#version 330 core

// The grid coordinates (x, y) where x < columns and y < rows
layout (location = 0) in vec2 grid_coord;

// The background color for this cell in RGBA (0 to 1.0)
layout (location = 1) in vec4 bg_color_in;

// The background color for this cell in RGBA (0 to 1.0)
flat out vec4 bg_color;

uniform vec2 cell_size;
uniform mat4 projection;

void main() {
    // Top-left cell coordinates converted to world space
    vec2 cell_pos = cell_size * grid_coord;

    // Turn the cell position into a vertex point depending on the
    // gl_VertexID. Since we use instanced drawing, we have 4 vertices
    // for each corner of the cell. We can use gl_VertexID to determine
    // which one we're looking at. Using this, we can use 1 or 0 to keep
    // or discard the value for the vertex.
    vec2 position;
    position.x = (gl_VertexID == 0 || gl_VertexID == 1) ? 1. : 0.;
    position.y = (gl_VertexID == 0 || gl_VertexID == 3) ? 0. : 1.;
    cell_pos = cell_pos + cell_size * position;

    gl_Position = projection * vec4(cell_pos, 1.0, 1.0);
    bg_color = vec4(bg_color_in.rgb / 255.0, 1.0);
}