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Fix _parallelogram_to_bounding_boxes #9181

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Fix _parallelogram_to_bounding_boxes #9181

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Fix `_parallelogram_to_bounding_boxes`
AntoineSimoulin Aug 12, 2025
0a2a419
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AntoineSimoulin Aug 12, 2025
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28 changes: 20 additions & 8 deletions torchvision/transforms/v2/functional/_geometry.py
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Original file line number Diff line number Diff line change
Expand Up @@ -462,21 +462,33 @@ def _parallelogram_to_bounding_boxes(parallelogram: torch.Tensor) -> torch.Tenso
dy12 = parallelogram[..., 1] - parallelogram[..., 3]
diag13 = torch.sqrt(dx13**2 + dy13**2)
diag24 = torch.sqrt(dx42**2 + dy42**2)
mask = diag13 > diag24

# Calculate rotation angle in radians
r_rad = torch.atan2(dy12, dx12)
cos, sin = torch.cos(r_rad), torch.sin(r_rad)

# Calculate width using the angle between diagonal and rotation
w = torch.where(
mask,
diag13 * torch.abs(torch.sin(torch.atan2(dx13, dy13) - r_rad)),
diag24 * torch.abs(torch.sin(torch.atan2(dx42, dy42) - r_rad)),
)
w13 = diag13 * torch.abs(torch.sin(torch.atan2(dx13, dy13) - r_rad))
delta_x13 = w13 * cos
delta_y13 = w13 * sin
w24 = diag24 * torch.abs(torch.sin(torch.atan2(dx42, dy42) - r_rad))
delta_x24 = w24 * cos
delta_y24 = w24 * sin

# Calculate the area of the triangle formed by the three points
# Area = 1/2 * |det([x1, y1, 1], [x2, y2, 1], [x3, y3, 1])|
# For points (x1, y1), (x1 - delta_x, y1 + delta_y), (x3, y3)
# This simplifies to 1/2 * |delta_x * (y3 - y1) - delta_y * (x3 - x1)|
area13 = 0.5 * torch.abs(delta_x13 * dy13 - delta_y13 * dx13)
# For points (x4, y4), (x4 - delta_x, y4 + delta_y), (x2, y2)
# This simplifies to 1/2 * |delta_x * (y2 - y4) - delta_y * (x2 - x4)|
area24 = 0.5 * torch.abs(delta_x24 * dy42 - delta_y24 * dx42)

# We keep the rectangle with the smallest area
mask = area13 < area24
delta_x = torch.where(mask, delta_x13, delta_x24)
delta_y = torch.where(mask, delta_y13, delta_y24)

delta_x = w * cos
delta_y = w * sin
# Update coordinates to form a rectangle
# Keeping the points (x1, y1) and (x3, y3) unchanged.
out_boxes[..., 2] = torch.where(mask, parallelogram[..., 0] + delta_x, parallelogram[..., 2])
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