用密度图直接预测分形参数,速度比传统方法快数十倍。
Amortized Set Prediction for Inverse IFS Reconstruction from Density Maps

- 通过密度图输入,用神经网络一次性预测分形变换参数。
- 30步优化后质量优于双倍预算的随机初始化方法,且平均提速12至2600倍。
- 适用于需要快速生成高质量分形图像的场景,如图像压缩或生成建模。
迭代函数系统(IFS)通过少量收缩仿射变换生成自相似分形。正向映射计算简单且明确,但逆问题——从图像估计变换参数——通常需逐图优化。本文提出一种学习型估算器,仅通过一次前向传播,即可直接从访问频率密度图预测仿射变换集合,从而摊销逆问题的计算成本。设计遵循两个约束:其一,密度图无法唯一确定参数,因此以重建效果为评估标准;采用匈牙利匹配处理无序参数集,并以真实参数作为稳定训练目标;其二,利用已知的精确正向模型,可生成完全合成的训练对,并支持仅图像输入的测试时微调。在分布内测试中,摊销初始化加少量精修,相较同等预算的随机初始化优化,处于更优的质量-速度前沿;30步精修(约0.56秒/样本)仍优于两倍预算基线。扩展至1000步显示,优势不仅在于速度:摊销初始化更频繁达到高质量重建。在真实图像(MNIST与Fashion-MNIST)上,该方法在密度指标上平均优于已有逐图优化器,且速度提升12至2600倍。
原文摘要 · Abstract (English)
Iterated Function Systems (IFS) generate self-similar fractals from a few contractive affine maps. The forward map from parameters to images is computationally inexpensive and well understood, whereas the inverse problem of estimating maps from an image is difficult and is typically handled by per-image optimization. We replace this loop with a single forward pass of a learned estimator that predicts the affine-map set directly from a visit-frequency density map, thereby amortizing the inverse problem. The design follows two constraints. First, density maps do not uniquely identify IFS parameters, so evaluation is based on reconstruction rather than parameter recovery; unordered map sets are handled by Hungarian matching, and ground-truth parameters provide a stable training surrogate. Second, the fully known forward model lets us generate exact synthetic training pairs and also supports image-only test-time refinement. On in-distribution tests, amortized initialization plus a few refinement steps lies on a better quality--speed frontier than equal-budget random-initialized per-image optimization, and a 30-step refinement (about $0.56$ s per sample) remains better than a doubled-budget baseline. Extending optimization to 1000 steps shows that the benefit is not only speed: amortized initialization reaches high-quality reconstructions more frequently than random starts. On real images (MNIST and Fashion-MNIST), it improves density metrics on average over a published per-image optimizer while being roughly 12 to 2600 times faster.
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