Problem-solving recipes

A step-by-step procedure for every problem type this course teaches — what to do first, what to do next, and how to tell which case you're in.

What are you being asked to do?

Start with what you're actually being asked to produce.

Given a flat, unsharp-looking image, need it to look crisper — unsharp masking.

Given a grayscale image, need thin, reliable edges (not just a raw gradient) — the Canny pipeline.

Given a photo of a document taken at an angle, need it flattened into a rectangle — perspective-correct it first. Need a clean, print-ready, black-on-white SCAN out of that same photo (not just the flat rectangle) — the document-scanner recipe, which itself starts by calling the perspective-correction recipe above.

Given a photographic image, need it compressed the real-world, standard way — the JPEG pipeline.

Sharpening and edge detection

Sharpen an image with unsharp masking

Use when: you have a grayscale image and want to emphasize edges and fine detail without a full frequency-domain filter

Goal: a sharpened image that amplifies detail while leaving flat regions nearly unchanged

Unsharp masking

Run the Canny edge detector

Use when: you need thin, well-placed, reliably found edges from a grayscale image, not just a raw gradient-magnitude threshold

Goal: a binary edge map: one pixel wide per genuine edge, noise rejected, faint-but-real edge segments kept

The full Canny pipeline, step by step

Geometric and document correction

Perspective-correct a photographed document (first pass)

Use when: a document was photographed at an angle instead of scanned flat, and you need a flat, rectangular result

Goal: a warped output image where the document's four corners land exactly on the output rectangle's four corners

Applied: perspective-correcting a photographed document

Build a document scanner

Use when: a document was photographed at an angle, under uneven real-world lighting, and you need a clean, thresholded, print-ready scan -- not just the flat rectangular warp Chapter 6 §9 stopped at

Goal: a binary (black text, white background) scanned-looking image, corrected for perspective, denoised, contrast-restored, and correctly thresholded despite uneven lighting

Applied: building a document scanner

Builds on the perspective-correction recipe above as its first step, then adds denoising, contrast restoration, and adaptive thresholding on top.

Compression

The JPEG compression pipeline, assembled

Use when: you need to compress a photo the standard, real-world way -- not a from-scratch technique, but the actual six-step process every JPEG encoder runs

Goal: a compressed byte stream that decodes back into a rebuilt image, lossy at exactly one named step, and small enough to actually save bandwidth

The JPEG pipeline, assembled