generate an image based on markdown blog entry

generate-cover.mjs

#!/usr/bin/env node
/**
 * generate-cover.mjs
 *
 * Generates a cover image for a markdown blog post using:
 *   1. Anthropic API  – crafts an optimal DALL·E image prompt from the post
 *   2. OpenAI DALL·E 3 – generates the image
 *
 * Output: <same-name-as-input>.webp alongside the markdown file
 *
 * Usage:
 *   node --env-file=.env generate-cover.mjs path/to/post.md
 *
 * Reads API keys from a .env file in the current working directory:
 *   ANTHROPIC_API_KEY=sk-ant-...
 *   OPENAI_API_KEY=sk-...
 *
 * Dependencies (install once):
 *   npm install @anthropic-ai/sdk openai sharp
 *
 * Requires Node.js 20.6+ (uses built-in --env-file flag)
 */

import fs from 'fs/promises';
import path from 'path';
import Anthropic from '@anthropic-ai/sdk';
import OpenAI from 'openai';
import sharp from 'sharp';

// ── Config ────────────────────────────────────────────────────────────────────

const IMAGE_SIZE = '1792x1024'; // DALL·E 3 wide-format (good for blog covers)
const IMAGE_QUALITY = 'standard'; // "standard" | "hd"  (hd doubles the cost)

// ── Helpers ───────────────────────────────────────────────────────────────────

function die(msg) {
  console.error(`\nError: ${msg}\n`);
  process.exit(1);
}

async function readMarkdown(filePath) {
  try {
    return await fs.readFile(filePath, 'utf8');
  } catch {
    die(`Cannot read file: ${filePath}`);
  }
}

/** Ask Claude to write a concise, vivid DALL·E prompt for this post. */
async function buildImagePrompt(anthropic, markdownContent) {
  console.log('→ Generating image prompt via Claude…');

  const message = await anthropic.messages.create({
    model: 'claude-opus-4-5',
    max_tokens: 300,
    messages: [
      {
        role: 'user',
        content: `You are an expert at writing prompts for DALL·E 3 image generation.

Read the blog post below and write a single, vivid image prompt (max 200 words) that:
- Captures the post's core theme or mood
- Works as an eye-catching blog cover image
- for the style cycle through the following styles at random:
  - Photorealistic
  - Tasteful illustrative
  - Cyberpunk
  - Vintage
  - Minimalistic
  - Comic book
- Contains NO text, letters, or words in the image
- Avoids clichés (no light bulbs, no handshakes, no generic office scenes)

Reply with ONLY the prompt — no preamble, no explanation.

--- BEGIN POST ---
${markdownContent.slice(0, 4000)}
--- END POST ---`
      }
    ]
  });

  return message.content[0].text.trim();
}

/** Call DALL·E 3 and return the image as a Buffer. */
async function generateImage(openai, prompt) {
  console.log('→ Generating image via DALL·E 3…');
  console.log(`   Prompt: ${prompt}\n`);

  const response = await openai.images.generate({
    model: 'dall-e-3',
    prompt,
    n: 1,
    size: IMAGE_SIZE,
    quality: IMAGE_QUALITY,
    response_format: 'b64_json'
  });

  const b64 = response.data[0].b64_json;
  return Buffer.from(b64, 'base64');
}

/** Convert PNG buffer → WebP and save next to the markdown file. */
async function saveWebP(pngBuffer, mdFilePath) {
  const dir = path.dirname(mdFilePath);
  const base = path.basename(mdFilePath, path.extname(mdFilePath));
  const outPath = path.join(dir, `${base}.webp`);

  await sharp(pngBuffer).webp({ quality: 85 }).toFile(outPath);

  return outPath;
}

/** Save the generated image prompt next to the markdown file. */
async function savePrompt(prompt, mdFilePath) {
  const dir = path.dirname(mdFilePath);
  const base = path.basename(mdFilePath, path.extname(mdFilePath));
  const outPath = path.join(dir, `${base}.prompt`);

  await fs.writeFile(outPath, `${prompt}\n`, 'utf8');

  return outPath;
}

/** Add heroImage to frontmatter if not already present. */
async function addHeroImage(mdFilePath) {
  const content = await fs.readFile(mdFilePath, 'utf8');

  // Already has heroImage — nothing to do
  if (/^heroImage:/m.test(content)) return;

  // Derive relative path: everything after "documents/" without extension
  const match = mdFilePath.match(/documents\/(.+)\.[^.]+$/);
  if (!match) return;
  const heroValue = match[1];

  // Insert heroImage before the closing --- of the frontmatter block
  const closingIdx = content.indexOf('\n---', content.indexOf('---') + 3);
  if (closingIdx === -1) return;
  const updated = content.slice(0, closingIdx) + `\nheroImage: ${heroValue}` + content.slice(closingIdx);
  await fs.writeFile(mdFilePath, updated, 'utf8');

  console.log(`✓ Added heroImage: ${heroValue}`);
}

// ── Main ──────────────────────────────────────────────────────────────────────

async function main() {
  const [, , inputFile] = process.argv;
  if (!inputFile) die('Usage: node generate-cover.mjs <path/to/post.md>');

  const mdPath = path.resolve(inputFile);

  const anthropicKey = process.env.ANTHROPIC_API_KEY;
  const openaiKey = process.env.OPENAI_API_KEY;
  if (!anthropicKey) die('ANTHROPIC_API_KEY is not set.');
  if (!openaiKey) die('OPENAI_API_KEY is not set.');

  const anthropic = new Anthropic({ apiKey: anthropicKey });
  const openai = new OpenAI({ apiKey: openaiKey });

  const markdown = await readMarkdown(mdPath);
  const prompt = await buildImagePrompt(anthropic, markdown);
  const pngBuffer = await generateImage(openai, prompt);
  const imagePath = await saveWebP(pngBuffer, mdPath);
  const promptPath = await savePrompt(prompt, mdPath);
  await addHeroImage(mdPath);

  console.log(`✓ Cover image saved: ${imagePath}`);
  console.log(`✓ Prompt saved: ${promptPath}`);
}

main().catch((err) => {
  console.error(err);
  process.exit(1);
});