Ben, I promised to answer some of your questions today. Some time ago I was studying SoftShadowMap code and I had the feeling that I understood it. But today when I tried to look at this again to find the answers for your questions about jitter scale and softness width I realized I am not so sure anymore. So its better if I will not try to spread confusion with some intuitions that may be incorrect. But I am ready to answer the question regarding AmbientBias.
I guess that AmbientBias was introduced in early osgdepthshadowmap example (osg 1.2 ?) to achieve the similar result as fixed pipeline TEXTURE_COMPARE_FAIL_VALUE_ARB parameter. See http://oss.sgi.com/projects/ogl-sample/registry/ARB/shadow_ambient.txt If you look at shadow fragment shader, fragment coloring formula looks like this: vec4 color = gl_Color * texture2D( osgShadow_baseTexture, gl_TexCoord[0].xy ); vec4 shadow = shadow2DProj( osgShadow_shadowTexture, gl_TexCoord[1] ); gl_FragColor = color * ( osgShadow_ambientBias.x + osgShadow_ambientBias.y * shadow ); AmbientBias variable is used in shadow & soft shadow mapping techiques. Its usually used to set up lower bound for shadowing factor. By default shadow2DProj returns value from range [0..1]. If such shadow factor was used directly, shadowed areas would be completely black. Sometimes we want to make them to be only a bit darker than lit areas. So essentially AmbientBias is used to define how much shadows darken the scene. By setting AmbientBias.x to some value from range 0..1 one limits minimal shadow value (hence AmbientBias because shadowed areas are lit only by ambient component). AmbientBias.y is usually set up to 1 - AmbientBias.x but I expect that it may be also set bit larger or smaller values used to make shadow range more dynamic or flat. Having said that, I should mention that I personally don't use AmbientBias and above shadowing formula. I often substitute default shaders with shaders using formula which is presented in many ShadowMapping papers. ie: uniform dimming; // similar to AmbientBias defines minimal Diffuse amount not darkened by shadow vec4 texColor = texture2D( osgShadow_baseTexture, gl_TexCoord[0].xy ); float shade = shadow2DProj( osgShadow_shadowTexture, gl_TexCoord[1] ).x; float diffuseShade = dimming + ( 1.0 - dimming ) * shade; gl_FragColor = ( ambient + diffuseShade * diffuse) * texColor + specular * shade; But such formula requires usage of vertex shader to pass diffuse and ambient components separately. Fixed pipeline provides only gl_Color containing diffuse and ambient terms added together. Cheers, Wojtek Lewandowski _______________________________________________ osg-users mailing list osg-users@lists.openscenegraph.org http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
