The final lesson closes the loop - encode a generated image to a valid JPEG, then decode it back and prove the pixels survive within tolerance and the file is structurally valid. This is the whole codec working as one.
Encode a generated gradient image to a JPEG, confirm the file is structurally valid, and decode it back within a stated per-channel tolerance.
This is the promise the whole project was built to keep: a codec that reads and writes real baseline JPEGs. Generate a smooth gradient image, Encode it to bytes, and Decode those bytes back. Two things are proven at once. The encoded file is structurally valid - it opens with SOI, closes with EOI, and your own segment walk parses APP0, DQT, SOF0, DHT, and SOS in order, so any conforming decoder could read it. And the pixels survive the round trip within a stated tolerance: because encode-quantize-decode is lossy, they are checked within a per-channel difference rather than for exact equality, which is the honest way to test a lossy codec.
Step back and look at what you built from two marker bytes: a marker-and-segment container, quantization and Huffman table parsing, the full baseline entropy scan with byte-stuffing, receive-and-extend, DC prediction, run-length AC coding, and restart markers, a separable inverse and forward DCT, chroma upsampling and downsampling, and color conversion in both directions - assembled into a decoder that turns a JPEG into pixels and an encoder that turns pixels into a valid JPEG. It is a teaching-grade codec, honest about its limits - baseline sequential only, standard tables, no progressive or arithmetic coding - but real in every byte it reads and writes. That is a genuine JPEG codec, and it is yours.
// 1. build im := gradient(16,16)// 2. data := Encode(im)// 3. assert data starts FFD8, ends FFD9, and walk(data) succeeds// 4. got, _ := Decode(data)// 5. assert got is 16x16 and every pixel within 15 of im (lossy tolerance)