Geometry shader doesn't emit geometry - c++

Question

I am setting the following pipeline:
Vertex shader gets as input 4 vertices to draw as full screen quad with triangle strip :
Vertex Shader:
#version 420 core
layout(location = 0) in vec4 position;
out gl_PerVertex
{
vec4 gl_Position;
}
;
void main()
{
gl_Position = position;
}
Then I want to do some work on them in geometry shader .But for now I just try to emit the same vertices as triangle strip from the shader just to make sure it work ok.
Geom Shader
#version 420 core
layout(invocations = 1,triangles) in;
layout(triangle_strip, max_vertices =4) out;
out gl_PerVertex
{
vec4 gl_Position;
};
void main()
{
gl_Position = vec4( 1.0, 1.0, 0.0, 1.0 );
EmitVertex();
gl_Position = vec4(-1.0, 1.0, 0.0, 1.0 );
EmitVertex();
gl_Position = vec4( 1.0,-1.0, 0.0, 1.0 );
EmitVertex();
gl_Position = vec4(-1.0,-1.0, 0.0, 1.0 );
EmitVertex();
EndPrimitive();
}
And the fragment shader:
#version 420 core
layout(binding=0) uniform sampler2D COLOR_MAP_0;
out vec4 OUTPUT;
void main(void) {
OUTPUT = vec4(1,0,0,1);
}
The client side setup is like this:
glm::vec4 va[4] ={glm::vec4(1.0, 1.0, 0.0, 1.0),glm::vec4(-1.0, 1.0, 0.0, 1.0),glm::vec4( 1.0,-1.0, 0.0, 1.0),glm::vec4(-1.0,-1.0, 0.0, 1.0)};
auto ss = sizeof(va);
glGenVertexArrays(1,&_dummyVao);
glBindVertexArray(_dummyVao);
glGenBuffers(1,&_dummyPoinBuff);
glBindBuffer(GL_ARRAY_BUFFER, _dummyPoinBuff);
glBufferData(GL_ARRAY_BUFFER, ss, &va[0], GL_STATIC_DRAW);
glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, 0, ((GLubyte *)NULL + (0)));
glEnableVertexAttribArray(0);
glBindVertexArray(0);
The draw call:
glBindVertexArray(_dummyVao);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
glBindVertexArray(0);
The weird thing is that it doesn't work!The output is empty.If I switch to emit points instead ,it does emit them.The vertex data is 100% correct.it draws the quad ok if I detach the geometry shader.What can be possibly wrong here?
UPDATE:
This variation does work.
const vec2 vert_data[4] = vec2[](
vec2(-1.0, 1.0),
vec2(-1.0, -1.0),
vec2(1.0, 1.0),
vec2(1.0, -1.0)
);
for(int i=0; i<4; i++)
{
gl_Position = vec4( vert_data[i].xy,0,1);///gl_Position;
EmitVertex();
}
EndPrimitive();
Redclare of inputs / outputs in geometry shader still didn't help:
in gl_PerVertex
{
vec4 gl_Position;
} gl_in[];
out gl_PerVertex
{
vec4 gl_Position;
};