I'd like to display a simple UV sphere (exported from Blender) and generate lines with normal coordinates using a unique geometry shader.
In a first time, I wrote a simple geometry shader which simply return the input vertices informations to the fragment shader. For a sake of simplicity (for the exemple) I erased the luminosity calculations in the fragment shader.
Vertex shader :
#version 400
layout (location = 0) in vec3 VertexPosition;
layout (location = 1) in vec3 VertexNormal;
uniform mat4 MVP;
out vec3 VPosition;
out vec3 VNormal;
void main(void)
{
VNormal = VertexNormal;
gl_Position = vec4(VertexPosition, 1.0f);
}
Geometry shader :
#version 400
layout(points) in;
layout(line_strip, max_vertices = 2) out;
uniform mat4 MVP;
in vec3 VNormal[];
out vec3 fcolor;
void main(void)
{
float size = 2.5f;
fcolor = vec3(0.0f, 0.0f, 1.0f);
gl_Position = MVP * gl_in[0].gl_Position;
EmitVertex();
fcolor = vec3(1.0f, 1.0f, 0.0f);
gl_Position = MVP * vec4(gl_in[0].gl_Position.xyz + vec3(
VNormal[0].x * size, VNormal[0].y * size, VNormal[0].z * size), 1.0f);
EmitVertex();
EndPrimitive();
}
And the fragment shader :
#version 400
in vec3 Position;
in vec3 Normal;
in vec2 TexCoords;
out vec4 FragColor;
in vec3 fcolor;
void main(void)
{
FragColor = vec4(fcolor, 1.0f);
}
Now in the C++ code the primitive type to draw (here triangles):
glDrawArrays(GL_TRIANGLES, 0, meshList[idx]->getVertexBuffer()->getBufferSize());
And finally the output :
Until here all is ok.
Now I want to generate strands on the sphere as normals. To do the job done I wrote the following geometry shader (the vertex and fragment shaders are the sames).
#version 400
layout(points) in;
layout(line_strip, max_vertices = 2) out;
uniform mat4 MVP;
in vec3 VNormal[];
out vec3 fcolor;
void main(void)
{
float size = 1.0f;
fcolor = vec3(0.0f, 0.0f, 1.0f);
gl_Position = MVP * gl_in[0].gl_Position;
EmitVertex();
fcolor = vec3(1.0f, 1.0f, 0.0f);
gl_Position = MVP * vec4(gl_in[0].gl_Position.xyz + vec3(
VNormal[0].x * size, VNormal[0].y * size, VNormal[0].z * size), 1.0f);
EmitVertex();
EndPrimitive();
}
The input primitive type being points I modified the C++ code to draw the scene :
glDrawArrays(GL_POINTS, 0, meshList[idx]->getVertexBuffer()->getBufferSize());
And the output:
Finally if I want to get a triangle input as input primitive and a line_strip as output primitive in the geometry shader I have the following shader:
#version 400
layout(triangles, invocations = 3) in;
layout(line_strip, max_vertices = 6) out;
uniform mat4 MVP;
in vec3 VNormal[];
out vec3 fcolor;
void main(void)
{
float size = 1.0f;
for (int i = 0; i < 3; i++)
{
fcolor = vec3(0.0f, 0.0f, 1.0f);
gl_Position = MVP * gl_in[i].gl_Position;
EmitVertex();
fcolor = vec3(1.0f, 1.0f, 0.0f);
gl_Position = MVP * vec4(gl_in[0].gl_Position.xyz + vec3(
VNormal[0].x * size, VNormal[0].y * size, VNormal[0].z * size), 1.0f);
EmitVertex();
EndPrimitive();
}
}
And the output is the following :
But my goal is to display in one output the scene (sphere + strands) using the same geometry shader. I'd like to know if it's possible to do this. I don't think so because a geometry shader must have just one type of input primitive and an other one in output and not several types. I want to be sure if it's possible or not.