The Flexible Rendering Pipeline

Introduction

The rendering pipeline defines the steps which are necessary to bring the abstract 3D data to a visible result on the screen. Horde3D has a configurable rendering pipeline which makes it possible to employ a plenty of different rendering techniques, including standard forward rendering and deferred shading. The system allows to define several render targets and to specify render commands to fill the targets with data. This gives much power and flexibility to the engine and enables the creation of most post processing effects like HDR, motion blur or depth of field as well as the usage of different rendering algorithms.

System Overview

Horde is heavily based on shaders which are small programs that are executed on the graphics card at different stages of the hardware pipeline. Horde uses vertex and fragment shaders. Vertex shaders influence directly the geometry which is rendered and determine e.g. the position and texture coordinates of the vertices. The fragment shaders are used for calculating the pixel colors in the rasterization process. Shader code in Horde is specified in the OpenGL Shading Language (GLSL). The engine has a XML based shader format which makes it possible to define different contexts. A context of a Horde shader is defined for a situation in the rendering process where the shader is executed. For example, a shader usually has a shadowmap context which is used when shadows are generated and a lighting context which is responsible for interactions of an object with light sources. For the rendering process shaders usually require some input data. This consists on the one hand of the geometric data of a model like the vertex positions, normals and texture coordinates. On the other hand there are textures used for rendering and arbitrary variables, so called uniforms, which can be defined by the user. For example a uniform could be used to define a force vector to do some wind physics for a tree model in the vertex shader. This mutable data is bound to the shader via the concept of materials. A material in Horde consists of a shader and a list of texture maps and uniforms which are assigned to that shader.

Lighting and Shadows

Horde supports basically two different approaches for lighting namely forward and deferred shading. Forward shading is the standard technique that is currently used in most applications. With this technique the geometry is rendered once for each light source using a special fragment shader that calculates the light contribution. The disadvantage here is of course that the geometry has to be drawn several times which can result in poor performance in scenes with many polygons and light sources. Deferred shading in contrast does the lighting as a post processing step. The idea is to store some attributes for each pixel on the screen in a special buffer often called the G-Buffer. These attributes usually include the position of a fragment, the normal and its color. To do the actual lighting it is only necessary to draw a fullscreen quad since the required information can easily be read from the G-Buffer. The advantage is that the lighting performance is completely independent of the geometry complexity now since each light just requires drawing a screen-space quad. Unfortunately this approach has also its drawbacks as it is difficult to do anti-aliasing or handle translucent geometry. Discussing the two techniques in detail here would be out of the scope of this manual so please refer to one of the many online and book resources available for this topic.

Light sources in Horde are defined as scene nodes. Each light node has two special attributes called lightingContext and shadowContext. The values of these attributes correspond to shader context names defined in the shader resources. When Horde is instructed to perform forward lighting the engine first builds a list of the geometry that needs to be drawn. After that it draws each object from that list using the shader context that is specified for the light source. If the context cannot be found for a material, the object using this material is ignored and thus not rendered. This is basically working the same way for doing the lighting and building the shadow map. For deferred shading you also need to specify a material for the light source. The shader defined in that material is used to draw the screen-space quads where the light's lightingContext attribute specifies the shader context which is used. The calculation of the shadows is working the same way as for forward shading where all geometry is rendered to the shadow map using the shadowContext. When doing forward shading you could also specify a material for a light source. The shader is ignored in this case but it is possible to bind several textures which can for example be used to create some sort of slide-projector.

With the shader context system it is possible to define arbitrary light source types. The behavior of a light (e.g. directional or spot) and its interaction with a material are defined entirely in the shaders. If you want to create a new type of light source you just have to assign a name to the lightingContext attribute and define an appropriate shader context for all the materials (respectively shaders) that shall interact with that light source.

Pipeline Configuration

The flexible rendering pipeline allows to define render targets and commands which determine the steps taken to render the scene. The commands are specified within a XML file. Most rendering commands use the attributes class and context. The class is defined in the materials and determines what geometry should be rendered. It is possible to use the tilde operator ~ as a logical NOT meaning that all geometry except the one with the specified material class will be drawn. The context is finally available for specifying the rendering technique which should be used for the current draw call.

Pipeline Syntax

The following XML elements and attributes are supported.

Pipeline root element of the document {1}
Setup initialization section of pipeline {0,1}
RenderTarget definition of a render target; child of Setup element {*}
id unique name of the render target {required}
depthBuf flag specifying whether depth buffer is used for target {required}; possible values: true, false
numColBufs number of color buffers {required}; possible values: 0, 1, 2, 3, 4
format pixel format of render target {optional}; possible values: RGBA8, RGBA16F, RGBA32F; default: RGBA8
width width of render target in pixels where 0 means width of the main framebuffer {optional}; default: 0
height height of render target in pixels where 0 means height of the main framebuffer {optional}; default: 0
scale scale factor which is multiplied with the size of the render target {optional}; default: 1.0
maxSamples the maximum number of samples used when anti-aliasing is enabled {optional}; default: 0
CommandQueue ordered list of commands {0, 1}
Stage definition of a set of render commands; child of CommandQueue element {*}
id unique name of the stage {required}
enabled flag indicating whether stage is enabled by default {optional}; default: true
link material resource used to bind stage-specific data {optional}; default: empty string
SwitchTarget command for setting the currently active render target to which data is rendered; the command implicitely calls the UnbindBuffers command before the desired render target is activated; child of Stage element {*}
target name of the render target which was defined in the Setup section or empty string to bind the output buffer assigned to the current camera {required}
BindBuffer command for binding a color or depth buffer of a render target as texture map; child of Stage element {*}
sampler name of texture sampler to which the buffer is bound {required}
sourceRT name of render target {required}
bufIndex index of color buffer or 32 as special value for binding the depth buffer {required}
UnbindBuffers command for unbinding all buffers that were bound to texture samplers before; child of Stage element {*}
ClearTarget command for clearing the currently bound render target; child of Stage element {*}
depthBuf flag specifying whether depth buffer is cleared {optional}; possible values: true, false; default: false
colBuf0 flag specifying whether first color buffer is cleared {optional}; possible values: true, false; default: false
colBuf1 flag specifying whether second color buffer is cleared {optional}; possible values: true, false; default: false
colBuf2 flag specifying whether third color buffer is cleared {optional}; possible values: true, false; default: false
colBuf3 flag specifying whether fourth color buffer is cleared {optional}; possible values: true, false; default: false
col_R red component of clear color {optional}; default: 0.0
col_G green component of clear color {optional}; default: 0.0
col_B blue component of clear color {optional}; default: 0.0
col_A alpha component of clear color {optional}; default: 0.0
DrawGeometry command for rendering the scene geometry; child of Stage element {*}
context name of the shader context used for rendering {required}
class material class used for including/excluding objects {optional}; default: empty string, meaning all classes
order rendering order (sorting) of scene nodes {optional}; values: NONE, STATECHANGES, FRONT_TO_BACK, BACK_TO_FRONT; default: STATECHANGES
DrawOverlays command for rendering all overlays; child of Stage element {*}
context name of the shader context used for rendering {required}
DrawQuad command for drawing a fullscreen quad to the screen; child of Stage element {*}
material material resource used for rendering {required}
context name of the shader context used for rendering {required}
DoForwardLightLoop command for performing forward lighting by rendering all affected geometry; the default shader context used for rendering is the lighting context attribute of the corresponding light source; child of Stage element {*}
class material class used for including/excluding objects {optional}; default: empty string, meaning all classes
context shader context used for doing lighting {optional}; default: empty string, meaning context assigned to light source
order rendering order (sorting) of scene nodes {optional}; values: NONE, FRONT_TO_BACK, BACK_TO_FRONT, STATECHANGES; default: NONE
noShadows flag for disabling shadowing {optional}; default: false
DoDeferredLightLoop command for performing deferred lighting by drawing screen-space quads; the default shader context used for rendering is the one which is stored as attribute of the corresponding light source; child of Stage element {*}
context shader context used for doing lighting {optional}; default: empty string, meaning context assigned to light source
SetUniform command for setting a material uniform to specified values; child of Stage element {*}
material material resource which contains the uniform to be set {required}
uniform name of the uniform {required}
a value of the first component {optional}; default: 0.0
b value of the second component {optional}; default: 0.0
c value of the third component {optional}; default: 0.0
d value of the fourth component {optional}; default: 0.0

Sample showing simple deferred shading pipeline

<Pipeline>

    <Setup>
        <RenderTarget id="GBUFFER" depthBuf="true" numColBufs="3" format="RGBA16F" scale="1.0" />
    </Setup>
        
    <CommandQueue>
        <Stage id="Attribpass">
            <SwitchTarget target="GBUFFER" />
            <ClearTarget depthBuf="true" colBuf0="true" />
            <DrawGeometry context="ATTRIBPASS" class="~Translucent" />
        </Stage>
		
        <Stage id="Lighting">
            <SwitchTarget target="" />
            <ClearTarget colBuf0="true" />
            <BindBuffer texUnit="8" target="GBUFFER" bufIndex="0" />
            <BindBuffer texUnit="9" target="GBUFFER" bufIndex="1" />
            <BindBuffer texUnit="10" target="GBUFFER" bufIndex="2" />
			
            <DrawQuad material="light.material.xml" context="AMBIENT" />
            <DoDeferredLightLoop />
        </Stage>
		
        <Stage id="Overlays">
            <DrawOverlays context="OVERLAY" />
        </Stage>
    </CommandQueue>
    
</Pipeline>

Shaders

Horde3D shaders are stored in text files with the extension .shader. A shader file consists of different sections: one FX section and an arbitrary number of named code sections. A section is introduced by the tag [[id]] where id is a unique identifier used as name of the section. A special identifier is FX which introduces an FX section.

Horde3D supports übershaders using static branching (compiler defines) and can automatically compile different shader combinations based on a set of given shader flags. This approach helps to solve a common problem: imagine you have one shader that does skinning and one that does parallax mapping. Now you want a shader that does both. Usually you would have to create a new shader file which combines the code of the two other shaders. It is quite obvious that you can quickly get many permutations of features and that writing different shader files for them can become very tedious or even unmanagable. With the help of übershaders, the shader creation workflow can be greatly simplified. You just need one shader file where the features are exposed with compiler defines. A feature is enabled by setting the corresponding flag (e.g. in a material file). Horde3D can check automatically if a desired combination is not yet existing and will compile it on the fly. Please note, even though übershaders are a powerful feature, they need to be used with care since it is easy to generate a huge number of combinations which would take a long time to load.

FX Section

The FX section defines and configures the shader contexts, custom texture samplers and custom (user-defined) uniforms used in hardware shaders. Please note that it is strictly required that all custom samplers and uniforms referenced in hardware shaders are declared in the FX section so that they can be used with the Horde3D material system.
The Horde3D FX syntax is similar to that of CgFX. Standard C++ block and single line comments are supported. All keywords and enums are case insensitive. However, identifiers like the name of a context are not case insensitive. Identifiers may only use alphanumeric characters and the underscore. Floating point numbers may not contain any whitespace.
Elements can optionally store meta information as annotations. This is useful for better tool support. For example, a uniform could store a minimum and maximum value range. It is suggested that annotations have the form required by CgFX, hence datatype identifier = value; However, this is not a requirement since annotations are just skipped by Horde's parser.

The following syntax is supported. Optional elements are put in square brackets and default values are highlighted as bold. Alternative values are separated by a slash.

sampler2D / samplerCube [< annotation(s) >] id
[ = sampler_state {
[ Texture = "TextureResName" ; ]
[ TexUnit = -1 / 0 / 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 10 / 11 ; ]
[ Address = Wrap / Clamp ; ]
[ Filter = None / Bilinear / Trilinear ; ]
[ MaxAnisotropy = 1 / 2 / 4 / 8 / 16 ; ]
} ] ;

Remarks:

float [< annotation(s) >] id [ = a ] ;
float4 [< annotation(s) >] id [ = { a, b, c, d } ] ;

Remarks:

context [< annotation(s) >] id
{
VertexShader = compile GLSL codesection ;
PixelShader = compile GLSL codesection ;
[ ZWriteEnable = false / true ; ]
[ ZEnable = true / false ; ]
[ ZFunc = Always / Equal / Less / LessEqual / Greater / GreaterEqual ; ]
[ BlendMode = Replace / Blend / Add / AddBlended / Mult ; ]
[ CullMode = Back / Front / None ; ]
[ AlphaToCoverage = false / true ; ]
}

Remarks:

Code Section

A code section contains the GLSL code. Besides the usual GLSL keywords, Horde3D introduces an additional preprocessor command: #include. The include directive is used to insert the content of a code resource at the corresponding location. The name of the code file needs to be enclosed by a pair of quotation marks.

The code section can also contain the shader flags used for the automatic combination generation. The shader flags have a special naming convention: _F<digit><digit>_<name>. The following would be a valid flag: _F06_MyFlag. The flag must have a number between 01 and 32 (note the leading zero). This number is exclusively used to identify the flag. The name is optional and just exists for convenience reasons in order to improve the code readability.

Sample

[[FX]]

sampler albedoMap = sampler_state
{
    Filter = Bilinear;
};

float4 brightness = {0.5, 0, 0, 0};

context OVERLAY
{
    VertexShader = compile GLSL VS_OVERLAY;
    PixelShader = compile GLSL FS_OVERLAY;
}  


[[VS_OVERLAY]]
// ============================================================================

uniform mat4 projMat;
attribute vec2 vertPos;
attribute vec2 texCoords0;
varying vec2 texCoords;

void main( void )
{
    texCoords = vec2( texCoords0.s, -texCoords0.t ); 
    gl_Position = projMat * vec4( vertPos.x, vertPos.y, 1, 1 );
}


[[FS_OVERLAY]]
// ============================================================================

uniform vec4 brightness;
uniform sampler2D albedoMap;
varying vec2 texCoords;

void main( void )
{
    vec4 albedo = texture2D( albedoMap, texCoords );
	
#ifdef _F07_BrightMult
    albedo *= brightness.a;
#endif
	
    gl_FragColor = albedo;
}

Predefined GLSL Uniforms and Attributes

Horde defines some standard uniforms and attributes which can be used by shaders. The engine automatically detects which input data is required and binds it to the shader programs.

Note: Not all uniforms and attributes are available for every pipeline step, e.g. light source parameters are only available when doing lighting calculations and particle specific data only when rendering emitters.

General vector/matrix uniforms

uniform vec2 frameBufSize dimensions (width and height) of the currently active frame buffer
uniform mat4 viewMat viewing matrix
uniform mat4 viewMatInv inverse viewing matrix
uniform mat4 projMat projection matrix
uniform mat4 viewProjMat premultiplied view and projection matrix
uniform mat4 viewProjMatInv inverse view-projection matrix
uniform vec3 viewerPos position of the viewer (virtual camera)
uniform vec4 lightPos position of the light source in xyz components and radius in w
uniform vec4 lightDir direction vector of the light source in xyz components and cosine of light FOV in w (for spotlights)
uniform vec3 lightColor (diffuse) color of the light source
uniform vec4 shadowSplitDists split distances determining which of the four shadow maps has to be sampled
uniform mat4 shadowMats[4] light transformation matrices for individual shadow maps
uniform float shadowMapSize size of the shadow map texture in pixels
uniform float shadowBias bias used for shadow mapping to reduce precision issues

General sampler uniforms

uniform sampler2D shadowMap shadow map texture

Per-instance vector/matrix uniforms

uniform mat4 worldMat matrix used for transforming vertex positions of currently rendered mesh to world space
uniform mat3 worldNormalMat matrix used for transforming tangent space basis of currently rendered mesh to world space
uniform float nodeId identifier value of currently rendered node (default value is node handle)
uniform vec4 customInstData[4] custom per-instance node data; only available for models
uniform vec4 skinMatRows[75*3] first three rows of skinning matrices for skeletal animation; fourth row is always (0, 0, 0, 1); only available for models

Overlay specific vector/matrix uniforms

uniform vec4 olayColor color of overlay

Particle specific vector/matrix uniforms

uniform vec3 parPosArray[64] position array of particle batch
uniform vec2 parSizeAndRotArray[64] combined size and rotation array of particle batch
uniform vec4 parColorArray[64] color array of particle batch

General vertex attributes

attribute vec3 vertPos vertex position
attribute vec2 texCoords0 first set of texture mapping coordinates

Model specific vertex attributes

attribute vec2 texCoords1 second set of texture mapping coordinates
attribute vec3 normal normal vector of vertex
attribute vec4 tangent tangent vector of vertex in xyz; handedness multiplicator (1.0, -1.0) of bitangent in w
attribute vec4 joints four joint indices referencing to skinning matrices
attribute vec4 weights four vertex weights for the four joint indices

Particle specific vertex attributes

attribute float parIdx index of current particle in position, size and color arrays

Remarks: