1. rcrnstn/glslvisnormals
  2. Commit ddbb4da27057883a8ac2f17a6fc6b6ebff7a62c0
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Add implementation

Robert Cranston authored on 22/12/2021 14:14:41
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README.md
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 A [GLSL][]/[OpenGL][] [\>=3.2][] [normal][] visualization library.
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+A [geometry shader][] is used to visualize the normals from the same geometry
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+as when drawing normally.
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+
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 [`glvisnormals`]: https://git.rcrnstn.net/rcrnstn/glvisnormals
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 [GLSL]: https://en.wikipedia.org/wiki/OpenGL_Shading_Language
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 [OpenGL]: https://en.wikipedia.org/wiki/OpenGL
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 [\>=3.2]: https://en.wikipedia.org/wiki/OpenGL#Version_history
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 [normal]: https://en.wikipedia.org/wiki/Normal_(geometry)
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+[geometry shader]: https://www.khronos.org/opengl/wiki/Geometry_Shader
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+
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+## Requirements
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+
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+[Geometry shader][]s require OpenGL [\>=3.2][] or the
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+[`ARB_geometry_shader4`][] extension.
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+
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+[`ARB_geometry_shader4`]: https://www.khronos.org/registry/OpenGL/extensions/ARB/ARB_geometry_shader4.txt
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+
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+## Usage
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+
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+Link the shader program with:
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+
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+-   `visnormals.vert`
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+-   `visnormals.geom`
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+-   `visnormals.frag`
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+
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+The following vertex attributes need to be set:
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+
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+-   `vec3 vert_position`
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+-   `vec3 vert_normal` (assumed to be of unit length)
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+-   `mat4 vert_model` (assumed to be an isometry, i.e. no scaling or skewing)
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+
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+The following uniforms need to be set:
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+
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+-   `mat4  view_projection`
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+-   `float scale`
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+-   `vec4  color`
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+
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+The following outputs are written by the fragment shader:
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+
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+-   ` vec4 frag_color`
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+
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+Make the shader program current and issue the same draw call as would be done
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+when drawing normally.
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 ## Build system
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assets/shaders/visnormals.frag
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+#version 150
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+
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+
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+uniform vec4 color;
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+
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+
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+out vec4 frag_color;
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+
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+
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+void main()
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+{
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+    frag_color = color;
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+}
assets/shaders/visnormals.geom
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+#version 150
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+
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+
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+layout(triangles) in;
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+layout(line_strip, max_vertices = 6) out;
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+
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+
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+uniform mat4  view_projection;
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+uniform float scale;
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+
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+
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+in vec4 geom_normal[3];
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+
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+
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+void primitive_(int i)
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+{
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+    gl_Position = view_projection *
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+        (gl_in[i].gl_Position);
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+    EmitVertex();
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+    gl_Position = view_projection *
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+        (gl_in[i].gl_Position + scale * geom_normal[i]);
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+    EmitVertex();
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+    EndPrimitive();
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+}
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+
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+
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+void main()
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+{
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+    primitive_(0);
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+    primitive_(1);
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+    primitive_(2);
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+}
assets/shaders/visnormals.vert
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+#version 150
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+
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+
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+in vec3 vert_position;
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+in vec3 vert_normal;
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+in mat4 vert_model;
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+
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+
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+out vec4 geom_normal;
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+
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+
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+void main()
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+{
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+    gl_Position = vert_model * vec4(vert_position, 1);
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+    geom_normal = vert_model * vec4(vert_normal,   0);
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+}