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Hidden canvas

As explained in D3.js in Action, Third Edition

A hidden canvas is an overlaid Canvas element rendered with unique per-shape colors and made transparent for pixel-based interaction detection.

A hidden canvas is an overlaid Canvas element that mirrors interactive shapes with unique colors while remaining invisible through CSS opacity. The visible canvas stays focused on rendering; the hidden layer supplies pixels for interaction detection.

Implement the interaction layer in four steps:

  • Use the returned RGB value to recover the shape and populate the tooltip. section 15.4

Canvas painting interaction

A painting visualization uses colored circles on a transparent overlay to identify the hovered painting, then displays its title, link, and tooltip position.

How D3.js in Action, Third Edition covers it

How deep it goes
Works through overlay creation, color mapping, CSS hiding, resolution correction, pixel lookup, and tooltip updates.
Examples
Uses an interactive painting visualization with circles, unique RGB colors, a transparent overlay, and hover tooltips.
What it assumes
Assumes Canvas drawing, JavaScript events, CSS positioning, and RGB values; demonstrates pixel-based interaction from implementation.
What you can do after
Build it from scratch with layered Canvas elements, color maps, getImageData(), and tooltip state.
How it connects
Connects Canvas, SVG, getImageData(), devicePixelRatio, Map, and tooltips.
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Related concepts

This explainer was generated by Manning's knowledge graph from the text of D3.js in Action, Third Edition. The section links point to the parts of the book it draws on.