * view * world matrix
mat4 g_mWV; // modelview matrix
mat3 g_mWVIT; // inverse transpose of modelview, used to transform direction vectors such as normals
mat4 g_mP; // projection matrix
vec3 g_vCamPos; // camera position in world space
vec3 g_vCamDir; // camera view direction in world space
vec3 g_cDiffuse; // diffuse color of material
vec3 g_cAmbient; // ambient color of material
vec3 g_cEmissive; // emissive color of material
vec3 g_cSpecular; // specular color of material
vec4 g_vMaterial; // other material parameters: vec4(shininess, alpha, 0, 0)

The light configuration is added to a shader if it is supposed to do lighting. If the shader descriptor defines that lighting has to be done via gouraud or flat shading, these are added to the vertex shader. Othwerise, in phong shading these are included in the fragment shader.

vec3 g_cLightDiffuse_X; // diffuse color of light X
vec3 g_cLightAmbient_X; // ambient color of light X
vec3 g_cLightSpecular_X; // specular color of light X

additionally for directional lights:

vec3 g_vLightDir_X; // light direction in view space

additionally for point lights:

vec3 g_vLightPos_X; // light position in view space
vec3 g_vLightAtten_X; // distance attentuation factors: vec3(linear, quadratic, cubic)

additionally for spot lights:

vec3 g_vLightPos_X; // light position in view space
vec3 g_vLightDir_X; // light direction in view space
vec3 g_vLightAtten_X; // distance attentuation factors: vec3(linear, quadratic, cubic)
vec2 g_vLightAngleExp_X; // angle attenuation factors: vec2(cutoff angle, exponent)

where X is replaced by the light index in range[0,.., numLight-1].

TextureSampler uniforms for a fragment shader are generated based on the number of specified textures in ShaderGenDesc:

samplerY g_TextureX; // X: texture stage id = 0,...,numTextures-1 Y: samplerType ie. 1D, 2D, Cube, 2DRect..

ShaderModifiers can add custom uniforms to a generated shader.

# Shader attribute IO

Custom attributes have to be handled manually and are ignored by the generator. In contrast, default attributes consisting of position, normal, texcoord and vertexcolor can be conveniently accessed via the SG_INPUT_X, SG_OUTPUT_X defines. Default attributes can be requested in a template or modifier via the SG_REQUEST_X defines. It does not matter in what stage the attribute request is defined; the attribute will be available in all stages. In general, all used default attributes in a program are passed down from the vertex shader through all following stages. The fragment shader has only one default output though: vec4 outFragment! Also, they are passed down without modifications from the vertex-shader; that is, if not done explicitly by a modifier or template afterwards.

However, this is difficult for geometry and tessellation shaders due to their freedom of having multiple inputs or outputs. There is a convenience attribute mapping function sg_MapIO() to help with this problem for each of these stages, which maps all default inputs to their corresponding outputs without any modifications.

The implementation of this function looks as follows for each stage:

Geometry shader:

void sg_MapIO(int inIdx)
{
gl_Position = gl_in[inIdx].gl_Position;
gl_PrimitiveID = gl_PrimitiveIDIn;
SG_OUTPUT_X = gl_in[inIdx].SG_INPUT_X
}

Tess-control shader:

void sg_MapIO(int inIdx)
{
gl_out[gl_InvocationID].gl_Position = gl_in[inIdx].gl_Position;
gl_out[gl_InvocationID].SG_OUTPUT_X = gl_in[inIdx].SG_INPUT_X
}

Tess-evaluation shader: This is somewhat tricky, since there is no "default" way to interpolate. Still, there are some helpers for barycentric interpolation for triangle patches and bilinear interpolation for quad patches:

void sg_MapIOBarycentric()
{
SG_OUTPUT_X = gl_TessCoord.x * SG_INPUT_X[0] + gl_TessCoord.y * SG_INPUT_X[1] + gl_TessCoord.z * SG_INPUT_X[2];
// same for gl_Position
}
void sg_MapIOBilinear()
{
// bilerp for SG_OUTPUT_X <- SG_INPUT_X[0..3] by gl_TessCoord.xy
// same for gl_Position
}

Full flexibility for the interpolation stage is possible with special keywords ("SG_INPUT", "SG_OUTPUT") recognized by the generator exclusively for the tess-eval stage: However, this only works if the whole instruction does not exceed more than a single line of code.

For instance, a barycentric interpolation of default attributes in a triangle can also be expressed as:

SG_OUTPUT = gl_TessCoord.x * SG_INPUT[0] + gl_TessCoord.y * SG_INPUT[1] + gl_TessCoord.z * SG_INPUT[2];

The generator then replaces the SG_OUTPUT, SG_INPUT keywords with each available default attribute.

Example 1: Displace vertices along normals in a vertex shader

// this is a vertex shader template!
#define SG_REQUEST_NORMALOS
uniform float factor;
void main()
{
SG_VERTEX_BEGIN;
SG_VERTEX_END;
vec4 newPosOS = SG_INPUT_POSOS + vec4(SG_INPUT_NORMALOS, 0) * factor;
vec4 newPosCS = g_mWVP * newPosOS;
SG_OUTPUT_POSCS = gl_Position = newPosCS;
#ifdef SG_OUTPUT_POSVS
SG_OUTPUT_POSVS = g_mWV * newPosOS;
#endif
#ifdef SG_OUTPUT_POSOS
SG_OUTPUT_POSOS = newPosOS;
#endif
}

Example 2: Write color-encoded normals in a fragment shader

Object-space normals have to be requested here! This example is implemented as a modifier for a change:

void modifyVertexIO(ACG::ShaderGenerator* _shader)
{
_shader->addDefine("SG_REQUEST_NORMALOS");
}
void modifyFragmentEndCode(QStringList* _code)
{
_code->push_back("outFragment = vec4(normalize(SG_INPUT_NORMALOS) * 0.5 + 0.5, 1);");
}

# Shader Templates

A shader template is glsl code with additional keywords that are interpreted by the shader generator.

Apart from the previously mentioned default defines, additional keywords are:

SG_VERTEX_BEGIN
SG_VERTEX_END
SG_FRAGMENT_BEGIN
SG_FRAGMENT_END

These keywords are location markers, where generated begin- or endcode of the vertex/fragment shader should be generated. However, it is not necessary to use them if the user wants to perform manual lighting and texturing etc. Templates for the tessellation and geometry stage do not have any exclusive keywords, but they can make use of defines, IO keywords and the mapping functions as described in the "Shader Attribute IO" section.

The vertex shader template without any user defined behaviour is:

void main()
{
SG_VERTEX_BEGIN;
SG_VERTEX_END;
}

Likewise, the fragment shader without special behaviour is:

void main()
{
SG_FRAGMENT_BEGIN;
SG_FRAGMENT_END;
}

# Vertex shader code generation

Some parts of a generated shader contain fixed code snippets, which are assembled according to specification in ShaderGenDesc.

A vertex shader has the following code layout.

void main(){
// ============================================================
// Generated begin code:
// read inputs from vertex buffer and initialize generated local variables
vec4 sg_vPosPS = g_mWVP * inPosition; // transform pos to projection space
vec4 sg_vPosVS = g_mWV * inPosition; // transform pos to view space
vec4 sg_cColor = vec4(g_cEmissive, ALPHA); // color (ALPHA: #define ALPHA g_vMaterial.y)
vec3 sg_vNormalVS = vec3(0,1,0); // unknown vertex normal at this point
#if SG_NORMALS
sg_vNormalVS = normalize(g_mWVIT * inNormal); // transform normal to view space
#endif
#if SG_TEXTURED
vecX sg_vTexCoord = inTexCoord;
#endif
#if SG_VERTEX_COLOR
sg_cColor = inColor;
#endif
#if SG_GOURAUD or SG_FLAT
sg_cColor += lightingCode();
#endif
// ============================================================
// Your ShaderModifier adds vertex shader code here.
// The modifier may overwrite any local variables from the shader generator.
for each modifier:
modifier->modifyVertexBeginCode();
// ============================================================
// Generated end code: pass local variables to next shader stage
gl_Position = sg_vPosPS;
outVertexPosCS = sg_vPosPS;
#if SG_TEXTURED
outVertexTexCoord = sg_vTexcoord;
#endif
#if SG_GOURAUD or SG_FLAT or SG_VERTEX_COLOR
outVertexColor = sg_cColor;
#endif
#if SG_PHONG
outVertexNormal = sg_vNormalVS;
outVertexPosVS = sg_vPosVS;
#endif
// ============================================================
// Shader modifiers are called again for the end code snippet.
// The modifers may write to gl_Position, outVertexNormal etc. and/or custom outputs
for each modifier:
modifier->modifyVertexEndCode();
}

Note
  Note that if directives are not generated at all if they do not change the compiled code.

# Tessellation/Geometry shader code generation

Tessellation and geometry shaders have a different layout and due to their complexity, there is no clear begin or end code marker for these shaders. The ShaderProgGenerator does not generate a main() function in these cases. However, it provides a convenient passthrough mapping function sg_MapIO() for default attribute IO. The sg_MapIO function can be called in a geometry shader template. The rest of the shader including the main() body has to be provided in form of a template file.

# Fragment shader generation

Fragment shaders are assembled similar to vertex shaders:

void main(){
// ============================================================
// Generated begin code:
// read inputs from vertex/geometry shader:
vec4 sg_vPosCS = outVertexPosCS; // or outGeometryPosCS
vec2 sg_vScreenPos = sg_vPosCS.xy / sg_vPosCS.w * 0.5 + vec2(0.5, 0.5); // projective texture coordinates
vec4 sg_cColor = vec4(g_cEmissive, ALPHA); // color (ALPHA: #define ALPHA g_vMaterial.y)
#if SG_GOURAUD or SG_FLAT or SG_VERTEX_COLOR
sg_cColor += outVertexColor; // or outGeometryColor
#endif
#if SG_PHONG
vec4 sg_vPosVS = outVertexPosVS;
vec3 sg_vNormalVS = outNormal;
sg_cColor += lightingCode();
#endif
#if SG_TEXTURED
vec4 sg_cTex = 0;
for each texture stage X:
sg_cTex += texture(g_TextureX, outVertexTexCoord); // additive combine texture stages
#if SG_UNLIT
sg_cColor += sg_cTex; // additive blending
#else
sg_cColor *= sg_cTex; // multiplicative blending
#endif
#if
sg_cColor = inColor;
#endif
// ============================================================
// Your ShaderModifier adds fragment shader code here.
// The modifier may overwrite any local variables from the shader generator.
for each modifier:
modifier->modifyFragmentBeginCode();
// ============================================================
// Generated end code: pass color to output
outFragment = sg_cColor;
// ============================================================
// Shader modifiers are called again for the end code snippet.
// The modifers may write to outFragment, and/or custom outputs
for each modifier:
modifier->modifyFragmentEndCode();
}

# Lighting code generation

The ShaderGenerator implements a forward lighting routine that supports any number of lights (within hardcoded SG_MAX_SHADER_LIGHTS limit) and any combination of light types. However, it is possible to extend or even completely replace the default lighting routine with ShaderModifiers.

The default lighting routine performs the following steps.

#for each light X:
#if X is directional:
sg_cColor += LitDirLight(sg_vPosVS, sg_vNormalVS, g_vLightDir_X, ..);
#else if X is point light:
sg_cColor += LitPointLight(sg_vPosVS, sg_vNormalVS, g_vLightPos_X, ..);
#else if X is spot light:
sg_cColor += LitSpotLight(sg_vPosVS, sg_vNormalVS, g_vLightPos_X, g_vLightDir_X, ..);

A ShaderModifier can now choose to keep, extend or completely replace the lighting routine provided by the generator. Keep in mind that since modifiers simply add more shader code to the existing routine, only commutative operations are supported correctly.

The modifyLightingCode function of a modifier receives a light index [0,...,numLights-1] and a light type [directional,point,spot] as arguments and is supposed to compute a lighting color based on the uniforms provided by the generator.

Refer to the toon renderer plugin for an example, that implements custom cel lighting functions with a modifier.

Finally, it is possible to perform default lighting in the fragment shader template via the SG_FRAGMENT_LIGHTING keyword.

SG_FRAGMENT_BEGIN;
sg_vNormalVS = g_mWV * (mTangentSpace * vNormalTS); // dot3
sg_cColor = vec4(g_cEmissive, SG_ALPHA);
SG_FRAGMENT_LIGHTING;
SG_FRAGMENT_END;