REVIEW 3 major objections 6 minor 2 cited by
HybridGS: High-Efficiency Gaussian Splatting Data Compression using Dual-Channel Sparse Representation and Point Cloud Encoder
T0 review · 3 major / 6 minor · reviewed 2026-08-16 · deepseek-v4-flash
Pith's one-line read HybridGS compresses 3D Gaussian Splatting scenes into standard point-cloud bitstreams with roughly two seconds of coding time, at reconstruction quality comparable to state-of-the-art generative compression methods like HAC.
desk verdict HybridGS is a useful, reproducible integration of quantization-aware 3DGS generation with GPCC, but the headline 0-2s encoding claim is not supported because it excludes the 70,000-epoch training stage that baselines include in their reported times. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing mechanism is the dual-channel sparse representation paired with a standard downstream encoder. 'Dual channel' denotes two kinds of sparsity imposed during generation: attribute sparsity, in which compressible features (color and rotation) are replaced by low-dimensional latent codes decoded by a one-hidden-layer MLP — a trained analogue of PCA — and precision sparsity, in which all attributes are quantized to a chosen bit depth inside the training loop using the Robust Quantizer, whose perturbation-injected affine transform at quantization and ridge-regression reconstruction at de-quantization keep quantization differentiable. For positions, the Learnable Quantizer-based Method (LQM) translates and rescales the scene into the integer range of a chosen bit depth, decomposes each integer coordinate as an inner product of a fixed basis vector and a learnable coding vector over $\{-1,0,1\}$, and enforces primitive uniqueness with progressive pruning, so positions render directly without de-quantization. The output is an explicit integer file whose per-primitive bit cost $3(\mathrm{BD}_p+\mathrm{BD}_s)+k_c\mathrm{BD}_c+\mathrm{BD}_o+k_r\mathrm{BD}_r$ is known in closed form; GPCC then encodes geometry by octree and attributes by RAHT, and that same formula is what makes rate control possible — prune primitives to hit a target count, or reduce attribute bit depth uniformly.
What would settle it
Run both pipelines end-to-end on the same hardware and scenes: HybridGS in full (70,000-epoch generation plus GPCC coding) and a state-of-the-art generative method such as HAC, measuring total wall-clock time to a compressed bitstream and the resulting PSNR. If the generation training is counted, the speed advantage reported in Table 2 (0.66 to 1.67 seconds versus 17 to 132 seconds) shrinks to roughly the training-time ratio, which directly decides whether the speed claim describes the whole compression system or only its last stage.
Extended reading notes
Core claim
The central claim is that 3DGS compression does not have to sacrifice speed for compactness. Because 3DGS is surjective — many distinct primitive sets render to nearly identical views — the generation stage can be steered toward a representation that a conventional codec handles well, and HybridGS does exactly that in two steps. First, a dual-channel sparse representation supervises the primitive position field and the attribute bit depth during training: color and rotation are reconstructed from low-dimensional latent codes through a lightweight one-hidden-layer decoder, a trainable analogue of PCA; attributes are quantized during training with a robust quantizer; and positions are converted to unique integers through a learnable basis-vector and coding-vector decomposition, with progressive pruning and deduplication governing the primitive count. Second, the resulting explicit file is encoded by GPCC, with positions in octree mode and attributes via RAHT. On five benchmark scenes the paper reports PSNR within a small margin of HAC and CompGS(MM) while cutting coding time from tens of seconds to well under two seconds. Because HybridGS deliberately omits modules that would improve rendering quality, its declared quality ceiling is vanilla 3DGS itself, which the authors treat as a feature: compression loss stays interpretable.
Load-bearing premise
The 0 to 2 second encoding claim counts only the final GPCC point-cloud coding pass; the roughly 70,000-epoch training run that produces the compact representation is not included in that time, even though the competing methods' published encoding times include their entire compression process.
Editorial extensions
If this is right
- Coding time for 3DGS scenes drops from the tens of seconds to minutes reported for HAC, CompGS(MM), and HGSC down to roughly 0 to 2 seconds, bringing interactive streaming of preprocessed Gaussian scenes into reach.
- Because the output is a standard point-cloud bitstream, 3DGS compression can ride on existing codec deployments and hardware rather than requiring a custom neural decoder at the receiver.
- Rate control becomes predictable: target bitrates are reached by pruning primitives or lowering feature bit depth against a known per-primitive bit budget, whereas RD-trained generative methods only discover their achieved rate after optimization.
- The deliberate absence of quality-improving modules caps HybridGS at vanilla 3DGS quality, and the paper reports that pruning can occasionally even raise PSNR slightly by removing redundant primitives — evidence that primitive count is not a strict quality ceiling.
Reading between the lines
- Read as an end-to-end system, HybridGS suits offline asset preparation: the 70,000-epoch training that creates each compact scene happens once, and the 0 to 2 second figure covers only the coding pass, so the practical niche is fast streaming or delivery of already-processed scenes, not live encoding.
- The paper's own numbers show GPCC's lossless ratio on 3DGS data (about 1.3 to 1.5 times) is far below its 3 to 4 times on dense point clouds; codec-side work tuned to 3DGS's locally dense, globally sparse geometry could roughly double the compression at no quality cost.
- Because position uniqueness is enforced during generation, the explicit integer files are also ready-made inputs for learning-based point cloud codecs, whose destructive deduplication of duplicated points would otherwise be a source of loss.
- The reported divergence between training-view and test-view PSNR under pruning suggests that compression-quality studies on 3DGS should report both; a method could look better on held-out views than it actually is on the views it was trained on.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes HybridGS, a two-stage 3D Gaussian Splatting (3DGS) compression framework. In the first stage, a dual-channel sparse representation is trained for 70,000 epochs, producing an explicit, compact 3DGS point cloud with quantized integer positions and low-dimensional latent features for color and rotation. In the second stage, the resulting point cloud is compressed with the standard GPCC codec, using octree coding for positions and RAHT for attributes, producing a standard bitstream. The paper claims that this design provides reconstruction quality comparable to state-of-the-art generative 3DGS compression methods while being much faster, with encoding and decoding 'typically ranging between 0s and 2s'. It also introduces two rate-control mechanisms based on primitive pruning and bit-depth reduction, supported by a formula that estimates bitrate using a fixed lossless compression ratio L=1.3.
Significance. If the speed and quality claims were supported, the work would be valuable for practical 3DGS streaming and standardization, since it would demonstrate that standard point cloud codecs can be used on compact, quantization-aware 3DGS representations. The paper includes several positive elements: the code is publicly available; the experiments cover multiple datasets and scenes; the appendix provides ablations for position uniqueness, outlier removal, and different quantizers; and the underlying idea of coupling quantization-aware training with a standard codec is coherent and worth pursuing. However, the central speed claim is not established by the reported measurements, and the reconstruction-quality comparison is overstated relative to the actual tables. These issues affect the paper's headline contributions and cannot be resolved by minor edits.
major comments (3)
- [Abstract and Section 4.2.2, Table 2] The claim that HybridGS encoding/decoding 'typically ranges between 0s and 2s' is not supported by the experiments. Section 4.2.2 states that the reported times are 'CPU computation time for HybridGS with GPCC' and that 'Data I/O time is excluded', so Table 2 measures only the GPCC tail of the pipeline after the 70,000-epoch 3DGS training stage (Section 4.1). For a compression system, encoding is the full path from source input to bitstream; the comparison methods' times, such as HAC's 85.03s and CompGS(MM)'s 36.29s, include their full end-to-end optimization. If the 70,000-epoch training is counted, HybridGS's encoding time is not 'between 0s and 2s' and is not 'evidently' faster than the baselines. Because the speed advantage is the paper's headline contribution, this is a load-bearing flaw.
- [Section 4.2.1 and Table 1] The abstract and Section 1 state that HybridGS 'provides comparable reconstruction performance against state-of-the-art methods', but the quantitative results do not support this. On 'playroom', HAC (λ=0.0005) achieves 30.84 dB PSNR at 6.86 MB, whereas HybridGS (kc=6, HR) achieves 29.89 dB at 16.08 MB; on 'bicycle', HAC (λ=0.004) achieves 24.81 dB at 26.99 MB, whereas HybridGS (kc=6, HR) achieves 24.10 dB at 30.21 MB. In these and most other rows, HybridGS is worse on both rate and distortion than HAC. The paper's own Limitations section concedes that 'the optimal compression efficiency of HybridGS is lower than end-to-end generation compression methods using RD loss as supervision.' The 'comparable' wording is therefore misleading and should be revised to a more precise statement of the trade-off.
- [Table 2 and Section 4.1] The coding-time comparison in Table 2 is not apples-to-apples even beyond the measurement-boundary issue. HybridGS timings were measured on the authors' machine ('Intel Core i9-14900HX, NVIDIA RTX 4090 Laptop', Section A.1), while the comparison times for HAC, CompGS(MM), and HGSC are taken from their original papers or official code (Section 4.1), which may involve different hardware, software stacks, and optimization levels. A fair speed comparison requires either re-running the baselines on the same machine with the same measurement protocol, or clearly stating the hardware configuration of each reported number. As presented, the speed advantage is confounded by both unequal scope and unequal platform.
minor comments (6)
- [Section 3.2.2 and Table 4] The rate-control formula in Eq. (8) uses a fixed lossless compression ratio L=1.3, but Table 4 shows rate errors such as a target of 10 MB for 'train' producing 8.59 MB, and the text attributes this to point density influencing L. The fixed-L assumption should be presented with an explicit caveat or replaced by a density-dependent estimate, since the authors already note that 'the denser the point cloud, the higher the compression ratio'.
- [Section 4.2.2] The statement 'Data I/O time is excluded' appears only in the experimental section. Since the abstract's 'between 0s and 2s' claim is central to the paper's framing, the measurement scope should be stated clearly wherever the speed claim is made, otherwise readers will reasonably interpret the numbers as end-to-end encoding time.
- [Table 2] Table 2 reports coding times for only 'bicycle' and 'room', although Table 1 includes results for five scenes. Adding timing results for the other scenes would strengthen the generality of the speed claim.
- [Table 3] Table 3's header 'Total Size' is ambiguous because it includes pre-GPCC sizes in parentheses and the bitstream size as the outer value. Clarifying that 'Total Size' is the GPCC output size and the parenthetical values are the explicit 3DGS file sizes would improve readability.
- [Appendix A.6, Table 7] The dataset name 'drjoshson' is a typo and should read 'drjohnson'.
- [Section 4.1] The sentence selecting 'the samples in the 50,000 and 70,000 epochs as High and Low (i.e., pruning 47% and 75% primitives)' would benefit from a brief explanation of why these two epochs were chosen, especially for reproducibility of the reported HR/LR points.
Circularity Check
No circularity found: the central claims rest on external baselines and disclosed measurements; the speed comparison asymmetry is a measurement-boundary issue, not a circular derivation.
full rationale
HybridGS's derivation chain does not reduce any claimed result to its inputs by construction. The compact-representation generation (LQM, robust quantization, latent-feature decoding) is trained against a rendering loss, and the output sizes and PSNRs are measured comparisons against external baselines (HAC, CompGS, GGSC, HGSC). The appendix honestly states a limitation: the optimal compression efficiency of HybridGS is lower than end-to-end generation compression methods using RD loss. The abstract's 0-2s encoding claim is an empirical measurement-boundary question, because Section 4.2.2 records only the GPCC CPU time and excludes data I/O and the 70,000-epoch generation stage, while baseline times include their full compression optimization; this asymmetry is a correctness or measurement-scope concern, not a circular reduction, since no equation or parameter is defined in terms of the claimed outcome. The only fitted constant, the lossless compression ratio L=1.3, is explicitly disclosed as a preliminary setting and is checked against measured real rates (Table 4), so it is not a hidden prediction. Self-citations (GGSC, point-cloud quality metrics) serve as prior-work baselines and metrics, and no load-bearing argument is justified solely by a self-citation chain. Consequently, no circular step can be exhibited and the correct finding is no significant circularity.
Assumptions & free parameters
free parameters (6)
- Latent feature dimensions kc, kr =
kc=3 or 6, kr=2
- Bit depth BD =
16 for position and all attributes
- RQ regularization lambda =
1e-2
- Lossless compression ratio estimate L =
1.3
- Training schedule (T, Td, Tp, Tu, intervals) =
T=70000, Td=15000, Tp=36000, Tu=66000, prune interval 2500, prune 0.1%/step
- Scaling and scaling-lr adjustment =
0.2*log(k)*scaling_lr(t1)
assumptions (6)
- domain assumption 3DGS is surjective: different primitive sets can render perceptually close views.
- domain assumption Color and rotation features are low-rank compressible; scaling is not.
- ad hoc to paper GPCC lossless compression ratio is roughly stable for a given point density (L=1.3 for 3DGS).
- domain assumption Integer 3DGS positions can be rendered without dequantization by correspondingly scaling and translating the camera.
- standard math Balanced ternary decomposition via basis vector [2^(N-2),...,1] and coding vector in {-1,0,1} can represent any integer position in range.
- standard math Straight-through estimator (STE) and robust quantizer (RQ) provide usable gradients for quantization-aware training.
Cite this review
Pith. "Pith review of HybridGS: High-Efficiency Gaussian Splatting Data Compression using Dual-Channel Sparse Representation and Point Cloud Encoder." pith.science (2026). https://pith.science/paper/EOT2UHS6
@misc{pith2026250501938,
author = {Pith},
title = {Pith review of: HybridGS: High-Efficiency Gaussian Splatting Data Compression using Dual-Channel Sparse Representation and Point Cloud Encoder},
year = {2026},
howpublished = {\url{https://pith.science/paper/EOT2UHS6}},
note = {Machine review of arXiv:2505.01938}
}
read the original abstract
Most existing 3D Gaussian Splatting (3DGS) compression schemes focus on producing compact 3DGS representation via implicit data embedding. They have long coding times and highly customized data format, making it difficult for widespread deployment. This paper presents a new 3DGS compression framework called HybridGS, which takes advantage of both compact generation and standardized point cloud data encoding. HybridGS first generates compact and explicit 3DGS data. A dual-channel sparse representation is introduced to supervise the primitive position and feature bit depth. It then utilizes a canonical point cloud encoder to perform further data compression and form standard output bitstreams. A simple and effective rate control scheme is proposed to pivot the interpretable data compression scheme. At the current stage, HybridGS does not include any modules aimed at improving 3DGS quality during generation. But experiment results show that it still provides comparable reconstruction performance against state-of-the-art methods, with evidently higher encoding and decoding speed. The code is publicly available at https://github.com/Qi-Yangsjtu/HybridGS.
Figures
Figures from the paper (12 more)
Forward citations
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Reviewed August 16, 2026 · model on record in the stance chip above.
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