Add StatePayload and serialization helpers for web viewer

PiperOrigin-RevId: 956400340
Change-Id: Ibe049afe10eca85e5fb6d718a78863a641b864bb

python/mujoco/experimental/studio/web/state_payload.h

@@ -0,0 +1,128 @@
+// Copyright 2026 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// This file defines the serialization format for the web viewer's browser
+// client render payload containing the data needed so that the browser can
+// render the scene using the following call:
+//
+// Render(model, data, perturb, camera, vis_options, width, height, extra_geoms)
+//
+// The arguments come from the Python process:
+//
+//   * model       : fetched once over HTTP as /model.mjb; its runtime-mutable
+//                   parts (opt/vis/stat) are re-sent in the render state block.
+//   * data        : streamed as the physics state vector (mjSTATE_INTEGRATION);
+//                   the browser recomputes the rest via mj_setState/mj_forward.
+//   * width/height: the browser canvas size.
+//   * extra_geoms : optional variable-size kTagExtraGeoms block.
+//   * ...         : the rest of the arguments are sent as a fixed-size block
+//
+// The payload (SerializeStatePayload) is a sequence of tagged blocks:
+//
+//   [StatePayloadHeader][u32 tag][u32 size][payload]...
+//
+// The payload is serialized by Python, sent over the /state WebSocket, and
+// parsed by the browser.
+//
+// TODO(matijak): Try shrinking the physics block: float32 (or quantized) values
+// instead of doubles, and/or delta-encoding against the client's last-acked
+// payload. The /state ack (web_server.py) tells the server which snapshot each
+// client last applied, which is the baseline that delta compression needs. For
+// 100humanoids.xml the payload is ~181 KB of doubles and dominates slow links.
+
+#ifndef MUJOCO_PYTHON_EXPERIMENTAL_STUDIO_WEB_STATE_PAYLOAD_H_
+#define MUJOCO_PYTHON_EXPERIMENTAL_STUDIO_WEB_STATE_PAYLOAD_H_
+
+#include <cstddef>
+#include <cstdint>
+#include <vector>
+
+#include <mujoco/mujoco.h>
+
+namespace mujoco::studio {
+
+// "MJWS" as little-endian bytes. This magic constant identifies the
+// StateServer WebSocket payload header and helps detect malformed or
+// misrouted messages.
+constexpr uint32_t kStatePayloadMagic =
+    'M' | ('J' << 8) | ('W' << 16) | ('S' << 24);
+constexpr uint16_t kStatePayloadVersion = 1;
+
+struct StatePayloadHeader {
+  uint32_t magic = kStatePayloadMagic;
+  uint16_t version = kStatePayloadVersion;
+  uint16_t nblocks = 0;
+  // CRC32 of the model's MJB bytes. When this changes, the browser must
+  // refetch /model.mjb before applying any further state.
+  uint32_t model_crc32 = 0;
+};
+static_assert(sizeof(StatePayloadHeader) == 12);
+
+// Block tags. Readers must skip unknown tags.
+enum StateBlockTag : uint32_t {
+  kTagPhysicsState = 1,  // [i32 mjtState spec signature][mjtNum values...]
+  kTagRenderState = 2,   // fixed-size block of kRenderStateSize bytes
+  kTagExtraGeoms = 3,    // n x mjvGeom (n = size / sizeof(mjvGeom))
+};
+
+struct StateBlockHeader {
+  uint32_t tag = 0;
+  uint32_t size = 0;
+};
+static_assert(sizeof(StateBlockHeader) == 8);
+
+// Fixed byte size of the render state block appended after physics state.
+// These are plain C structs of int/float/double members whose total size is
+// fixed, independent of the model and generally negligible compared to the size
+// of the physics state
+constexpr size_t kRenderStateSize =
+    sizeof(mjvCamera) + sizeof(mjvPerturb) + sizeof(mjvOption) +
+    sizeof(mjOption) + sizeof(mjVisual) + sizeof(mjStatistic) + mjNRNDFLAG;
+
+// Maximum number of extra geoms serialized per frame. Bounds the shared
+// memory buffer the StateServer allocates; WebViewer truncates longer lists.
+constexpr uint32_t kMaxExtraGeoms = 1024;
+
+// Upper bound of a serialized payload, used to size the StateServer's shared
+// memory buffer. `physics_bytes` is mj_stateSize(...) * sizeof(mjtNum).
+size_t MaxStatePayloadSize(size_t physics_bytes);
+
+// Serialize the complete state payload sent over the state WebSocket.
+std::vector<std::byte> SerializeStatePayload(
+    uint32_t model_crc32, int32_t physics_spec, const void* physics,
+    size_t physics_bytes, const mjvCamera& camera, const mjvPerturb& perturb,
+    const mjvOption& vis_options, const mjOption& opt, const mjVisual& vis,
+    const mjStatistic& stat, const std::vector<uint8_t>& render_flags,
+    const mjvGeom* extra_geoms, size_t extra_geom_count);
+
+// Parsed view into a serialized payload. Pointers alias the input buffer and
+// are NOT guaranteed to be aligned; so you must memcpy the data out before use.
+struct StatePayloadView {
+  uint32_t model_crc32 = 0;
+  int32_t physics_spec = 0;
+  const std::byte* physics = nullptr;
+  size_t physics_bytes = 0;
+  const std::byte* render_state = nullptr;  // kRenderStateSize bytes when non-null
+  const std::byte* extra_geoms = nullptr;   // extra_geom_count * sizeof(mjvGeom)
+  size_t extra_geom_count = 0;
+};
+
+// Parses a payload produced by SerializeStatePayload. Returns false if the
+// buffer is malformed (bad magic/version or out-of-bounds block). Blocks
+// with unknown tags are skipped.
+bool ParseStatePayload(const void* data, size_t size, StatePayloadView* out);
+
+}  // namespace mujoco::studio
+
+#endif  // MUJOCO_PYTHON_EXPERIMENTAL_STUDIO_WEB_STATE_PAYLOAD_H_