opengl - Faulty geometry shaders to visualise normals -

somehing wrong visualising normals.

as can see in video, normals faulty , seem move. duck , sphere loaded .dea files (with assimp) , hard coded cube myself.

here shaders:

vertex shader:

#version 430  layout(location = 0) in vec3 inpos; layout(location = 1) in vec3 innormal; out vec4 vertex_normal; void main(void) {     gl_position = vec4(inpos, 1.0);     vertex_normal = vec4(innormal, 1.0f); } 

geometry shader:

#version 430  layout(triangles) in; layout(line_strip, max_vertices = 6) out;  uniform mat4 projectionmatrix; uniform mat4 viewmatrix; uniform mat4 modelmatrix; uniform mat4 normalmatix;  in vec4 vertex_normal[3]; out vec4 outcolor;  void main(void) {        //the "normalviewprojection" matrix normals     mat4 nvp = projectionmatrix * viewmatrix * normalmatix;     //the modelviewprojection matrix vertices     mat4 mvp = projectionmatrix * viewmatrix * modelmatrix;      //normals transformed screen space     const vec4 normals[] = {      normalize(nvp * vertex_normal[0]),     normalize(nvp * vertex_normal[0]),     normalize(nvp * vertex_normal[0]),     };      const float normallength = 1.2f;      //gl_in.length() = 3, since working triangles                for(int = 0; < gl_in.length(); i++)     {         outcolor = normals[i];         const vec4 position = mvp * gl_in[i].gl_position;         //first vertex         gl_position = position;         emitvertex();         //second vertex         gl_position = position + normals[i] * normallength;         emitvertex();                //send line fragment shader         endprimitive();         } } 

fragment shader:

#version 430  layout(location = 0) out vec4 fragcolor; in vec4 outcolor; void main(void) {         fragcolor = vec4(outcolor.xyz, 1.0f); } 

i have tried multiple tutorials. tutorial1 tutorial2 , few more out results.

one has distinguish between 2 different types of vectors here:

• position vectors, describe position in 3d space. example vertices, and
• direction vectors, describe direction, not position. normals example such direction vectors.

projections gives correct results position vectors not direction vectors. , problem in code lies: treating normals (directions) positions.

in opinion, correct code has somehow this:

... for(int = 0; < gl_in.length(); i++) {     const vec4 position = mvp * gl_in[i].gl_position;     //first vertex     gl_position = position;     emitvertex();     //second vertex     const vec4 position2 = mvp * vec4(gl_in[i].gl_position.xyz + vertex_normal[i].xyz, 1.0);     gl_position = position2;     emitvertex(); } ... 

here position of tip of normal first calculated in model space (gl_in[i].gl_position.xyz + vertex_normal[i].xyz) , projected.

edit: when using scalings in model matrix , want have normals @ same lenght, might have transform normals , positions world space before adding:

mat4 vp = projectionmatrix * viewmatrix;  ... for(int = 0; < gl_in.length(); i++) {     const vec4 position_ws = modelmatrix * gl_in[i].gl_position;     //first vertex     gl_position = vp * position_ws;     emitvertex();     //second vertex     const vec4 normal_ws = normalize(normalmatrix * vertex_normal[i]);     gl_position = vp * vec4(position_ws.xyz + normal_ws.xyz, 1.0);     emitvertex(); } ...