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An Overlay is the result of comparing two Blocks from different drawing revisions. When you compare two drawings, Bedrock matches sheets by sheet number, then matches blocks within each sheet, aligns the images, and generates change visualization. Each overlay contains:
  • Combined overlay — Both versions with changes highlighted
  • Additions image — Only new elements (green)
  • Deletions image — Only removed elements (red)
  • Aligned images — Both versions registered to the same coordinate system
  • Similarity score — How similar the two blocks are (0-1)

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Comparison flow

Drawing Rev A                    Drawing Rev B
  └─→ Sheets                      └─→ Sheets
        └─→ Matched by sheet number ──→
              └─→ Blocks matched ──→ Overlay created
You can compare at two levels:
  • Drawing comparison — Compares all matching sheets between two revisions
  • Sheet comparison — Compares two specific sheets directly

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Similarity scores

ScoreInterpretation
0.95+Nearly identical, minor changes
0.80-0.95Moderate changes
0.60-0.80Significant changes
< 0.60Major revisions or different content