Skip to content

ogp.md

Latest commit

 

History

History
107 lines (81 loc) · 3.2 KB

ogp.md

File metadata and controls

107 lines (81 loc) · 3.2 KB

OGP Image Generation

Overview

gohan generates OGP (Open Graph Protocol) thumbnail images at build time using pure Go. Each article gets a unique og:image derived from its title, eliminating the need for manual image creation.

Output

public/
└── ogp/
    └── {slug}.png    # 1200×630px OGP image per article

The og:image tag in each article page resolves to:

{{.Config.Site.BaseURL}}/ogp/{{.Article.FrontMatter.Slug}}.png

Listing pages (index, tag, category) fall back to a user-supplied default image:

{{.Config.Site.BaseURL}}/assets/images/ogp-default.png

Configuration

Add an ogp block to config.yaml:

ogp:
  enabled: true
  background_color: "#1e1e2e"
  text_color: "#cdd6f4"
  font_file: "assets/fonts/NotoSansJP-Bold.ttf"   # TTF/OTF, required for CJK
  logo_file: "assets/images/logo.png"              # optional overlay
  width: 1200
  height: 630

The font file must be bundled by the user. Any TTF/OTF font is supported.

Data Model

Add OGPConfig to model.go:

// OGPConfig holds settings for build-time OGP image generation.
type OGPConfig struct {
    Enabled         bool   `yaml:"enabled"`
    BackgroundColor string `yaml:"background_color"`
    TextColor       string `yaml:"text_color"`
    FontFile        string `yaml:"font_file"`
    LogoFile        string `yaml:"logo_file"` // empty means no logo
    Width           int    `yaml:"width"`
    Height          int    `yaml:"height"`
}

Add to Config:

type Config struct {
    // ... existing fields ...
    OGP OGPConfig `yaml:"ogp"`
}

Implementation

internal/generator/ogp.go (new file)

  • Implement OGPGenerator satisfying OutputGenerator
  • Use stdlib image, image/color, image/png, image/draw
  • Use golang.org/x/image/font and golang.org/x/image/font/opentype for TrueType rendering
  • Use golang.org/x/image/math/fixed for fixed-point arithmetic
  • Rendering pipeline: fill background → draw logo (if configured) → draw word-wrapped title text centered vertically and horizontally
  • Skip generation if ogp.enabled: false
  • Skip per-article generation if the output .png already exists and the source article is unchanged (cache-aware via ChangeSet)

internal/generator/generator.go

  • Invoke OGPGenerator.Generate() as part of the build pipeline when cfg.OGP.Enabled is true

Template usage (user-side):

<!-- article.html -->
<meta property="og:image"
  content="{{.Config.Site.BaseURL}}/ogp/{{.Article.FrontMatter.Slug}}.png">
<meta name="twitter:image"
  content="{{.Config.Site.BaseURL}}/ogp/{{.Article.FrontMatter.Slug}}.png">

<!-- index.html, tag.html, category.html -->
<meta property="og:image"
  content="{{.Config.Site.BaseURL}}/assets/images/ogp-default.png">

Dependencies

Package Purpose
image, image/png, image/draw stdlib — canvas creation and PNG encoding
golang.org/x/image/font Font face interface and text drawing
golang.org/x/image/font/opentype Load TTF/OTF font files
golang.org/x/image/math/fixed Fixed-point arithmetic for font rendering

golang.org/x/image is already transitively pulled in by many Go projects and adds negligible build overhead.