HiEI: A Universal Framework for Generating High-quality Emerging Images from Natural Images

Emerging images (EIs) are a type of stylized image that consists of discrete speckles with irregular shapes and sizes, colored only in black and white. EIs have significant applications that can contribute to the study of perceptual organization in cognitive psychology and serve as a CAPTCHA mechanism. However, generating high-quality EIs from natural images faces the following challenges: 1) color quantization--how to minimize perceptual loss when reducing the color space of a natural image to 1-bit; 2) perceived difficulty adjustment--how to adjust the perceived difficulty for object discovery and recognition. This paper proposes a universal framework HiEI to generate high-quality EIs from natural images, which contains three modules: the human-centered color quantification module (TTNet), the perceived difficulty control (PDC) module, and the template vectorization (TV) module. TTNet and PDC modules are specifically designed to address the aforementioned challenges. Experimental results show that compared to the existing EI generation methods, HiEI can generate EIs with superior content and style quality while offering more flexibility in controlling perceived difficulty. In particular, we experimently demonstrate that EIs generated by HiEI can effectively defend against attacks from deep network-based visual models, confirming their viability as a CAPTCHA mechanism.