#version 330 core
// Feature 9: PPI Shoreline / Terrain
// Each vertex encodes (bearingDeg, rangeMiles) in polar radar coordinates.
// The vertex shader converts to NDC and computes sweep-based fade on the GPU,
// so the geometry buffer is static — only uniforms change each frame.
layout(location = 0) in vec2 aBearingRange;  // x = bearing °CW-from-N, y = range miles

uniform vec2  uCenter;      // NDC centre of PPI scope
uniform float uPpiRx;       // ppiR * 2.0 / W  (one full-radius in NDC, x axis)
uniform float uPpiRy;       // ppiR * 2.0 / H  (one full-radius in NDC, y axis)
uniform float uMaxRange;    // current max range (miles)
uniform float uSweepAngle;  // current sweep angle (degrees CW from N)
uniform float uSweepDegPS;  // sweep speed (degrees / second)
uniform float uPersist;     // phosphor persistence duration (seconds)

out float vFade;    // 0..1 brightness multiplier
out float vActive;  // 1.0 when sweep head is over this point

const float PI     = 3.14159265358979;
const float THRESH = 3.5;   // degrees — active window around sweep head

void main()
{
    float bearDeg   = aBearingRange.x;
    float rangeMi   = aBearingRange.y;
    float rangeFrac = rangeMi / uMaxRange;

    // Push out-of-range points outside the NDC clip volume
    if (rangeFrac > 1.0) {
        gl_Position = vec4(2.0, 2.0, 2.0, 1.0);
        vFade   = 0.0;
        vActive = 0.0;
        return;
    }

    // Convert polar radar coords → NDC
    // North (0°) = +Y in NDC; East (90°) = +X in NDC
    float bearRad = bearDeg * PI / 180.0;
    float nx = uCenter.x + rangeFrac * uPpiRx * sin(bearRad);
    float ny = uCenter.y + rangeFrac * uPpiRy * cos(bearRad);
    gl_Position  = vec4(nx, ny, 0.0, 1.0);
    gl_PointSize = 2.0;

    // Degrees the sweep has travelled past this point since last illumination
    float angBehind    = mod(uSweepAngle - bearDeg + 360.0, 360.0);
    float timeSinceLit = angBehind / uSweepDegPS;
    vFade   = clamp(1.0 - timeSinceLit / uPersist, 0.0, 1.0);
    vActive = (angBehind < THRESH) ? 1.0 : 0.0;
}