#version 150

uniform mat4 modelViewProjectionMatrix;
uniform float gridSize;

in vec2 texcoord;
in vec4 position;

out vec2 tex_c;

void main()
{
    tex_c = texcoord;

    vec2 resolution = vec2(1280.0, 960.0);

    // Transform the vertex position to clip space
    vec4 clipPos = modelViewProjectionMatrix * position;
    
    // Perform perspective division
    vec3 ndcPos = clipPos.xyz / clipPos.w;
    
    // Convert to screen space
    vec2 screenPos = (ndcPos.xy + 1.0) * 0.5 * resolution;
    
    // Snap to grid
    screenPos = floor(screenPos / gridSize) * gridSize;
    
    // Convert back to NDC space
    ndcPos.xy = (screenPos / resolution) * 2.0 - 1.0;
    
    // Reconstruct the clip space position
    clipPos = vec4(ndcPos * clipPos.w, clipPos.w);
    
    gl_Position = clipPos;
}