osgEarth全球霧效的實現解決方法

爲了模擬全球霧效，可以利用osgearth着色器合成實現，通過引入VirtualProgram屬性執行運行時着色器合成，實現覆蓋場景圖中較高位置的着色器。實現效果如圖1所示。

                                                                                           圖1 全球霧效

實現osgEarth全球霧效的關鍵代碼如下：

char s_hazeVertShader[] =
			"#version " GLSL_VERSION_STR "\n"
			"varying vec3 v_pos; \n"
			"void setup_haze(inout vec4 VertexVIEW) \n"
			"{ \n"
			"    v_pos = vec3(VertexVIEW); \n"
			"} \n";
char s_hazeFragShader[] =
			"#version " GLSL_VERSION_STR "\n"
			"varying vec3 v_pos; \n"
			"void apply_haze(inout vec4 color) \n"
			"{ \n"
			"    float dist = clamp( length(v_pos)/1000.0, 0.0, 0.80 ); \n"
			"    color = mix(color, vec4(0.6, 0.6, 0.6, 1.0), dist); \n"
			"} \n";
vp->setFunction( "setup_haze", s_hazeVertShader, osgEarth::ShaderComp::LOCATION_VERTEX_VIEW );
vp->setFunction( "apply_haze", s_hazeFragShader, osgEarth::ShaderComp::LOCATION_FRAGMENT_LIGHTING );