Shader Composition

osgEarth uses GLSL shaders in several of its rendering modes. By default, osgEarth will detect the capabilities of your graphics hardware and automatically select an appropriate mode to use.

Since osgEarth relies on shaders, and since you as the developer may wish to use your own shader code as well, osgEarth provides a shader composition framework. This allows you a great deal of flexibility when incorporating your own shaders into osgEarth.

There are several ways to integrate your own shader code into osgEarth. We discuss these below. But first it is important to understand the basics of osgEarth’s shader composition framework.

Framework Basics

osgEarth installs default shaders for rendering. The default shaders are shown below. The LOCATION_* designators allow you to inject functions at various points in the shader’s execution.

Here is the pseudo-code for osgEarth’s built-in shaders:

// VERTEX SHADER:

void main(void)
{
    vec4 vertex = gl_Vertex;

    // "LOCATION_VERTEX_MODEL" user functions are called here:
    model_func_1(vertex);
    ...

    vertex = gl_ModelViewMatrix * vertex;

    // "LOCATION_VERTEX_VIEW" user functions are called here:
    view_func_1(vertex);
    ...

    vertes = gl_ProjectionMatrix * vertex;

    // "LOCATION_VERTEX_CLIP" user functions are called last:
    clip_func_1(vertex);
    ...

    gl_Position = vertex;
}


// FRAGMENT SHADER:

void main(void)
{
    vec4 color = gl_Color;
    ...

    // "LOCATION_FRAGMENT_COLORING" user functions are called here:
    coloring_func_1(color);
    ...

    // "LOCATION_FRAGMENT_LIGHTING" user functions are called here:
    lighting_func_1(color);
    ...

    gl_FragColor = color;
}

VirtualProgram

osgEarth include an OSG state attribute called VirtualProgram that performs the runtime shader composition. Since VirtualProgram is an osg::StateAttribute, you can attach one to any node in the scene graph. Shaders that belong to a VirtualProgram can override shaders lower down on the attribute stack (i.e., higher up in the scene graph). In the way you can override individual shader functions in osgEarth.

The sections below on integration will demonstrate how to use VirtualProgram.

Integrating Custom Shaders

There are two ways to use shader composition in osgEarth.

  • Injecting user functions
  • Overriding osgEarth’s built-in functions with a custom ShaderFactory

Injecting User Functions

In the core shader code above, osgEarth calls into user functions. These don’t exist in the default shaders that osgEarth generates; rather, they represent code that you as the developer can “inject” into various locations in the built-in shaders.

For example, let’s use User Functions to create a simple “haze” effect. (NOTE: see this example in its entirety in osgearth_shadercomp.cpp):

static char s_hazeVertShader[] =
    "varying vec3 v_pos; \n"
    "void setup_haze(inout vec4 vertexVIEW) \n"
    "{ \n"
    "    v_pos = vec3(vertexVIEW); \n"
    "} \n";

static char s_hazeFragShader[] =
    "varying vec3 v_pos; \n"
    "void apply_haze(inout vec4 color) \n"
    "{ \n"
    "    float dist = clamp( length(v_pos)/10000000.0, 0, 0.75 ); \n"
    "    color = mix(color, vec4(0.5, 0.5, 0.5, 1.0), dist); \n"
    "} \n";

osg::StateAttribute*
createHaze()
{
    osgEarth::VirtualProgram* vp = new osgEarth::VirtualProgram();

    vp->setFunction( "setup_haze", s_hazeVertShader, osgEarth::ShaderComp::LOCATION_VERTEX_VIEW);
    vp->setFunction( "apply_haze", s_hazeFragShader, osgEarth::ShaderComp::LOCATION_FRAGMENT_LIGHTING);

    return vp;
}

...
sceneGraph->getOrCreateStateSet()->setAttributeAndModes( createHaze() );

In this example, the function setup_haze is called from the core vertex shader after the built-in vertex functions. The apply_haze function gets called from the core fragment shader after the built-in fragment functions.

There are FIVE injection points, as follows:

Location Shader Type Signature
ShaderComp::LOCATION_VERTEX_MODEL VERTEX void func(inout vec4 vertex)
ShaderComp::LOCATION_VERTEX_VIEW VERTEX void func(inout vec4 vertex)
ShaderComp::LOCATION_VERTEX_CLIP VERTEX void func(inout vec4 vertex)
ShaderComp::LOCATION_FRAGMENT_COLORING FRAGMENT void func(inout vec4 color)
ShaderComp::LOCATION_FRAGMENT_LIGHTING FRAGMENT void func(inout vec4 color)

Each VERTEX locations let you operate on the vertex in a particular coordinate space. You can alter the vertex, but you must leave it in the same space.

MODEL:Vertex is the raw, untransformed values from the geometry.
VIEW:Vertex is relative to the eyepoint, which lies at the origin (0,0,0) and points down the -Z axis. In VIEW space, the orginal vertex has been transformed by gl_ModelViewMatrix.
CLIP:Post-projected clip space. CLIP space lies in the [-w..w] range along all three axis, and is the result of transforming the original vertex by gl_ModelViewProjectionMatrix.

Shader Variables

There are some built-in shader variables that osgEarth installs and that you can access from your shader functions.

Important: Shader variables starting with the prefix ``oe_`` or ``osgearth_`` are reserved for osgEarth internal use.

Uniforms:

oe_tile_key:(vec4) elements 0-2 hold the x, y, and LOD tile key values; element 3 holds the tile’s bounding sphere radius (in meters)
oe_layer_tex:(sampler2D) texture applied to the current tile
oe_layer_texc:(vec4) texture coordinate for current tile
oe_layer_tilec:(vec4) unit coordinates for the current tile (0..1 in x and y)
oe_layer_uid:(int) Unique ID of the active layer
oe_layer_order:(int) Render order of the active layer
oe_layer_opacity:
 (float) Opacity [0..1] of the active layer

Vertex attributes:

oe_terrain_attr:
 (vec4) elements 0-2 hold the unit height vector for a terrain vertex, and element 3 holds the raw terrain elevation value
oe_terrain_attr2:
 (vec4) element 0 holds the parent tile’s elevation value; elements 1-3 are currently unused.

Shared Image Layers

By default, osgEarth gives you access to the layer it’s currently drawing (via the oe_layer_tex uniform; see above). But sometimes you want to access more than one layer at a time. For example, you might have a masking layer that indicates land vs. water. You may not actually want to draw this layer, but you want to use it to modulate another visible layer.

You can do this using shared image layers. In the Map, mark an image layer as shared (using ImageLayerOptions::shared()) and the renderer will make it available to all the other layers in a secondary sampler.

Please refer to osgearth_sharedlayer.cpp for a usage example!

Customizing the Shader Factory

This is amore advanced topic. If you want to replace osgEarth’s built-in shader functions, you can install a custom ShaderFactory. The ShaderFactory is stored in the osgEarth Registry and contains all the methods for creating the built-in functions. You can install your own ShaderFactory like so:

#include <osgEarth/ShaderFactory>
...

class CustomShaderFactory : public osgEarth::ShaderFactory
{
    ... override desired methods here ...
};
...

osgEarth::Registry::instance()->setShaderFactory( new CustomShaderFactory() );

This method is good for replacing osgEarth’s built-in lighting shader code. HOWEVER: be aware that override the built-in texturing functions may not work. This is because osgEarth’s image layer composition mechanisms override these methods themselves to perform layer rendering.