diff --git a/src/presa_mano/perception/depth_align.py b/src/presa_mano/perception/depth_align.py
new file mode 100644
index 0000000..7c3fbfd
--- /dev/null
+++ b/src/presa_mano/perception/depth_align.py
@@ -0,0 +1,98 @@
+"""Forward-project an IR depth image into RGB pixel coordinates.
+
+Vectorised equivalent of librealsense `rs.align(stream.color)`. Each IR pixel
+with valid depth is back-projected to 3D in the IR camera frame, transformed to
+the RGB camera frame via the rigid extrinsics, and projected with RGB
+intrinsics. On collisions the closest point wins (z-buffer).
+
+Output is a uint16 array in millimeters, shape (rgb_height, rgb_width), with 0
+on pixels that received no IR projection.
+"""
+from __future__ import annotations
+
+import numpy as np
+
+from .depth_to_rgb import CameraRegistration
+
+_SENTINEL = np.iinfo(np.uint32).max
+
+
+def align_depth_to_rgb(
+    depth_ir_mm: np.ndarray,
+    reg: CameraRegistration,
+) -> np.ndarray:
+    """Forward-project IR depth → RGB pixel grid.
+
+    Parameters
+    ----------
+    depth_ir_mm : (H_ir, W_ir) uint16
+        Depth in millimeters, IR camera frame. Zero means "no depth".
+    reg : CameraRegistration
+        IR and RGB intrinsics + IR→RGB rigid transform (translation in meters).
+
+    Returns
+    -------
+    (reg.rgb_height, reg.rgb_width) uint16
+        Depth in millimeters in RGB pixel coordinates; 0 where no IR pixel
+        projected.
+    """
+    if depth_ir_mm.ndim != 2:
+        raise ValueError(f"depth_ir_mm must be 2D, got shape {depth_ir_mm.shape}")
+    if depth_ir_mm.shape != (reg.ir_height, reg.ir_width):
+        raise ValueError(
+            f"depth_ir_mm shape {depth_ir_mm.shape} ≠ IR resolution "
+            f"({reg.ir_height}, {reg.ir_width}) from registration"
+        )
+
+    H_rgb, W_rgb = reg.rgb_height, reg.rgb_width
+    H_ir, W_ir = reg.ir_height, reg.ir_width
+
+    valid = depth_ir_mm > 0
+    if not valid.any():
+        return np.zeros((H_rgb, W_rgb), dtype=np.uint16)
+
+    v_idx, u_idx = np.where(valid)
+    z_ir_m = depth_ir_mm[v_idx, u_idx].astype(np.float64) / 1000.0
+
+    # Back-project IR pixel → 3D point in IR camera frame.
+    K_ir_inv = np.linalg.inv(reg.K_ir)
+    pixels_h = np.vstack([u_idx.astype(np.float64),
+                          v_idx.astype(np.float64),
+                          np.ones_like(u_idx, dtype=np.float64)])
+    rays_ir = K_ir_inv @ pixels_h
+    P_ir = rays_ir * z_ir_m
+
+    # IR camera frame → RGB camera frame.
+    P_rgb = reg.R_ir_to_rgb @ P_ir + reg.t_ir_to_rgb.reshape(3, 1)
+
+    z_rgb = P_rgb[2]
+    in_front = z_rgb > 1e-6
+    P_rgb = P_rgb[:, in_front]
+    if P_rgb.shape[1] == 0:
+        return np.zeros((H_rgb, W_rgb), dtype=np.uint16)
+
+    z_rgb = P_rgb[2]
+    u_rgb = reg.K_rgb[0, 0] * P_rgb[0] / z_rgb + reg.K_rgb[0, 2]
+    v_rgb = reg.K_rgb[1, 1] * P_rgb[1] / z_rgb + reg.K_rgb[1, 2]
+
+    u_int = np.round(u_rgb).astype(np.int64)
+    v_int = np.round(v_rgb).astype(np.int64)
+    z_mm = np.round(z_rgb * 1000.0).astype(np.int64)
+
+    in_bounds = (
+        (u_int >= 0) & (u_int < W_rgb)
+        & (v_int >= 0) & (v_int < H_rgb)
+        & (z_mm > 0) & (z_mm < 65535)
+    )
+    u_int = u_int[in_bounds]
+    v_int = v_int[in_bounds]
+    z_mm = z_mm[in_bounds].astype(np.uint32)
+    if z_mm.size == 0:
+        return np.zeros((H_rgb, W_rgb), dtype=np.uint16)
+
+    # Z-buffer: initialise to sentinel, reduce-min, then sentinel→0.
+    zbuf = np.full(H_rgb * W_rgb, _SENTINEL, dtype=np.uint32)
+    flat_idx = v_int * W_rgb + u_int
+    np.minimum.at(zbuf, flat_idx, z_mm)
+    zbuf[zbuf == _SENTINEL] = 0
+    return zbuf.reshape(H_rgb, W_rgb).astype(np.uint16)
diff --git a/tests/test_depth_align.py b/tests/test_depth_align.py
new file mode 100644
index 0000000..095ac27
--- /dev/null
+++ b/tests/test_depth_align.py
@@ -0,0 +1,88 @@
+"""Unit tests for align_depth_to_rgb."""
+from __future__ import annotations
+
+import numpy as np
+
+from presa_mano.perception.depth_align import align_depth_to_rgb
+from presa_mano.perception.depth_to_rgb import CameraRegistration
+
+
+def _identity_reg(W: int = 64, H: int = 48) -> CameraRegistration:
+    """Same K for RGB and IR, no extrinsic offset."""
+    K = np.array([[60.0, 0.0, W / 2.0],
+                  [0.0,  60.0, H / 2.0],
+                  [0.0,  0.0,  1.0]], dtype=np.float64)
+    return CameraRegistration(
+        K_rgb=K.copy(), K_ir=K.copy(),
+        R_ir_to_rgb=np.eye(3, dtype=np.float64),
+        t_ir_to_rgb=np.zeros(3, dtype=np.float64),
+        rgb_width=W, rgb_height=H,
+        ir_width=W, ir_height=H,
+    )
+
+
+def test_identity_alignment_preserves_depth():
+    """K_ir == K_rgb and no extrinsic → every IR pixel maps to itself."""
+    reg = _identity_reg()
+    depth_ir = np.full((reg.ir_height, reg.ir_width), 1000, dtype=np.uint16)
+    depth_rgb = align_depth_to_rgb(depth_ir, reg)
+    assert depth_rgb.shape == (reg.rgb_height, reg.rgb_width)
+    assert depth_rgb.dtype == np.uint16
+    nonzero = depth_rgb[depth_rgb > 0]
+    # Coverage close to full (rounding can drop a few border pixels).
+    assert nonzero.size >= 0.95 * depth_rgb.size, f"coverage too low: {nonzero.size}/{depth_rgb.size}"
+    # All filled pixels equal the input depth (±1 mm rounding).
+    assert np.all(np.abs(nonzero.astype(int) - 1000) <= 1)
+
+
+def test_zero_depth_pixels_stay_zero():
+    """IR pixels with depth=0 must not produce any RGB output."""
+    reg = _identity_reg()
+    depth_ir = np.zeros((reg.ir_height, reg.ir_width), dtype=np.uint16)
+    depth_rgb = align_depth_to_rgb(depth_ir, reg)
+    assert depth_rgb.shape == (reg.rgb_height, reg.rgb_width)
+    assert int(depth_rgb.sum()) == 0
+
+
+def test_sparse_pixels_preserved():
+    """Two isolated IR depths produce exactly two filled RGB pixels (identity reg)."""
+    reg = _identity_reg(W=64, H=48)
+    depth_ir = np.zeros((reg.ir_height, reg.ir_width), dtype=np.uint16)
+    depth_ir[0, 0] = 1000
+    depth_ir[reg.ir_height - 1, reg.ir_width - 1] = 1000
+    depth_rgb = align_depth_to_rgb(depth_ir, reg)
+    assert (depth_rgb > 0).sum() == 2
+
+
+def test_z_buffer_keeps_closer_point():
+    """When two IR pixels collide on the same RGB pixel, the smaller depth wins.
+
+    Force a collision with K_ir = 2 × K_rgb (same resolution), so adjacent IR
+    pixels round to the same RGB pixel."""
+    W = H = 16
+    K_rgb = np.array([[60.0, 0.0, W / 2.0],
+                      [0.0,  60.0, H / 2.0],
+                      [0.0,  0.0,  1.0]], dtype=np.float64)
+    K_ir = K_rgb.copy()
+    K_ir[0, 0] *= 2.0   # double focal length → two IR pixels collapse to one RGB pixel
+    K_ir[1, 1] *= 2.0
+    reg = CameraRegistration(
+        K_rgb=K_rgb, K_ir=K_ir,
+        R_ir_to_rgb=np.eye(3, dtype=np.float64),
+        t_ir_to_rgb=np.zeros(3, dtype=np.float64),
+        rgb_width=W, rgb_height=H, ir_width=W, ir_height=H,
+    )
+    depth_ir = np.zeros((H, W), dtype=np.uint16)
+    # Two IR pixels (5, 5) and (6, 5) both project to the same RGB pixel near (4, 5)
+    # (when projected through K_ir with depth 1m and re-projected through K_rgb).
+    depth_ir[5, 5] = 1500
+    depth_ir[5, 6] = 800
+    depth_rgb = align_depth_to_rgb(depth_ir, reg)
+    nonzero = depth_rgb[depth_rgb > 0]
+    # At least one RGB pixel must receive the closer (800) value; nothing further
+    # may be ≥ 1500 if a collision actually occurred. If they didn't collide, we
+    # see exactly two values [800, 1500]; if they did, we see only [800].
+    assert nonzero.size in (1, 2)
+    assert int(nonzero.min()) == 800
+    if nonzero.size == 1:
+        assert int(nonzero[0]) == 800   # collision happened, closer point won