The Geometry Shader is duplicating Shapes (in Processing) - opengl

Question

I'm trying to generate a simple shape with a Geometry Shader, but the shape is rendering twice and I don't know why.
First we have a really simple Vertex Shader
#version 150
in vec4 position;
void main() {
gl_Position = position;
}
Then there's a Geometry Shader thats generating a simple triangle.
#version 150
layout (triangles) in;
layout (triangle_strip, max_vertices = 5) out;
out FragData {
vec4 color;
} FragOut;
void main(){
//RED TOP LEFT
FragOut.color = vec4(1.0, 0.0, 0.0, 1.0);
gl_Position = gl_in[0].gl_Position + vec4( -1.0, 0.0, 0.0, 0.0);
EmitVertex();
//BLUE BOTTOM LEFT
FragOut.color = vec4(0., 0., 1., 1.);
gl_Position = gl_in[0].gl_Position + vec4( -1.0, -1.0, 0.0, 0.0);
EmitVertex();
//GREEN BOTTOM RIGHT
FragOut.color = vec4(0.0, 1.0, 0.0, 1.0);
gl_Position = gl_in[0].gl_Position + vec4( 1.0, -1.0, 0.0, 0.0);
EmitVertex();
EndPrimitive();
}
And finally a simple Fragment Shader
#version 150
in FragData {
vec4 color;
} FragIn;
out vec4 fragColor;
void main() {
fragColor = FragIn.color;
}
The result should be a triangle, but TWO triangles are being rendered:
Here's the result

Answer 1

The Geometry Shader is executed once for each primitive. A rect() consists of 2 triangles, so the geometry shader is executed twice and generates 2 triangle_strip primitives.
Draw a single POINTS primitive instead of the rectangle:
beginShape(POINTS);
vertex(x, y);
endShape();
Note that you need to change the Primitive input specification:
layout (triangles) in;
layout (points) in;