利用GeometryShader绘制一个立方体

张利维

我想用几何着色器画一个立方体，大致思路是从osg传入一个顶点的坐标，然后再在几何着色器中进行偏移。于是我首先想以绘制一个正方形测试一下这个思路是否可行。（在几何着色器中通过绘制4个顶点的triangle_strip实现绘制一个正方形的效果）。

但结果发现当使用一个顶点作为输入时，没有任何内容显示出来，而传入两个顶点时却可以显示出两个正方形（但也存在一些问题，比如正方形在漫游过程中会被奇怪的裁剪掉一部分）。这让我很是纠结，为什么传入一个顶点的时候不行呢？

下面是我通过传入一个顶点，在几何着色器中绘制正方形的代码，不知道为啥就是显示不出来-->_<--。求大神指点迷津
另外，我看英文版的论坛上也有一个和我一样的问题，但是没有解决问题地址

1. 
   
2. #include <osg/Geode>
   
3. #include <osg/Geometry>
   
4. #include <osgViewer/Viewer>
   
5. #include <osg/MatrixTransform>
   
6. #include <osgDB/ReadFile>
   
7. #include <osgDB/WriteFile>
   
8. #include <osg/ShapeDrawable>
   
9. #include <osg\PolygonMode>
   
10. #include <osg\LineWidth>
    
11. #include <osg/ComputeBoundsVisitor>
    
12. #include <osgUtil\Optimizer>
    
13. #include <osg/CullFace>
    
14. #include <osgViewer/ViewerEventHandlers>
    
15. #include <osgGA/TrackballManipulator>
    
16. 
    
17. #include "Voxelization.h"
    
18. #include "DataAcquireNodeVisitor.h"
    
19. #include "DrawUtils.h"
    
20. 
    
21. #include <iostream>
    
22. #include <fstream>
    
23. 
    
24. const int OSG_WIDTH = 1280;
    
25. const int OSG_HEIGHT = 960;
    
26. const osg::Vec4f FOG_COLOR = osg::Vec4f(0.6f, 0.6f, 0.7f, 1.f);
    
27. 
    
28. int main() {
    
29. 
    
30.         osgViewer::Viewer viewer;
    
31.         osg::ref_ptr<osg::Group> root = new osg::Group;
    
32.         root->addChild(createVoxel(viewer.getCamera()));
    
33.         viewer.setSceneData(root.get());
    
34.         viewer.setUpViewInWindow(100, 100, OSG_WIDTH, OSG_HEIGHT);
    
35. 
    
36.         //viewer.getCamera()->setClearColor(osg::Vec4(1.0f, 1.0f, 1.0f, 1.0f));
    
37.         /* depth settings */
    
38.         osg::StateSet* state = root->getOrCreateStateSet();
    
39.         state->setMode(GL_DEPTH_TEST, osg::StateAttribute::ON);
    
40. 
    
41.         osg::Camera* camera = viewer.getCamera();
    
42.         camera->setClearColor(FOG_COLOR);
    
43.         camera->setClearMask(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    
44. 
    
45.         viewer.setCameraManipulator(new osgGA::TrackballManipulator);
    
46.         viewer.realize();
    
47.         while (!viewer.done()) {
    
48.                 camera->setClearMask(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    
49.                 viewer.frame();
    
50.         }
    
51. 
    
52.         return 0;
    
53. }

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传入一个顶点

1. 
   
2. osg::ref_ptr<osg::Geode> createVoxel(osg::Camera* camera)
   
3. {
   
4.         osg::ref_ptr<osg::Vec3Array> vertices = new osg::Vec3Array;
   
5.         //vertices->push_back(osg::Vec3(0.0f, 1.0f, 0.0f));
   
6.         vertices->push_back(osg::Vec3(1.0f, 0.0f, 0.0f));
   
7. 
   
8.         // create geometry
   
9.         osg::ref_ptr<osg::Geometry> geom = new osg::Geometry;
   
10.         geom->addPrimitiveSet(new osg::DrawArrays(GL_POINTS, 0, vertices->size()));
    
11.         geom->setUseDisplayList(false);
    
12.         geom->setVertexArray(vertices);
    
13.         // set attributes
    
14.         geom->setVertexAttribArray(0, vertices, osg::Array::BIND_PER_VERTEX);
    
15.        
    
16.         // create shader
    
17.         osg::ref_ptr<osg::Program> program = new osg::Program;
    
18. 
    
19.         osg::ref_ptr<osg::Shader> vertShader = new osg::Shader(osg::Shader::VERTEX);
    
20.         if (!vertShader->loadShaderSourceFromFile("Shaders/voxelDraw.vert"))
    
21.                 std::cerr << "Could not read VERTEX shader from file" << std::endl;
    
22.         program->addShader(vertShader);
    
23. 
    
24.         osg::ref_ptr<osg::Shader> geomShader = new osg::Shader(osg::Shader::GEOMETRY);
    
25.         if (!geomShader->loadShaderSourceFromFile("Shaders/voxelDraw.geom"))
    
26.                 std::cerr << "Could not read GEOMETRY shader from file" << std::endl;
    
27.         program->addShader(geomShader);
    
28. 
    
29.         osg::ref_ptr<osg::Shader> fragShader = new osg::Shader(osg::Shader::FRAGMENT);
    
30.         if (!fragShader->loadShaderSourceFromFile("Shaders/voxelDraw.frag"))
    
31.                 std::cerr << "Could not read FRAGMENT shader from file" << std::endl;
    
32.         program->addShader(fragShader);
    
33. 
    
34.         // geode
    
35.         osg::ref_ptr<osg::Geode> geode = new osg::Geode;
    
36.         geode->addDrawable(geom.get());
    
37. 
    
38.         osg::StateSet* state = geode->getOrCreateStateSet();
    
39.         state->setAttributeAndModes(program.get(), osg::StateAttribute::ON);
    
40. 
    
41.         // add uniforms
    
42.         osg::Uniform* modelViewProjectionMatrix = new osg::Uniform(osg::Uniform::FLOAT_MAT4, "ModelViewProjectionMatrix");
    
43.         modelViewProjectionMatrix->setUpdateCallback(new ModelViewProjectionMatrixCallback(camera));
    
44.         state->addUniform(modelViewProjectionMatrix);
    
45. 
    
46.         osg::Uniform* viewportVector = new osg::Uniform(osg::Uniform::FLOAT_VEC2, "Viewport");
    
47.         viewportVector->setUpdateCallback(new ViewportCallback(camera));
    
48.         state->addUniform(viewportVector);
    
49. 
    
50.         osg::Uniform* cameraEye = new osg::Uniform(osg::Uniform::FLOAT_VEC4, "CameraEye");
    
51.         cameraEye->setUpdateCallback(new CameraEyeCallback(camera));
    
52.         state->addUniform(cameraEye);
    
53. 
    
54.         // state settings
    
55.         osg::LineWidth* lw = new osg::LineWidth;
    
56.         lw->setWidth(10.f);
    
57.         state->setAttribute(lw, osg::StateAttribute::ON);
    
58.         return geode.release();
    
59. }
    

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顶点着色器

1. 
   
2. #version 330
   
3. 
   
4. uniform mat4 ModelViewProjectionMatrix;
   
5. 
   
6. layout(location = 0) in vec4 position;
   
7. out mat4 transform;
   
8. out vec4 pos;
   
9. 
   
10. void main()
    
11. {
    
12.         gl_Position = ModelViewProjectionMatrix * position;
    
13.         transform = ModelViewProjectionMatrix;
    
14.         pos = position;
    
15.        
    
16. }
    

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几何着色器

1. 
   
2. #version 330
   
3. 
   
4. layout(points) in;
   
5. layout(triangle_strip,max_vertices = 8) out;
   
6. 
   
7. uniform vec2 Viewport;
   
8. in mat4 transform[];
   
9. in vec4 pos[];
   
10. out vec4 color;
    
11. void main()
    
12. {
    
13.         //这里我是想在世界坐标系下进行偏移，然后再进行视图变换和投影
    
14.         gl_Position = transform[0] * (pos[0] + vec4(0.0,0.0,0.0,1.0));EmitVertex();
    
15.         gl_Position = transform[0] * (pos[0] + vec4(0.5,0.0,0.0,1.0));EmitVertex();
    
16.         gl_Position = transform[0] * (pos[0] + vec4(0.0,0.0,0.5,1.0));EmitVertex();
    
17.         gl_Position = transform[0] * (pos[0] + vec4(0.5,0.0,0.5,1.0));EmitVertex();
    
18.         EndPrimitive();
    
19. 
    
20.         color = pos[0];
    
21. }

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片段着色器

1. 
   
2. #version 330
   
3. in vec4 color;
   
4. void main()
   
5. {
   
6.         gl_FragColor = vec4(color.xyz,1.0);
   
7. }

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下面是之前提到的，正方形被诡异的裁剪了的情况