cv-raft-optical-flow-extraction

Overview

For any task where motion matters more than appearance — collision detection, action recognition, anomaly localization — feeding a CNN raw RGB pairs is wasteful: the network has to rediscover motion from scratch on every frame pair. Pretrained RAFT (Recurrent All-Pairs Field Transforms) gives you dense optical flow as a precomputed input channel, with state-of-the-art accuracy on Sintel/KITTI and a one-call API. The output is an HxWx2 field where each pixel holds (dx, dy) displacement in pixels. Stack it as channels 4-5 of an RGB+flow input, or convert to magnitude/angle and use as a standalone heatmap.

Quick Start

import torch, cv2, numpy as np
from raft.core.raft import RAFT
from raft.core.utils.utils import InputPadder
from raft.config import RAFTConfig

config = RAFTConfig(dropout=0, alternate_corr=False, small=False, mixed_precision=False)
model = RAFT(config).to(device).eval()
model.load_state_dict(torch.load('raft-sintel.pth', map_location=device))

def to_tensor(img):
    return torch.from_numpy(img).permute(2, 0, 1).float()[None].to(device)

@torch.no_grad()
def flow_between(f1, f2, iters=20):
    i1, i2 = to_tensor(f1), to_tensor(f2)
    padder  = InputPadder(i1.shape)
    i1, i2  = padder.pad(i1, i2)
    _, flow = model(i1, i2, iters=iters, test_mode=True)
    return flow[0].permute(1, 2, 0).cpu().numpy()      # (H, W, 2)

Workflow

1. Download the RAFT checkpoint matching your domain — raft-sintel.pth for natural video, raft-kitti.pth for driving / outdoor scenes
2. Read consecutive frames with cv2.VideoCapture and convert to float tensors
3. Wrap inputs in InputPadder — RAFT requires dimensions divisible by 8
4. Run with iters=20 for highest accuracy, iters=12 for 1.6x faster inference at minor quality cost
5. Cache the flow alongside the original frames; recomputing per epoch is the dominant training cost
6. Either stack as 5-channel input (RGB + flow_x + flow_y) or compute magnitude np.linalg.norm(flow, axis=-1) for a single-channel motion map

Key Decisions

• Pretrained, not trained from scratch: RAFT needs millions of synthetic flow pairs to train; transfer always wins on real video.
• small=False: the small model is 5x faster but ~30% worse on fast motion; not worth it for offline preprocessing.
• Cache aggressively: flow is 8 bytes/pixel and rebuilds slowly; store as .npy files keyed by (video, frame).
• Backward flow if needed: call with (f2, f1) for backward flow — useful for occlusion handling.
• InputPadder not manual padding: RAFT's stride-8 requirement varies by resolution; the helper handles it.
• iters=20 at inference, not training: training the full RAFT downstream is rare; use it as a frozen feature extractor.

References

• Optical flow estimation using RAFT