2 files changed, 110 insertions, 0 deletions
diff --git a/crates/fparkan-render/src/lib.rs b/crates/fparkan-render/src/lib.rs
index 2ad4789..db0171f 100644
--- a/crates/fparkan-render/src/lib.rs
+++ b/crates/fparkan-render/src/lib.rs
@@ -1,6 +1,8 @@
 #![forbid(unsafe_code)]
 //! Camera mathematics and graphics pipeline state.
 
+pub mod water;
+
 /// A 64-byte transform block returned by the original Terrain camera ABI.
 ///
 /// The original uses two selector-dependent transform pointers.  Their exact
diff --git a/crates/fparkan-render/src/water.rs b/crates/fparkan-render/src/water.rs
new file mode 100644
index 0000000..0c7ad2a
--- /dev/null
+++ b/crates/fparkan-render/src/water.rs
@@ -0,0 +1,108 @@
+//! The GOG Terrain EMBM wave map and its world-space animation clock.
+
+/// Side length of the native signed Du/Dv texture.
+pub const EMBM_BUMP_SIZE: u32 = 128;
+/// Native default wave tiling across the bounding rectangle of reflective faces.
+pub const EMBM_BUMP_TILE: f32 = 100.0;
+/// Native diagonal bump matrix, without luminance modulation.
+pub const EMBM_COEFFICIENTS: [f32; 2] = [0.01, 0.01];
+
+/// Generates the signed bytes written by Terrain.dll RVA 0x2BF50 with CW 0x007F.
+#[must_use]
+pub fn legacy_embm_bump_map() -> Vec<[i8; 2]> {
+    let mut texels = Vec::with_capacity((EMBM_BUMP_SIZE * EMBM_BUMP_SIZE) as usize);
+    for y in 0..EMBM_BUMP_SIZE {
+        for x in 0..EMBM_BUMP_SIZE {
+            // The native constant is slightly smaller than mathematical 4*pi.
+            let angle = ((y as f32 / EMBM_BUMP_SIZE as f32 - 0.5)
+                + (x as f32 / EMBM_BUMP_SIZE as f32 - 0.5))
+                * f32::from_bits(0x4149_0fd0);
+            texels.push([
+                (angle.cos() * 64.0).round_ties_even() as i8,
+                (angle.sin() * 64.0).round_ties_even() as i8,
+            ]);
+        }
+    }
+    texels
+}
+
+/// Queue clock modulo the native ten-second wave scroll period.
+#[must_use]
+pub fn legacy_embm_phase(clock_ms: u32) -> f32 {
+    (clock_ms % 10_000) as f32 / 10_000.0
+}
+
+/// Native reflection coordinates: U reverses world Y and V reverses world X.
+#[must_use]
+pub fn legacy_embm_uv(world_xy: [f32; 2], bounds: [f32; 4]) -> Option<[f32; 2]> {
+    let span = [bounds[2] - bounds[0], bounds[3] - bounds[1]];
+    if !world_xy
+        .into_iter()
+        .chain(bounds)
+        .chain(span)
+        .all(f32::is_finite)
+        || span.iter().any(|&value| value <= 0.0)
+    {
+        return None;
+    }
+    Some([
+        1.0 - (world_xy[1] - bounds[1]) / span[1],
+        1.0 - (world_xy[0] - bounds[0]) / span[0],
+    ])
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn wave_bytes_match_native_generation_and_clock_wraps_without_float_time_loss() {
+        let texels = legacy_embm_bump_map();
+        assert_eq!(texels.len(), 128 * 128);
+        assert_eq!(
+            &texels[..16],
+            &[
+                [64, 0],
+                [64, 6],
+                [63, 12],
+                [61, 19],
+                [59, 24],
+                [56, 30],
+                [53, 36],
+                [49, 41],
+                [45, 45],
+                [41, 49],
+                [36, 53],
+                [30, 56],
+                [24, 59],
+                [19, 61],
+                [12, 63],
+                [6, 64],
+            ]
+        );
+        assert_eq!(texels[16], [0, 64]);
+        assert_eq!(texels[32], [-64, 0]);
+        assert_eq!(texels[48], [0, -64]);
+        let native_hash = texels
+            .iter()
+            .flatten()
+            .fold(0xcbf29ce484222325u64, |hash, &byte| {
+                (hash ^ u64::from(byte as u8)).wrapping_mul(0x100000001b3)
+            });
+        assert_eq!(native_hash, 0x0b92dc6a9e482625);
+        assert_eq!(legacy_embm_phase(0), 0.0);
+        assert_eq!(legacy_embm_phase(5_000), 0.5);
+        assert_eq!(legacy_embm_phase(10_000), 0.0);
+        assert_eq!(legacy_embm_phase(u32::MAX), 0.7295);
+    }
+
+    #[test]
+    fn mapping_uses_the_reflective_rectangle_and_keeps_axis_order() {
+        let bounds = [100.0, 200.0, 300.0, 600.0];
+        assert_eq!(legacy_embm_uv([100.0, 200.0], bounds), Some([1.0, 1.0]));
+        assert_eq!(legacy_embm_uv([300.0, 600.0], bounds), Some([0.0, 0.0]));
+        assert_eq!(legacy_embm_uv([150.0, 500.0], bounds), Some([0.25, 0.75]));
+        assert_eq!(legacy_embm_uv([0.0; 2], [1.0; 4]), None);
+        assert_eq!(legacy_embm_uv([f32::NAN, 0.0], bounds), None);
+    }
+}