/*
 * MIT License
 * Author: Mark Allyn
 *
 * graticule.vert — vertex shader for the incandescent bearing graticule,
 * the yellow cursor, and any other 2-D screen-space line geometry.
 *
 * Vertices are supplied in window pixels (origin top-left, y down).
 * The shader converts them to OpenGL NDC (origin bottom-left, y up).
 */
#version 330 core

layout(location = 0) in vec2 aPos;   // screen pixels, top-left origin

uniform vec2 u_viewportSize;         // (WINDOW_WIDTH, WINDOW_HEIGHT)

void main() {
    // Convert: pixel → NDC
    // ndc.x: 0 → -1, width → +1
    // ndc.y: 0 → +1 (top), height → -1 (bottom)
    vec2 ndc = vec2(
         aPos.x / u_viewportSize.x * 2.0 - 1.0,
        -aPos.y / u_viewportSize.y * 2.0 + 1.0
    );
    gl_Position = vec4(ndc, 0.0, 1.0);
}