Every segment writer is built - time to lay them out in order and wrap the entropy scan into a complete JPEG. Today you assemble a valid baseline file.
Assemble a full baseline JPEG - SOI, APP0, DQT, SOF0, DHT, SOS, the byte-stuffed entropy scan, and EOI - into one byte stream.
This is the encoder’s assembly point. In order, the file is: SOI (FF D8); an APP0/JFIF header identifying it; the DQT segment with the quantization tables; the SOF0 frame header; the DHT segment with the Huffman tables; the SOS scan header; then the entropy scan bytes the bit writer produced (already byte-stuffed); and finally EOI (FF D9). That segment order is the conventional one every decoder expects, and each piece was built in the preceding lessons.
The simplest correct encoder writes a single-component (grayscale) baseline file - one quantization table, one DC and one AC Huffman table, one component sampled 1x1 - which sidesteps chroma downsampling and MCU interleaving while still exercising the whole DCT, quantize, and entropy path; emitting full-color 4:2:0 is a natural extension once this works. The proof that it worked is structural: the assembled bytes start with SOI, end with EOI, and your own chapter-one segment walk steps through APP0, DQT, SOF0, DHT, and SOS cleanly before reaching the scan. That is a genuine baseline JPEG file - not just something your decoder can read, but a file conforming to the container other decoders parse. Whether the pixels survive the lossy round trip is the capstone’s question; that the file is valid is settled here.
// Encode: SOI, APP0/JFIF, DQT, SOF0, DHT, SOS header, then the entropy// scan bytes (from the bit writer, already stuffed), then EOI.// out := []byte{0xFF, 0xD8}; append each segment; ... ; 0xFF, 0xD9func Encode(im *Image) []byte { }