MeshFlow

MeshFlow is a video stabilization algorithm described in the 2016 paper MeshFlow: Minimum Latency Online Video Stabilization by S. Liu et al. This Python implementation of MeshFlow stabilizes video offline (after recording).

At a high level, MeshFlow stabilizes a video in four steps.

1. MeshFlow estimates the unstabilized video's motion by placing a mesh over it and tracking how each mesh vertex moves. Each vertex moves with its nearby features.
2. MeshFlow computes how each mesh vertex should move in the stabilized video by minimizing an energy function.
3. MeshFlow stabilizes the video by warping it so that each mesh vertex follows its stabilized motion.
4. MeshFlow crops and resizes the video to fit its initial dimensions.

Because MeshFlow works with sparse mesh vertex motions instead of dense pixel motions, the algorithm is relatively computationally cheap. Note that this implementation was built for experimentation and is not optimized for speed.

Usage

See requirements.txt for the implementation's dependencies.

Basic usage

Example

Stabilize a video by constructing a MeshFlowStabilizer object as shown below.

stabilizer = MeshFlowStabilizer()

input_path = 'videos/video-1/video-1.m4v'
output_path = 'videos/video-1/stabilized-method-original.m4v'
stabilizer.stabilize(input_path, output_path)

Advanced usage

Constructor

The MeshFlowStabilizer constructor takes the following optional arguments.

• mesh_row_count: The number of rows contained in the mesh. Note that there are 1 + mesh_row_count vertices per row. Defaults to 16.
• mesh_col_count: The number of columns contained in the mesh. Note that there are 1 + mesh_col_count vertices per column. Defaults to 16.
• mesh_outlier_subframe_row_count: The height in rows of each subframe when breaking down the image into subframes to eliminate outlying features. Defaults to 4.
• mesh_outlier_subframe_col_count: The width of columns of each subframe when breaking down the image into subframes to eliminate outlying features. Defaults to 4.
• feature_ellipse_row_count: The height in rows of the ellipse drawn around each feature to match it with vertices in the mesh. Defaults to 10.
• feature_ellipse_col_count: The width in columns of the ellipse drawn around each feature to match it with vertices in the mesh. Defaults to 10.
• homography_min_number_corresponding_features: The minimum number of features that must correspond between two frames to perform a homography. Defaults to 4.
• temporal_smoothing_radius: In the energy function used to smooth the image, the number of frames to inspect both before and after each frame when computing that frame's regularization term. Thus, the regularization term involves a sum over up to 2 * temporal_smoothing_radius frame indexes. Note that this constant is denoted as $\Omega_{t}$ in the original paper. Defaults to 10.
• optimization_num_iterations: The number of iterations of the Jacobi method to perform when minimizing the energy function. Defaults to 100.
• color_outside_image_area_bgr: The color, expressed in BGR, to display behind the stabilized footage in the output. Note that this color should be removed during cropping, but is customizable just in case. Defaults to (0, 0, 255).
• visualize: Whether or not to display a video loop of the unstabilized and cropped, stabilized videos after saving the stabilized video. Pressing Q closes the window. Defaults to False.

MeshFlow variants

In addition, stabilize takes an optional adaptive_weights_definition argument. This argument specifies how to define the energy function's adaptive weights $\lambda_t$. The argument's four MeshFlowStabilizer.possible values, listed below, each describe a "variant" of MeshFlow.

• MeshFlowStabilizer.ADAPTIVE_WEIGHTS_DEFINITION_ORIGINAL (default): Calculate the adaptive weights using the linear model presented in the original paper. I made assumptions where the paper's description was vague.
• MeshFlowStabilizer.ADAPTIVE_WEIGHTS_DEFINITION_FLIPPED: Calculate the adaptive weights using a variant of the original model in which one of the terms has had its sign flipped. Suggested here by GitHub user @LeeVinteuil.
• MeshFlowStabilizer.ADAPTIVE_WEIGHTS_DEFINITION_CONSTANT_HIGH: Set the adaptive weights to $100$. This definition appears in the MeshFlow implementation by GitHub user @sudheerachary.
• MeshFlowStabilizer.ADAPTIVE_WEIGHTS_DEFINITION_CONSTANT_LOW: Set the adaptive weights to $1$. This model is based on the authors' claim that smaller adaptive weights lead to less cropping and wobbling. Here both terms in the energy equation have equal weight.

Performance metrics

Finally, stabilize returns a tuple (cropping_ratio, distortion_score, stability_score) of three performance metrics describing the stabilized video. These metrics are described in the original paper. I made assumptions where the paper's definitions were vague.

An example of more advanced usage is shown below.

Example

stabilizer = MeshFlowStabilizer(mesh_row_count=20, mesh_col_count=20, visualize=True)

input_path = 'videos/video-1/video-1.m4v'
output_path = 'videos/video-1/stabilized-method-original.m4v'
stabilizer.stabilize(
    input_path,
    output_path,
    adaptive_weights_definition=MeshFlowStabilizer.ADAPTIVE_WEIGHTS_DEFINITION_CONSTANT_HIGH
)

Demos

Demo videos are available in the videos directory. Each subdirectory contains five videos: an unstabilized video, and four stabilized versions stabilized by the four MeshFlow variants described above.

Video credits are available in videos/credits.txt.