OpenGL Vertex shader transformation and output - opengl

Question

I have a simple vertex shader
#version 330 core
uniform mat4 projectionMatrix;
uniform mat4 viewMatrix;
uniform mat4 modelMatrix;
in vec3 in_Position;
out vec3 pass_Color;
void main(void)
{
//gl_Position = projectionMatrix * viewMatrix * modelMatrix * vec4(in_Position, 1.0);
gl_Position = vec4(in_Position, 1.0);
pass_Color = vec3(1,1,1);
}
In my client code i have
glm::vec4 vec1(-1,-1,0,1);//first
glm::vec4 vec2(0,1,0,1);//second
glm::vec4 vec3(1,-1,0,1);//third
glm::mat4 m = projectionMatrix * viewMatrix * modelMatrix;
//translate on client side
vec1 = m * vec1;
vec2 = m * vec2;
vec3 = m * vec3;
//first vertex
vertices[0] = vec1.x;
vertices[1] = vec1.y;
vertices[2] = vec1.z;
//second
vertices[3] = vec2.x;
vertices[4] = vec2.y;
vertices[5] = vec2.z;
//third
vertices[6] = vec3.x;
vertices[7] = vec3.y;
vertices[8] = vec3.z;
Now my question if i use no matrix multiplication in the shader and none in client code this will render me a nice triangle which strectch the whole screen, so i take it the vertex shader maps cordinates its given to the screen in a cordinate system with x=-1..1 and y=-1..1
If i do the matrix multiplication in the shader everything works nice. But if i comment out the code in the shader like shown and do it on the client i get odd results. Shouldnt it yield the same result?
Have i gotten it wrong thinking the output of the vertex shader gl_Position is 2D cordinates despite being a vec4?
Thanks for any help. I really like to understand what exactly the output of the vertex shader is in terms of vertex position.

Answer 1

The problem is in your shader as it accepts only 3 components of position. It is OK to set the forth coordinate to 1 (like you do it) if the coordinate is not in projection space yet.
When you are doing the transformation in client space, the results are correct 4-component homogeneous vectors. You just need to use them as is in your vertex shader:
in vec4 in_Position.
...
gl_Position = in_Position.