The file needs a SOF0 frame header and a SOS scan header describing the image and its table selectors. Today you serialize both, mirroring the parsers from the frame and scan chapters.
Serialize a SOF0 frame header and a SOS scan header with correct markers, lengths, and per-component fields.
The SOF0 header declares the image to the decoder: marker FF C0, a length, the precision byte (8), the height and width, the component count, and three bytes per component (id, packed sampling factors, quant selector). For a 16-by-16 grayscale image with one 1x1 component the length is 8 + 3 = 11 (00 0B). The SOS header names the scan’s components and their Huffman selectors: marker FF DA, a length, the component count, a (Td<<4)|Ta selector byte per component, and the three baseline spectral bytes 00 3F 00.
These are the exact structures the frame and scan chapters parsed, written in reverse, and their lengths follow the same self-counting rule as every other segment. Emitting them with the right selectors is what tells the decoder which of the tables you wrote earlier to apply to each component. Every segment the file needs can now be produced individually; the final lesson strings them together, wraps the entropy scan, and closes the file.
// SOF0: FF C0, len, precision(8), H(2), W(2), Nf, per comp: id,(H<<4|V),Tq// len = 8 + 3*Nf// SOS: FF DA, len, Ns, per comp: id,(Td<<4|Ta), then Ss(0),Se(63),00// len = 6 + 2*Nsfunc writeFrameAndScan(f *Frame) []byte { }