go-widgets/svg

Snapshot: turn any RGBA byte buffer (the output of a go-widgets/toolkit widget composition, or any other pixel producer) into a portable, self-contained SVG document.

Pure Go, stdlib only, BSD-3-Clause. 100% statement coverage.

Why

Widgets in go-widgets/toolkit are pixel-blitting by design — every Draw call writes bytes into a []byte at (x, y). That is great for rendering, less great for embedding a screenshot in a README: raw PNGs travel poorly across dark-mode readers, don't scale on Retina displays, and lose their crispness at any zoom level.

svg.Snapshot wraps the render in an SVG envelope. The SVG declares a viewBox pinned to the render's pixel dimensions and carries the pixels as a base-64 PNG under image-rendering: pixelated. The result:

• Bit-exact: every pixel you see in the browser is the same pixel the widget produced. No re-vectorisation, no font substitution, no antialiasing drift.
• Scales crisply: any zoom keeps hard edges (pixelated hint). Perfect for a docs site that hosts one asset but serves it at multiple sizes.
• Self-contained: one file, zero external assets. Drops into any Markdown renderer that already accepts SVG images (GitHub, GitLab, mkdocs, Hugo, ...).

Use

The root package works against any RGBA byte producer:

import (
    "os"
    "github.com/go-widgets/svg"
)

surface := make([]byte, 4*w*h)  // fill with anything
// ...your draw code here...

f, _ := os.Create("out.svg")
defer f.Close()
svg.Snapshot(f, surface, w, h, "my image")

For go-widgets/toolkit widgets specifically, the sibling widget subpackage removes the manual make + Draw + Snapshot dance:

import (
    "os"
    "github.com/go-widgets/svg/widget"
    "github.com/go-widgets/toolkit"
)

btn := toolkit.NewButton("Click me", nil)
btn.SetBounds(toolkit.Rect{X: 0, Y: 0, W: 200, H: 40})

f, _ := os.Create("button.svg")
defer f.Close()
widget.Snapshot(f, btn, 200, 40, toolkit.DefaultLight(), "widget: button")

// Or a raw PNG (no SVG envelope):
p, _ := os.Create("button.png")
defer p.Close()
widget.PNG(p, btn, 200, 40, toolkit.DefaultLight())

The label argument on Snapshot becomes the SVG's <title> + aria-label for a11y + previewer tooltips. Pass "" to omit.

API

Root package (svg):

func Snapshot(w io.Writer, surface []byte, width, height int, label string) (int, error)

• Returns the byte count written + the first error, mirroring io.Writer.
• Errors on dimension ≤ 0 or len(surface) != 4*width*height.
• Errors bubble up from w.Write calls (header, title, body).
• All XML output escapes the five predefined XML entities in label (both inside <title> and in the aria-label attribute), so a label containing <b>&"'</b> remains valid XML.

Widget subpackage (svg/widget):

func Snapshot(w io.Writer, wg toolkit.Widget, width, height int, theme *toolkit.Theme, label string) (int, error)
func PNG(w io.Writer, wg toolkit.Widget, width, height int, theme *toolkit.Theme) (int, error)

• Renders wg into a fresh width×height RGBA surface via wg.Draw(surface, width, theme), then serialises. Snapshot wraps in an SVG envelope; PNG emits raw PNG bytes.
• Errors on non-positive dimensions, nil widget, nil theme.
• Split into a subpackage so the root svg stays dep-free — a consumer that only uses svg.Snapshot([]byte) for a home-grown pixel producer does not pull go-widgets/toolkit into its module graph.

License

BSD-3-Clause.