Hi Robert, I'm happy to see your new work on shader composition. Naturally I have some questions :)
In my opinion the biggest problem that shader composition (SC) needs to address is the code maintenance nightmare of having to copy, paste, or edit other peoples' shaders in order to add your own functionality. So I am eager to understand how you will address that. How can I add a new shader component without editing the main() functions to support it? For example, take the shaders in the terrain displacement code. I want to add some gamma correction code into the fragment stage. Will I be able to do that without editing your shaders too? Glenn Waldron / @glennwaldron On Fri, Feb 13, 2015 at 1:01 PM, Robert Osfield <[email protected]> wrote: > Hi All, > > Attached is a screenshot of running (clock.osgt is in OpenSceneGraph-Data): > > osgshadercomposition clock.osgt > > A very simple example that provides a single osg::Program that have five > different shader combinations used on the same subgraph to produce > different results. From left to right they are: > > 1) White unlit box - is the default implementation with no define's > supplied from osg::StateSet's so no texturing or lighting > > 2) White lit box - "GL_LIGHTING" define is provided to the shaders via > the stateset->setDefine("GL_LIGHTING"); > > 3) Textured unlit box, "GL_TEXTURE_2D" define is provide to the shader > via stateset->setDefine("GL_TEXTURE_2D"); > > 4) Textured lit box, both "GL_LIGHTING" and "GL_TEXTURE_2D" defines > are provide via stateset->setDefine("GL_TEXTURE_2D"); etc. > > 5) As for 4 but a macro function is passed to the shader via : > > stateset->setDefine("VERTEX_FUNC(v)" , "vec4(v.x, v.y, v.z > * sin(osg_SimulationTime), v.w)"); > > In each of these instances the define you provide via > StateSet::setDefine(DefineString) is mapped to a set of: > > #define DefineString > > The is passed to the shader compilation. When > StateSet::setDefine(DefineString, DefineValue) is used it's mapped to: > > #define DefineString DefineValue > > So for the case 5 the Define's passed to the StateSet map to a set of > shader lines that are inserted before the body of the shader thus: > > #define GL_LIGHTING > #define GL_TEXTURE_2D > #define VERTEX_FUNC(v) vec4(v.x, v.y, v.z * sin(osg_SimulationTime), > v.w) > > These #defines are then used by the three shaders attached to the > osg::Program in the osgshadercomposition example, the first shader is > OpenSceneGraph-Data/shader/osgshadercomposition.vert which looks like: > > -- start of osgshadercomposition.vert -- > > #pragma import_defines ( GL_LIGHTING, GL_TEXTURE_2D, VERTEX_FUNC(v) ) > > #ifdef GL_LIGHTING > // forward declare lighting computation, provided by lighting.vert shader > void directionalLight( int lightNum, vec3 normal, inout vec4 color ); > #endif > > #ifdef GL_TEXTURE_2D > varying vec2 texcoord; > #endif > > #ifdef VERTEX_FUNC > uniform float osg_SimulationTime; > #endif > > varying vec4 basecolor; > > void main(void) > { > basecolor = gl_Color; > > #ifdef GL_LIGHTING > directionalLight( 0, gl_Normal.xyz, basecolor); > #endif > > #ifdef GL_TEXTURE_2D > // if we want texturing we need to pass on texture coords > texcoord = gl_MultiTexCoord0.xy; > #endif > > #ifdef VERTEX_FUNC > gl_Position = gl_ModelViewProjectionMatrix * VERTEX_FUNC(gl_Vertex); > #else > gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex; > #endif > > } > > -- end of osgshadercomposition.vert -- > > First thing of note is the first line: > > #pragma import_defines(GL_LIGHTING, GL_TEXTURE_2D, VERTEX_FUNC(v)) > > This #pragma is read and used by the OSG to tell it what Define's to look > up and apply if they are provided in the parental chain of StateSet's, note > the matching is case sensitive and in the case of macro VERTEX_FUNC(v) you > must use exactly the same parameters including name as the matching is > doing straight std::string match internally. When using import_defines() > the defines are all treated as optional, so if they aren't supplied by the > StateSet's then nothing is passed along. Also if you switch off a define > via setDefine("GL_LIGHTING", osg::StateAttribute::OFF); then this value > will not be passed on. > > It is intended that the shaders themselves will use #ifdef blocks to > enable/disable various features so that all the various paths make sense. > This approach make it possible to implement optional varying and uniform > usage as well as operations within the main or functions. It also ensures > that the shader compilation done by the drive just compiles what is > required, it doesn't have extra uniform variables that might be constant or > varyings and uniforms that might not be used in all code paths. > > -- > > The second shader is OpenSceneGraph-Data/shaders/lighting.vert that > provides the directionalLight() function that is used by the > osgshadercomposition.vert main when GL_LIGHTING is enabled via > StateSet::setDefine("GL_LIGHTING"). Since this shader is not required when > GL_LIGHTING is not defined it's not appropriate to link the shader to > Program, so here we provide a #pragma requres(GL_LIGHTING) as a directive > to the OSG so it know when to link it. The shader looks : > > -- start of lighting.vert > > #pragma requires(GL_LIGHTING) > > void directionalLight( int lightNum, vec3 normal, inout vec4 color ) > { > vec3 n = normalize(gl_NormalMatrix * normal); > > float NdotL = dot( n, normalize(gl_LightSource[lightNum].position.xyz) > ); > NdotL = max( 0.0, NdotL ); > > float NdotHV = dot( n, gl_LightSource[lightNum].halfVector.xyz ); > NdotHV = max( 0.0, NdotHV ); > #if 1 > color *= gl_LightSource[lightNum].ambient + > gl_LightSource[lightNum].diffuse * NdotL; > #else > color *= gl_FrontLightModelProduct.sceneColor + > gl_FrontLightProduct[lightNum].ambient + > gl_FrontLightProduct[lightNum].diffuse * NdotL; > #endif > #if 0 > if ( NdotL * NdotHV > 0.0 ) > color += gl_FrontLightProduct[lightNum].specular * pow( NdotHV, > gl_FrontMaterial.shininess ); > #endif > } > > -- end of lighting.vert > > Since this shader requires GL_LIGHTING it doesn't make any sense to had an > optional code paths using this define so the shader doesn't have any. If > you did want to add in use of defines as in the osgshadercomposition.vert > shader you'd need to specify them with an additional #pragma > import_defines(...) line as per the osgshadercomposition.vert shader. > > The optional linking facility provided by #pragma requires(..) enables on > to optionally pull in whole chunks of the shader, so if you wanted you > could optional link in a geometry shader, or disable vertex shaders and > fallback to use fixed function pipeline if you so wished. It also allows > you to attach multiple shaders to the main osg::Program and have these > shaders provide the same functions but each with a different > implementation, but then use different define's to pull in the > implementation you want and no link the unneeded ones without any conflicts. > > -- > > Finally we have the osgshadercomposition.frag shader that does the > optional texture mapping, the shader looks like: > > -- start of osgshadercomposition.frag > #pragma import_defines ( GL_TEXTURE_2D ) > > #ifdef GL_TEXTURE_2D > uniform sampler2D texture0; > > varying vec2 texcoord; > #endif > > varying vec4 basecolor; > > void main(void) > { > #ifdef GL_TEXTURE_2D > gl_FragColor = texture2D( texture0, texcoord) * basecolor; > #else > gl_FragColor = basecolor; > #endif > } > -- end of osgshadercomposition.frag > > Note the #pragma import_defines pulls in the optional GL_TEXTURE_2D, and > when it's provided both the uniform sample2D texture0; and the varying > texcoords are compiled in, along with the texture2D(..) fetch in the main(). > > -- > > The use of OSG specific #pragma enables the shaders themselves to be > passed as-is to the driver to compile and have these custom #pragma's > simply ignored by the driver. This means one can also use the same shaders > in non OSG applications, such as developing the shaders in a 3rd party > tool. In this tool you'd need to manually add in the #define's to test the > different path ways out and then comment them out/remove them for the final > shaders you pass to the OSG so the StateSet::setDefine() magic can provide > all the different paths you need. > > This ability to have shaders that are readable and usable as-is without > code insertion/mucking about by the application is a real bonus. When I > was modifying the old osgshadercomposition code paths (now placed in > examples/osgshadercomposition/oldshadercomposition.cpp) I was struck by how > hard is was to work on what on earth was going on with how the C++ code > passing strings for code injection during shader composition to the final > GLSL. Basically the old way is complicated and so obfuscated it's now > seems an obviously bad approach - a case of hard problems needing complex > solutions. > > My hope is that the new "#pragma(tic) shader composition" approach is > simple to understand and use, I've certainly found it fun to start using in > - it's now used in the new osgTerrain::DisplacementTechnique to enable > toggling between different features. It's actually so simple and powerful > I can't believe I never thought of this approach years ago. I guess that > is the way sometimes with hard problems, you spend too close to a problem > that the you can't see through the complexity to find a simple solution, > but once the simple and elegant solution is presented to you is like it's > so OBVIOUS to hard to fathom why it was overlooked for so long. > > Given how much user friendly the new approach is over the previous > attempts at shader composition like the old osg::ShaderComposition > framework and the VirtualProgram approach I believe it's appropriate to > deprecate the ShaderComposition, in fact my preference would be to remove > it from the OSG as it's just adding weight and complexity to the OSG that > offers less power than the new approach. I know there are some users out > there who've worked/modified osg::ShaderComposition framework to work > better, so removing it completely will be an initial loss, but I feel > keeping the API simpler is worth that bit of pain. Porting to #pragm(tic) > shader composition should be hopefully be relatively straight forward. The > osgvirtualprogram provides another previous attempt that now feel is > eclipsed by the new approach so again I'm keen to simple remove it so user > don't got learning the wrong things. > > For osgEarth, it has it's own VirtualProgram infrastructure that works > across a range of OSG versions, but I don't expect any problems so once > OSG-3.4 with the new shader composition functionality both will happily > work together. In fact the #pragma's and StateSet::setDefine(..) > functionality should hopefully work out of the box with VirtualProgram and > make it more flexible and powerful. Longer term it could be mean that > osgEarth::VirtualProgram could be deprecated and new shaders composition > used instead, but I don't see any hurry to do this. > > Now I've written this baby, it's over to you community, see what fun > things you can dream up to do with it ;-) > > > > > > _______________________________________________ > osg-users mailing list > [email protected] > http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org > >
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