Initial code for screen differ that divides screen into blocks and calculate

a 'minimal' set of rectangles that covers the changed region.

BUG=none
TEST=new unittests added

Review URL: http://codereview.chromium.org/2714007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@49325 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 226 insertions, 0 deletions

diff --git a/remoting/host/differ.cc b/remoting/host/differ.cc
new file mode 100644
index 0000000..e0a8125
--- /dev/null
+++ b/remoting/host/differ.cc
@@ -0,0 +1,226 @@
+// Copyright (c) 2010 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "remoting/host/differ.h"
+
+#include "base/logging.h"
+
+namespace remoting {
+
+Differ::Differ(int width, int height, int bpp) {
+  // Dimensions of screen.
+  width_ = width;
+  height_ = height;
+  bytes_per_pixel_ = bpp;
+  bytes_per_row_ = width_ * bytes_per_pixel_;
+
+  // Calc number of blocks (full and partial) required to cover entire image.
+  // One additional row/column is added as a boundary on the right & bottom.
+  diff_info_width_ = (width_ + kBlockSize - 1) / kBlockSize + 1;
+  diff_info_height_ = (height_ + kBlockSize - 1) / kBlockSize + 1;
+  diff_info_size_ = diff_info_width_ * diff_info_height_ * sizeof(DiffInfo);
+  diff_info_.reset(new uint8[diff_info_size_]);
+}
+
+void Differ::CalcDirtyRects(const void* prev_buffer, const void* curr_buffer,
+                            DirtyRects* rects) {
+  if (!rects) {
+    return;
+  }
+  rects->clear();
+
+  if (!prev_buffer || !curr_buffer) {
+    return;
+  }
+
+  // Identify all the blocks that contain changed pixels.
+  MarkDirtyBlocks(prev_buffer, curr_buffer);
+
+  // Now that we've identified the blocks that have changed, merge adjacent
+  // blocks to minimize the number of rects that we return.
+  MergeBlocks(rects);
+}
+
+void Differ::MarkDirtyBlocks(const void* prev_buffer, const void* curr_buffer) {
+  memset(diff_info_.get(), 0, diff_info_size_);
+
+  // Calc number of full blocks.
+  int x_full_blocks = width_ / kBlockSize;
+  int y_full_blocks = height_ / kBlockSize;
+
+  // Calc size of partial blocks which may be present on right and bottom edge.
+  int partial_column_width = width_ - (x_full_blocks * kBlockSize);
+  int partial_row_height = height_ - (y_full_blocks * kBlockSize);
+
+  // Offset from the start of one block-column to the next.
+  int block_x_offset = bytes_per_pixel_ * kBlockSize;
+  // Offset from the start of one block-row to the next.
+  int block_y_stride = (width_ * bytes_per_pixel_) * kBlockSize;
+  // Offset from the start of one diff_info row to the next.
+  int diff_info_stride = diff_info_width_ * sizeof(DiffInfo);
+
+  const uint8* prev_block_row_start = static_cast<const uint8*>(prev_buffer);
+  const uint8* curr_block_row_start = static_cast<const uint8*>(curr_buffer);
+  uint8* diff_info_row_start = static_cast<uint8*>(diff_info_.get());
+
+  for (int y = 0; y < y_full_blocks; y++) {
+    const uint8* prev_block = prev_block_row_start;
+    const uint8* curr_block = curr_block_row_start;
+    uint8* diff_info = diff_info_row_start;
+
+    for (int x = 0; x < x_full_blocks; x++) {
+      DiffInfo diff = DiffBlock(prev_block, curr_block, bytes_per_row_);
+      if (diff != 0) {
+        // Mark this block as being modified so that it gets incorporated into
+        // a dirty rect.
+        *diff_info = diff;
+      }
+      prev_block += block_x_offset;
+      curr_block += block_x_offset;
+      diff_info += sizeof(DiffInfo);
+    }
+
+    // If there is a partial column at the end, handle it.
+    if (partial_column_width != 0) {
+      // TODO(garykac): Handle last partial column.
+    }
+
+    // Update pointers for next row.
+    prev_block_row_start += block_y_stride;
+    curr_block_row_start += block_y_stride;
+    diff_info_row_start += diff_info_stride;
+  }
+  if (partial_row_height != 0) {
+    // TODO(garykac): Handle last partial row.
+  }
+}
+
+DiffInfo Differ::DiffBlock(const uint8* prev_buffer, const uint8* curr_buffer,
+                           int stride) {
+  const uint8* prev_row_start = prev_buffer;
+  const uint8* curr_row_start = curr_buffer;
+
+  // Number of uint64s in each row of the block.
+  // This must be an integral number.
+  int int64s_per_row = (kBlockSize * bytes_per_pixel_) / sizeof(uint64);
+  DCHECK(((kBlockSize * bytes_per_pixel_) % sizeof(uint64)) == 0);
+
+  for (int y = 0; y < kBlockSize; y++) {
+    const uint64* prev = reinterpret_cast<const uint64*>(prev_row_start);
+    const uint64* curr = reinterpret_cast<const uint64*>(curr_row_start);
+
+    // Check each row in uint64-sized chunks.
+    // Note that this check may straddle multiple pixels. This is OK because
+    // we're interested in identifying whether or not there was change - we
+    // don't care what the actual change is.
+    for (int x = 0; x < int64s_per_row; x++) {
+      if (*prev++ != *curr++) {
+        return 1;
+      }
+    }
+    prev_row_start += stride;
+    curr_row_start += stride;
+  }
+  return 0;
+}
+
+DiffInfo Differ::DiffPartialBlock(const uint8* prev_buffer,
+                                  const uint8* curr_buffer,
+                                  int stride, int width, int height) {
+  const uint8* prev_row_start = prev_buffer;
+  const uint8* curr_row_start = curr_buffer;
+
+  for (int y = 0; y < height; y++) {
+    const uint8* prev = prev_row_start;
+    const uint8* curr = curr_row_start;
+    for (int x = 0; x < width; x++) {
+      for (int b = 0; b < bytes_per_pixel_; b++) {
+        if (*prev++ != *curr++) {
+          return 1;
+        }
+      }
+    }
+    prev_row_start += bytes_per_row_;
+    curr_row_start += bytes_per_row_;
+  }
+  return 0;
+}
+
+void Differ::MergeBlocks(DirtyRects* rects) {
+  DCHECK(rects);
+  rects->clear();
+
+  uint8* diff_info_row_start = static_cast<uint8*>(diff_info_.get());
+  int diff_info_stride = diff_info_width_ * sizeof(DiffInfo);
+
+  for (int y = 0; y < diff_info_height_; y++) {
+    uint8* diff_info = diff_info_row_start;
+    for (int x = 0; x < diff_info_width_; x++) {
+      if (*diff_info != 0) {
+        // We've found a modified block. Look at blocks to the right and below
+        // to group this block with as many others as we can.
+        int left = x * kBlockSize;
+        int top = y * kBlockSize;
+        int width = 1;
+        int height = 1;
+        *diff_info = 0;
+
+        // Group with blocks to the right.
+        // We can keep looking until we find an unchanged block because we
+        // have a boundary block which is never marked as having diffs.
+        uint8* right = diff_info + 1;
+        while (*right) {
+          *right++ = 0;
+          width++;
+        }
+
+        // Group with blocks below.
+        // The entire width of blocks that we matched above much match for
+        // each row that we add.
+        uint8* bottom = diff_info;
+        bool found_new_row;
+        do {
+          found_new_row = true;
+          bottom += diff_info_stride;
+          right = bottom;
+          for (int x2 = 0; x2 < width; x2++) {
+            if (*right++ == 0) {
+              found_new_row = false;
+            }
+          }
+
+          if (found_new_row) {
+            height++;
+
+            // We need to go back and erase the diff markers so that we don't
+            // try to add these blocks a second time.
+            right = bottom;
+            for (int x2 = 0; x2 < width; x2++) {
+              *right++ = 0;
+            }
+          }
+        } while (found_new_row);
+
+        // Add rect to list of dirty rects.
+        width *= kBlockSize;
+        if (left + width > width_) {
+          width = width_ - left;
+        }
+        height *= kBlockSize;
+        if (top + height > height_) {
+          height = height_ - top;
+        }
+        rects->push_back(gfx::Rect(left, top, width, height));
+      }
+
+      // Increment to next block in this row.
+      diff_info++;
+    }
+
+    // Go to start of next row.
+    diff_info_row_start += diff_info_stride;
+  }
+}
+
+}  // namespace remoting