first build attempt

shaders/sweep.frag

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+/*
+ * MIT License
+ * Author: Mark Allyn
+ *
+ * sweep.frag — phosphor accumulation update shader.
+ *
+ * For each texel in the 1024×1024 phosphor FBO:
+ *   1. Decay the previous frame's energy by u_decayFactor.
+ *   2. If the texel's PPI bearing falls within the current sweep arc
+ *      [u_beamAnglePrev, u_beamAngle], add contributions from:
+ *        - range rings (beam-painted per the P7 spec)
+ *        - target echoes
+ *   3. Output the updated single-channel energy value.
+ *
+ * PPI convention: north = +y, east = +x; bearing = atan2(x, y)
+ * in degrees, clockwise from north.
+ */
+#version 330 core
+
+in vec2 vTexCoord;
+
+layout(location = 0) out vec4 fragOut;  // .r = energy; .gba unused
+
+uniform sampler2D u_prevPhosphor;  // previous frame's energy texture (GL_R32F)
+uniform float u_decayFactor;       // exp(-decay_rate * dt)
+uniform float u_beamAngle;         // current beam angle, degrees CW from north
+uniform float u_beamAnglePrev;     // beam angle at previous frame
+uniform float u_sweepBg;           // ambient sweep-line energy (makes beam visible)
+uniform float u_halfBeamDeg;       // half-beamwidth for target blobs (display widening)
+
+// Targets: .x = range_norm (0-1), .y = bearing_deg, .z = brightness, .w = size_norm
+uniform vec4  u_targets[32];
+uniform int   u_targetCount;
+
+// Range rings: up to 4 normalised radii
+uniform float u_ringRadii[4];
+uniform int   u_ringCount;
+uniform float u_ringWidth;         // half-width in normalised range units
+uniform float u_ringBrightness;
+
+// ----------------------------------------------------------------
+
+// Smallest unsigned angular distance between two bearings [0,360)
+float angleDiff(float a, float b) {
+    float d = mod(abs(a - b), 360.0);
+    return (d > 180.0) ? (360.0 - d) : d;
+}
+
+// True if bearing b is inside the arc [prev, curr] swept this frame.
+// Handles the 0/360 wraparound when the sweep crosses north.
+bool inSweep(float b, float prev, float curr) {
+    if (curr >= prev) {
+        return (b >= prev && b <= curr);
+    }
+    // Wraparound: arc crosses 360→0
+    return (b >= prev || b <= curr);
+}
+
+// ----------------------------------------------------------------
+
+void main() {
+    vec2 pos   = vTexCoord * 2.0 - 1.0;   // PPI coords: (-1,-1) SW … (+1,+1) NE
+    float rng  = length(pos);
+
+    if (rng > 1.0) {
+        fragOut = vec4(0.0);
+        return;
+    }
+
+    // Bearing: clockwise from north — atan2(east, north) = atan2(x, y)
+    float brg = degrees(atan(pos.x, pos.y));
+    if (brg < 0.0) brg += 360.0;
+
+    // Decay previous value
+    float energy = texture(u_prevPhosphor, vTexCoord).r * u_decayFactor;
+
+    if (inSweep(brg, u_beamAnglePrev, u_beamAngle)) {
+
+        float contrib = u_sweepBg;   // beam passage gives a faint ambient glow
+
+        // ---- Range rings (painted at every bearing as beam sweeps) ----
+        for (int i = 0; i < u_ringCount; i++) {
+            float d = abs(rng - u_ringRadii[i]);
+            if (d < u_ringWidth) {
+                float w = 1.0 - d / u_ringWidth;
+                contrib = max(contrib, u_ringBrightness * w * w);
+            }
+        }
+
+        // ---- Target echoes ----
+        for (int i = 0; i < u_targetCount; i++) {
+            float tRng  = u_targets[i].x;
+            float tBrg  = u_targets[i].y;
+            float tBrt  = u_targets[i].z;
+            float tSize = u_targets[i].w;
+
+            if (tRng <= 0.0 || tBrt <= 0.0) continue;
+
+            // Angular proximity: beam must be sweeping over the target's bearing
+            float dBrg = angleDiff(brg, tBrg);
+            if (dBrg >= u_halfBeamDeg) continue;
+
+            // Range proximity: pixel must be within the target blob
+            float dRng = abs(rng - tRng);
+            if (dRng >= tSize) continue;
+
+            float bw = 1.0 - dBrg / u_halfBeamDeg;         // angular taper
+            float rw = 1.0 - dRng / tSize;                  // range taper
+            contrib = max(contrib, tBrt * bw * rw);
+        }
+
+        energy = max(energy, contrib);
+    }
+
+    fragOut = vec4(clamp(energy, 0.0, 1.0), 0.0, 0.0, 1.0);
+}