# Image Storage in DNA by an Extensible Quaternary Codec System
## 4 Discussion  
With the rapid development of the internet and digital technologies, images have become one of the most critical data types on the internet, playing a pivotal role in fields such as medical diagnosis, visual positioning, facial recognition, and autonomous driving. The demand for storing high-resolution and high-color-depth images has surged, and images also serve as the foundation for video storage. DNA storage, with its ultra-high density and scalability, has emerged as one of the most promising media capable of meeting these demanding storage requirements. We have designed a true quaternary DNA storage encoding system that divides a color image into three RGB color channels to reduce redundant data and enhance storage density. Our system employs a lossless compression method, ensuring that the decompressed image is completely identical to the original with no information loss. To guarantee data accuracy and integrity, error correction codes are indispensable in DNA storage systems [34]. Therefore, we incorporated RS error correction codes into a quaternary Huffman tree [35], transcending the \"so-called\" quaternary encoding framework established by DNA fountain codes. Both in vitro and simulation experiments were successfully conducted. In the in vitro experiment, constrained by the current precision of DNA synthesis, the synthesized sequences were limited to a length of 150 nt, resulting in a net storage density of 1.41 bits/nt. In the in silico experiment, the net storage density exceeded the theoretical maximum of 2 bits/nt established by [PERSON] et al. [10]. While our method was specifically applied to color image files, for other file types such as text or audio, the ETQ encoding steps cannot be directly applied due to the absence of color channels. However, the combination of quaternary Huffman compression and RS decoding remains applicable. By omitting the steps of color channel division and flattening, the ETQ design is still effective for these data types. Hence, our research provides significant insights into the development and optimization of DNA digital storage systems and contributes to improving their performance. Unlike DNA Fountain's probabilistic encoding [10], which achieves theoretical density at the cost of complex reassembly, the ETQ guarantees deterministic reconstruction through fixed-length quaternary mapping. The ETQ's net density after adding error correction redundancy is 1.41 bits/nt in biochemical implementation--surpassing Organick's random access codec (1.16 bits/nt) [11] and approaching [PERSON]'s code benchmarks (1.55 bits/nt) [10]--validates quaternary encoding's potential despite current synthesis constraints. Notably, our in silico achievement of 2.22 bits/nt exceeds [PERSON]'s theoretical 2 bits/nt limit. Additionally, the two mapping in the coding scheme ensures smooth decoding and the addition of RS error correction redundancy is more robust than the quadratic system designed by [PERSON] et al. [35]. A limitation of this study lies in its reliance on the RS error correction algorithm, which has been widely adopted in wireless communication. Unlike communication systems, DNA storage systems are composed of four characters (A, T, C, G) and are subject to substitution, insertion, and deletion errors [14]. These unique characteristics pose significant challenges to error correction, particularly because insertions and deletions (indels) can cause frame shifts in sequence reading. Indels not only introduce localized errors but also produce downstream effects that compromise the integrity of the entire sequence. At present, research on quaternary error correction codes specifically tailored for DNA storage remains scarce. The few existing studies, such as [PERSON] et al. [36], involve complex and highly redundant encoding steps. Given the high cost of DNA synthesis, the feasibility of such approaches is limited.