DNA fingerprint

Each longer k-mer refines the quadtree: A top-left, C top-right, T bottom-left, G bottom-right. At depth 1 the image is a 2×2 count of A/C/T/G; at depth 2 a 4×4 grid of dinucleotide counts (AA, AC, …, GG), and so on. Every output pixel sums the hierarchical counts from all depths that cover that position. The image is greyscale: values are normalized linearly to 0–255 (optional log scale before normalization).