Kestrel

A native, real-time 2D Gaussian Splatting viewer for robotics and simulation pipelines

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Kestrel is a local, single-window viewer for 2D Gaussian Splatting (surfel) models.

This project was born out of the lack of optimized Gaussian Splatting visualizers. While good viewers exist (SIBR, Viser), they rarely allow fine-grained control over the splats, making scene inspection and rendering computationally expensive even on capable hardware.

Kestrel addresses this with a set of optimization and compression techniques that significantly reduce memory usage and render time, while keeping visual quality mostly unchanged.

Features

• Frustum culling — only the splats visible in the current view are passed to the rasterizer, cutting render time proportionally to the fraction of the scene culled.
• Model compression — three levels of offline compression reduce memory footprint with minimal visual impact.
• Real-time rendering — RGB, depth, normals, curvature and more at interactive frame rates.
• Split-view — compare any two render types side by side with a draggable divider.
• Crop box — axis-aligned bounding box filter to isolate regions of interest.
• Persistent config — global defaults + per-model camera pose, compression level, and all settings saved automatically.
• Orbit + FPS camera — mouse orbit/pan/zoom, WASD translate, arrow-key FPS look.

Requirements

Python 3.9+
torch (CUDA)
numpy
plyfile
PyQt5

The 2DGS CUDA rasterizer must be built from the submodule:

pip install -e 2d_gaussian_splatting/submodules/diff-surfel-rasterization
pip install -e 2d_gaussian_splatting/submodules/simple-knn

Install Python dependencies:

pip install -r requirements.txt

Usage

Load a PLY file

python main.py path/to/scene.ply

Load from a training output directory

python main.py path/to/model_dir/
python main.py path/to/model_dir/ --iterations 30000   # specify checkpoint

Kestrel resolves <model_dir>/point_cloud/iteration_<N>/point_cloud.ply automatically.

Build the frustum-culling index at startup

python main.py scene.ply --build-index

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Overview

The window is split into a sidebar on the left and the render canvas on the right. All controls live in the sidebar; the canvas captures mouse and keyboard input for camera navigation.

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Camera Controls

Input	Action
LMB drag	Orbit around the look-at point
RMB drag	Pan the look-at point
Scroll wheel	Zoom (change distance)
W / S	Move look-at forward / backward
A / D	Strafe look-at left / right
E / Q	Translate camera + look-at along world-up
Arrow keys	FPS-style look in place (yaw / pitch)
R	Reset camera to origin

Mouse inversion, orbit speed, move speed, and world-up axis (+Z, +Y, +X, …) are all configurable in the Camera sidebar panel and persisted per model.

Frustum Culling

Kestrel builds a CPU-side octree over the splat positions. Before each frame, it runs a 5-plane Gribb–Hartmann frustum test over all leaf nodes and uploads a single boolean mask to the GPU — skipping geometry that cannot be visible.

On a ~5M-splat scene, frustum culling reduces the active splat count from ~2.2M to ~700K per frame, bringing rasterization time from ~140ms down to ~20ms.

Click Build Index in the Frustum Culling panel once; the index is saved to .kestrel/<stem>.idx next to the PLY file and reloaded automatically.

Note: the far plane is intentionally omitted from culling. Aerial scenes contain splats at large distances that the rasterizer handles correctly but a hard far-clip would discard.

Rebuild at any time if you change the leaf size (--leaf-max, default 5 000).

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Compression

L0 (Original)

L1 compression

L2 compression

L3 compression

Four compression levels are available in the Compression panel:

Level	Storage	SH degree cap	File size	Quality loss
L0	original PLY	full	1.2 GB	none
L1	fp16	full	1.2 GB	minimal (fp16 rounding)
L2	fp16 + SH degree 1	1	488 MB	minor view-dependent lighting loss
L3	fp16 + int8	0	312 GB	no view-dependent colour

Benchmarked on a 5M-splat aerial scene. L2 is the recommended default — the SH degree reduction has negligible visual impact on most outdoor scenes while halving memory usage.

Switching level compresses the PLY on a background thread and reloads the model without blocking the UI. Compressed files are cached in .kestrel/ and reused on subsequent loads — compression runs only once per level per model.

Note: performance figures shown are from benchmarks on a representative set of scenes. Actual speedup depends on scene size, splat density, and camera field of view.

Render Types

Select the output image via the Type dropdown:

Name	Description
RGB	Standard colour render
Edge	Gradient-magnitude edge map
Alpha	Accumulated opacity
Normal	World-space surface normals
View-Normal	Normals in camera space
Depth	Rendered surface depth
Depth-Distort	Depth distortion map
Depth-to-Normal	Normals derived from depth
Depth-to-Curvature	Surface curvature from depth

Split View

Enable Split View in the Render Options panel to compare two render types side by side. Independent Left and Right dropdowns pick what each half shows; the Split Pos slider (0 – 1) moves the vertical divider.

Gaussian Model Controls

The Gaussian Model panel exposes per-frame render parameters:

Control	Effect
SH Degree	Active spherical-harmonic bands (0 = diffuse only)
Opacity Threshold	Cull splats below this opacity — removes near-transparent debris
Sparsity	Render every Nth splat; useful for a quick preview on large models
Scale	Global splat size multiplier
Pointcloud	Render splat centres as points instead of Gaussians
Disk mode	Render oriented disk outlines (requires Pointcloud enabled)
Point Size	Radius of pointcloud markers

Crop Box

Enable Crop Box and drag the X / Y / Z min–max sliders to restrict rendering to an axis-aligned bounding box. Useful for isolating a single object, a floor plane, or a specific room.

Resolution & Aspect Ratio

Choose a named preset (360p – 4K) or type a custom width × height. Lock AR keeps the aspect ratio fixed when editing either dimension. The Aspect combo snaps to standard ratios (16:9, 4:3, 16:10, 21:9, …).

The render resolution is independent of the window size — the canvas scales the output image to fill the available area.

Persistent Configuration

Global config (~/.config/kestrel/config.json) — default settings shared across all models: FOV, speeds, mouse inversion, render type, overlays, etc.

Per-model config (.kestrel/<stem>_view.json) — everything in the global config plus the camera pose (look-at, distance, yaw, pitch), active compression level, split-view state, crop box, and pointcloud toggle. Saved on close, restored on open.

.kestrel/ Directory

All viewer artifacts for a given model are stored alongside the PLY file:

File	Contents
<stem>.idx	Octree frustum-culling index
<stem>_L1.ply	fp16-compressed PLY cache
<stem>_L2.ply	fp16 + SH-degree-1 PLY cache
<stem>_L3.ply	fp16 + int8-quantised PLY cache
<stem>_view.json	Per-model persistent state

CLI Reference

python main.py <ply_path> [options]

positional:
  ply_path              Path to .ply file or model directory

options:
  --iterations N        Checkpoint iteration when resolving model dir (default: 30000)
  --sh_degree N         SH degree override (-1 = auto-detect from PLY header)
  --build-index         Build / rebuild octree index at startup
  --leaf-max N          Max splats per octree leaf (default: 5000)
  --no-culling          Disable frustum culling even if an index exists
  --no-profiling        Suppress per-frame GPU timing output
  --fp16-load           Transfer tensors via fp16 during PLY load

Acknowledgements

• 2D Gaussian Splatting — Huang et al., 2024. 2D Gaussian Splatting for Geometrically Accurate Radiance Fields.
• 2D-GS-Viser-Viewer — the viser-based viewer that inspired Kestrel.