最简单的几何着色器（Geometry Shader）【OpenGL】【GLSL】

以绘制模型的法线的为例，效果如图：

Torus:

Dragon:

关键代码如下：

1.顶点着色器

        static const char * vs_source[] =        {            "#version 410 core                                                  \n"            "                                                                   \n"            "layout (location = 0) in vec4 position;                            \n"            "layout (location = 1) in vec3 normal;                              \n"            "                                                                   \n"            "out VS_OUT                                                         \n"            "{                                                                  \n"            "    vec3 normal;                                                   \n"            "    vec4 color;                                                    \n"            "} vs_out;                                                          \n"            "                                                                   \n"            "void main(void)                                                    \n"            "{                                                                  \n"            "    gl_Position = position;                                        \n"            "    vs_out.color = position * 2.0 + vec4(0.5, 0.5, 0.5, 0.0);      \n"            "    vs_out.normal = normalize(normal);                             \n"            "}                                                                  \n"        };

2.几何着色器

        static const char * gs_source[] ={"#version 410 core                                                      \n""                                                                       \n""layout (triangles) in;                                                 \n" // 输入的图元类型  "layout (line_strip, max_vertices = 7) out;                             \n" // 输出的图元类型和最大的顶点数  "                                                                       \n""uniform mat4 mv_matrix;                                                \n""uniform mat4 proj_matrix;                                              \n""                                                                       \n""in VS_OUT                                                              \n" // 顶点着色器输出的顶点属性  "{                                                                      \n""    vec3 normal;                                                       \n""    vec4 color;                                                        \n""} gs_in[];                                                             \n""                                                                       \n""out GS_OUT                                                             \n" // 几何着色器输出的顶点属性  "{                                                                      \n""    vec3 normal;                                                       \n""    vec4 color;                                                        \n""} gs_out;                                                              \n""                                                                       \n""uniform float normal_length = 0.4;                                     \n""                                                                       \n""void main(void)                                                        \n""{                                                                      \n""    mat4 mvp = proj_matrix * mv_matrix;                                \n""    vec3 ab = gl_in[1].gl_Position.xyz - gl_in[0].gl_Position.xyz;     \n""    vec3 ac = gl_in[2].gl_Position.xyz - gl_in[0].gl_Position.xyz;     \n""    vec3 face_normal = normalize(cross(ab, ac));                      \n""                                                                       \n""    vec4 tri_centroid = (gl_in[0].gl_Position +                        \n""                         gl_in[1].gl_Position +                        \n""                         gl_in[2].gl_Position) / 3.0;                  \n""                                                                       \n""    gl_Position = mvp * tri_centroid;                                  \n""    gs_out.normal = gs_in[0].normal;                                   \n""    gs_out.color = gs_in[0].color;                                     \n""    EmitVertex();                                                      \n"     // 生成新顶点  "                                                                       \n""    gl_Position = mvp * (tri_centroid +                                \n" // 三角图元的中心法线  "                         vec4(face_normal * normal_length, 0.0));      \n""    gs_out.normal = gs_in[0].normal;                                   \n""    gs_out.color = gs_in[0].color;                                     \n""    EmitVertex();                                                      \n""    EndPrimitive();                                                    \n"     // 结束本图元（以下为新图元）  "                                                                       \n"//"    gl_Position = mvp * gl_in[0].gl_Position;                          \n"   // 第一个顶点的法线  //"    gs_out.normal = gs_in[0].normal;                                   \n"  //"    gs_out.color = gs_in[0].color;                                     \n"  //"    EmitVertex();                                                      \n"  //"                                                                       \n"  //"    gl_Position = mvp * (gl_in[0].gl_Position +                        \n"  //"                         vec4(gs_in[0].normal * normal_length, 0.0));  \n"  //"    gs_out.normal = gs_in[0].normal;                                   \n"  //"    gs_out.color = gs_in[0].color;                                     \n"  //"    EmitVertex();                                                      \n"  //"    EndPrimitive();                                                    \n"  "                                                                       \n""    gl_Position = mvp * gl_in[0].gl_Position;                          \n" // 三角形的三个顶点  "    gs_out.normal = gs_in[0].normal;                                   \n""    gs_out.color = vec4(1.);                                     \n""    EmitVertex();                                                      \n""                                                                       \n""    gl_Position = mvp * gl_in[1].gl_Position;                          \n""    gs_out.normal = gs_in[1].normal;                                   \n""    gs_out.color = vec4(1.);                                     \n""    EmitVertex();                                                      \n""                                                                       \n""    gl_Position = mvp * gl_in[2].gl_Position;                          \n""    gs_out.normal = gs_in[2].normal;                                   \n""    gs_out.color = vec4(1.);                                     \n""    EmitVertex();                                                      \n""    EndPrimitive();                                                    \n""}                                                                      \n"};

3.片元着色器

        static const char * fs_source[] =        {            "#version 410 core                                                  \n"            "                                                                   \n"            "out vec4 color;                                                    \n"            "                                                                   \n"            "in GS_OUT                                                          \n"            "{                                                                  \n"            "    vec3 normal;                                                   \n"            "    vec4 color;                                                    \n"            "} fs_in;                                                           \n"            "                                                                   \n"            "void main(void)                                                    \n"            "{                                                                  \n"            "    color = fs_in.color * abs(normalize(fs_in.normal).z);            \n"            "}                                                                  \n"        };