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diff --git a/third_party/libwebrtc/video/alignment_adjuster.cc b/third_party/libwebrtc/video/alignment_adjuster.cc
new file mode 100644
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+++ b/third_party/libwebrtc/video/alignment_adjuster.cc
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+/*
+ *  Copyright (c) 2020 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "video/alignment_adjuster.h"
+
+#include <algorithm>
+#include <limits>
+
+#include "absl/algorithm/container.h"
+#include "rtc_base/logging.h"
+
+namespace webrtc {
+namespace {
+// Round each scale factor to the closest rational in form alignment/i where i
+// is a multiple of `requested_alignment`. Each resolution divisible by
+// `alignment` will be divisible by `requested_alignment` after the scale factor
+// is applied.
+double RoundToMultiple(int alignment,
+                       int requested_alignment,
+                       VideoEncoderConfig* config,
+                       bool update_config) {
+  double diff = 0.0;
+  for (auto& layer : config->simulcast_layers) {
+    double min_dist = std::numeric_limits<double>::max();
+    double new_scale = 1.0;
+    for (int i = requested_alignment; i <= alignment;
+         i += requested_alignment) {
+      double dist = std::abs(layer.scale_resolution_down_by -
+                             alignment / static_cast<double>(i));
+      if (dist <= min_dist) {
+        min_dist = dist;
+        new_scale = alignment / static_cast<double>(i);
+      }
+    }
+    diff += std::abs(layer.scale_resolution_down_by - new_scale);
+    if (update_config) {
+      RTC_LOG(LS_INFO) << "scale_resolution_down_by "
+                       << layer.scale_resolution_down_by << " -> " << new_scale;
+      layer.scale_resolution_down_by = new_scale;
+    }
+  }
+  return diff;
+}
+}  // namespace
+
+// Input: encoder_info.requested_resolution_alignment (K)
+// Input: encoder_info.apply_alignment_to_all_simulcast_layers (B)
+// Input: vector config->simulcast_layers.scale_resolution_down_by (S[i])
+// Output:
+// If B is false, returns K and does not adjust scaling factors.
+// Otherwise, returns adjusted alignment (A), adjusted scaling factors (S'[i])
+// are written in `config` such that:
+//
+// A / S'[i] are integers divisible by K
+// sum abs(S'[i] - S[i]) -> min
+// A integer <= 16
+//
+// Solution chooses closest S'[i] in a form A / j where j is a multiple of K.
+
+int AlignmentAdjuster::GetAlignmentAndMaybeAdjustScaleFactors(
+    const VideoEncoder::EncoderInfo& encoder_info,
+    VideoEncoderConfig* config,
+    absl::optional<size_t> max_layers) {
+  const int requested_alignment = encoder_info.requested_resolution_alignment;
+  if (!encoder_info.apply_alignment_to_all_simulcast_layers) {
+    return requested_alignment;
+  }
+
+  if (requested_alignment < 1 || config->number_of_streams <= 1 ||
+      config->simulcast_layers.size() <= 1) {
+    return requested_alignment;
+  }
+
+  // Update alignment to also apply to simulcast layers.
+  const bool has_scale_resolution_down_by = absl::c_any_of(
+      config->simulcast_layers, [](const webrtc::VideoStream& layer) {
+        return layer.scale_resolution_down_by >= 1.0;
+      });
+
+  if (!has_scale_resolution_down_by) {
+    // Default resolution downscaling used (scale factors: 1, 2, 4, ...).
+    size_t size = config->simulcast_layers.size();
+    if (max_layers && *max_layers > 0 && *max_layers < size) {
+      size = *max_layers;
+    }
+    return requested_alignment * (1 << (size - 1));
+  }
+
+  // Get alignment for downscaled layers.
+  // Adjust `scale_resolution_down_by` to a common multiple to limit the
+  // alignment value (to avoid largely cropped frames and possibly with an
+  // aspect ratio far from the original).
+  const int kMaxAlignment = 16;
+
+  for (auto& layer : config->simulcast_layers) {
+    layer.scale_resolution_down_by =
+        std::max(layer.scale_resolution_down_by, 1.0);
+    layer.scale_resolution_down_by =
+        std::min(layer.scale_resolution_down_by, 10000.0);
+  }
+
+  // Decide on common multiple to use.
+  double min_diff = std::numeric_limits<double>::max();
+  int best_alignment = 1;
+  for (int alignment = requested_alignment; alignment <= kMaxAlignment;
+       ++alignment) {
+    double diff = RoundToMultiple(alignment, requested_alignment, config,
+                                  /*update_config=*/false);
+    if (diff < min_diff) {
+      min_diff = diff;
+      best_alignment = alignment;
+    }
+  }
+  RoundToMultiple(best_alignment, requested_alignment, config,
+                  /*update_config=*/true);
+
+  return std::max(best_alignment, requested_alignment);
+}
+}  // namespace webrtc