From b540909f188cf02faac79075bae23e1b11af0d90 Mon Sep 17 00:00:00 2001
From: Haroon Qureshi <haroonq@google.com>
Date: Fri, 15 May 2026 10:43:16 -0700
Subject: [PATCH] Ensure swap chains are resized appropriately.

The swap chain dimensions are a union of all render request
viewport sizes that use the swap chain.

PiperOrigin-RevId: 916078573
Change-Id: If26a9e1486006ed346227120a361773bfa23f292
---
 .../filament/filament/filament_context.cc     | 71 ++++++++++++++-----
 .../filament/filament/filament_context.h      |  3 +-
 2 files changed, 57 insertions(+), 17 deletions(-)

diff --git a/src/experimental/filament/filament/filament_context.cc b/src/experimental/filament/filament/filament_context.cc
index fdee15c2..e381d402 100644
--- a/src/experimental/filament/filament/filament_context.cc
+++ b/src/experimental/filament/filament/filament_context.cc
@@ -14,6 +14,7 @@
 
 #include "experimental/filament/filament/filament_context.h"
 
+#include <algorithm>
 #include <cstdint>
 #include <memory>
 #include <span>
@@ -86,6 +87,13 @@ mjrFrameHandle FilamentContext::Render(
     mju_error("Only one read request is supported for now.");
   }
 
+  if constexpr (UTILS_HAS_THREADING) {
+    // Wait until previous frame is completed before requesting a new frame.
+    engine_->flushAndWait();
+  }
+
+  ValidateSwapChains(requests);
+
   bool render_began = false;
   mjrRenderTarget* current_target = nullptr;
   for (const mjrRenderRequest& request : requests) {
@@ -100,22 +108,6 @@ mjrFrameHandle FilamentContext::Render(
         mju_error("Cannot read pixels from the window.");
       }
 
-      if constexpr (UTILS_HAS_THREADING) {
-        // Wait until previous frame is completed before requesting a new frame.
-        engine_->flushAndWait();
-      }
-
-      // If the window size has changed, we need to reacquire the swap chain.
-      if (request.viewport.width != window_width_ ||
-          request.viewport.height != window_height_) {
-        if (window_width_ != 0 && window_height_ != 0) {
-          engine_->destroy(window_swap_chain_);
-          window_swap_chain_ = engine_->createSwapChain(config_.native_window);
-        }
-        window_width_ = request.viewport.width;
-        window_height_ = request.viewport.height;
-      }
-
       if (!render_began) {
         render_began = renderer_->beginFrame(window_swap_chain_);
       }
@@ -196,4 +188,51 @@ void FilamentContext::GetFrameStats(mjrFrameHandle frame,
   }
 }
 
+void FilamentContext::ValidateSwapChains(
+    std::span<const mjrRenderRequest> requests) {
+  // Determine the maximum extents of the requests.
+  int max_window_width = 0;
+  int max_window_height = 0;
+  int max_offscreen_width = 0;
+  int max_offscreen_height = 0;
+
+  for (const mjrRenderRequest& request : requests) {
+    const int width = request.viewport.width + request.viewport.left;
+    const int height = request.viewport.height + request.viewport.bottom;
+    if (request.target == nullptr) {
+      max_window_width = std::max(max_window_width, width);
+      max_window_height = std::max(max_window_height, height);
+    } else {
+      max_offscreen_width = std::max(max_offscreen_width, width);
+      max_offscreen_height = std::max(max_offscreen_height, height);
+    }
+  }
+
+  if (max_window_width && max_window_height) {
+    if (max_window_width != window_width_ ||
+        max_window_width != window_height_) {
+      engine_->destroy(window_swap_chain_);
+      window_width_ = max_window_width;
+      window_height_ = max_window_height;
+      if (config_.native_window) {
+        window_swap_chain_ = engine_->createSwapChain(config_.native_window);
+      } else {
+        window_swap_chain_ =
+            engine_->createSwapChain(window_width_, window_height_);
+      }
+    }
+  }
+
+  if (max_offscreen_width && max_offscreen_height) {
+    if (max_offscreen_width != config_.width ||
+        max_offscreen_height != config_.height) {
+      engine_->destroy(offscreen_swap_chain_);
+      config_.width = max_offscreen_width;
+      config_.height = max_offscreen_height;
+      offscreen_swap_chain_ =
+          engine_->createSwapChain(config_.width, config_.height);
+    }
+  }
+}
+
 }  // namespace mujoco
diff --git a/src/experimental/filament/filament/filament_context.h b/src/experimental/filament/filament/filament_context.h
index d1b175ce..d1dfbf43 100644
--- a/src/experimental/filament/filament/filament_context.h
+++ b/src/experimental/filament/filament/filament_context.h
@@ -68,8 +68,9 @@ class FilamentContext : public mjrfContext {
   }
 
  private:
-  mjrFilamentConfig config_;
+  void ValidateSwapChains(std::span<const mjrRenderRequest> render_requests);
 
+  mjrFilamentConfig config_;
   filament::Engine* engine_ = nullptr;
   filament::Renderer* renderer_ = nullptr;
   filament::SwapChain* window_swap_chain_ = nullptr;