f524c1b9d8
Replace the charCount * fontSize * 0.55 text sizing heuristic with canvas-based measurement (graceful fallback when native deps unavailable). Add validate.mjs for automated spatial checks (text overflow, arrow-text collisions, element overlap). Update element format reference with sizing rules, label guidelines, and arrow routing conventions. Add verification step to SKILL.md workflow. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
142 lines
6.2 KiB
Markdown
142 lines
6.2 KiB
Markdown
---
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name: excalidraw-png-export
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description: "This skill should be used when creating diagrams, architecture visuals, or flowcharts and exporting them as PNG files. It uses the Excalidraw MCP to render hand-drawn style diagrams locally and Playwright to export them to PNG without sending data to any remote server. Triggers on requests like 'create a diagram', 'make an architecture diagram', 'draw a flowchart and export as PNG', or any request that needs a visual diagram delivered as an image file."
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---
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# Excalidraw PNG Export
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Create hand-drawn style diagrams with the Excalidraw MCP and export them locally to PNG files. All rendering happens on the local machine. Diagram data never leaves the user's computer.
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## Prerequisites
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### First-Time Setup
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Run the setup script once per machine to install Playwright and Chromium headless:
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```bash
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bash <skill-path>/scripts/setup.sh
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```
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This creates a `.export-runtime` directory inside `scripts/` with the Node.js dependencies. The setup is idempotent and skips installation if already present.
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### Required MCP
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The Excalidraw MCP server must be configured. Verify availability by checking for `mcp__excalidraw__create_view` and `mcp__excalidraw__read_checkpoint` tools.
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## Workflow
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### Step 1: Design the Diagram Elements
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Translate the user's request into Excalidraw element JSON. Load [excalidraw-element-format.md](./references/excalidraw-element-format.md) for the full element specification, color palette, and sizing guidelines.
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Key design decisions:
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- Choose appropriate colors from the palette to distinguish different components
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- Use `label` on shapes instead of separate text elements
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- Use `roundness: { type: 3 }` for rounded corners on rectangles
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- Include `cameraUpdate` as the first element to frame the view (MCP rendering only)
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- Use arrow bindings (`startBinding`/`endBinding`) to connect shapes
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### Step 2: Render with Excalidraw MCP
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Call `mcp__excalidraw__create_view` with the element JSON array. This renders an interactive preview in the Claude Code UI.
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```
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mcp__excalidraw__create_view({ elements: "<JSON array string>" })
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```
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The response includes a `checkpointId` for retrieving the rendered state.
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### Step 3: Extract the Checkpoint Data
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Call `mcp__excalidraw__read_checkpoint` with the checkpoint ID to get the full element JSON back.
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```
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mcp__excalidraw__read_checkpoint({ id: "<checkpointId>" })
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```
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### Step 4: Convert Checkpoint to .excalidraw File
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Use the `convert.mjs` script to transform raw MCP checkpoint JSON into a valid `.excalidraw` file. This handles all the tedious parts automatically:
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- Filters out pseudo-elements (`cameraUpdate`, `delete`, `restoreCheckpoint`)
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- Adds required Excalidraw defaults (`seed`, `version`, `fontFamily`, etc.)
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- Expands `label` properties on shapes/arrows into proper bound text elements
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```bash
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# Save checkpoint JSON to a file first, then convert:
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node <skill-path>/scripts/convert.mjs <input.json> <output.excalidraw>
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```
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The input JSON should be the raw checkpoint data from `mcp__excalidraw__read_checkpoint` (the `{"elements": [...]}` object).
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**For batch exports**: Write each checkpoint to a separate raw JSON file, then convert each one:
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```bash
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node <skill-path>/scripts/convert.mjs raw1.json diagram1.excalidraw
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node <skill-path>/scripts/convert.mjs raw2.json diagram2.excalidraw
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```
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**Manual alternative**: If you need to write the `.excalidraw` file by hand (e.g., without the convert script), each element needs these defaults:
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```
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angle: 0, roughness: 1, opacity: 100, groupIds: [], seed: <unique int>,
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version: 1, versionNonce: <unique int>, isDeleted: false,
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boundElements: null, link: null, locked: false
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```
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Text elements also need: `fontFamily: 1, textAlign: "left", verticalAlign: "top", baseline: 14, containerId: null, originalText: "<same as text>"`
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Bound text (labels on shapes/arrows) needs: `containerId: "<parent-id>"`, `textAlign: "center"`, `verticalAlign: "middle"`, and the parent needs `boundElements: [{"id": "<text-id>", "type": "text"}]`.
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### Step 5: Export to PNG
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Run the export script. Determine the runtime path relative to this skill's scripts directory:
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```bash
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cd <skill-path>/scripts/.export-runtime && node <skill-path>/scripts/export_png.mjs /tmp/excalidraw-export/diagram.excalidraw /tmp/excalidraw-export/output.png
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```
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The script:
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1. Starts a local HTTP server serving the `.excalidraw` file and an HTML page
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2. Launches headless Chromium via Playwright
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3. The HTML page loads the Excalidraw library from esm.sh (library code only, not user data)
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4. Calls `exportToBlob` on the local diagram data
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5. Extracts the base64 PNG and writes it to disk
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6. Cleans up temp files and exits
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The script prints the output path on success. Verify the result with `file <output.png>`.
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### Step 5.5: Validate and Iterate
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Run the validation script on the `.excalidraw` file to catch spatial issues:
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```bash
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node <skill-path>/scripts/validate.mjs /tmp/excalidraw-export/diagram.excalidraw
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```
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Then read the exported PNG back using the Read tool to visually inspect:
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1. All label text fits within its container (no overflow/clipping)
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2. No arrows cross over text labels
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3. Spacing between elements is consistent
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4. Legend and titles are properly positioned
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If the validation script or visual inspection reveals issues:
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1. Identify the specific elements that need adjustment
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2. Edit the `.excalidraw` file (adjust coordinates, box sizes, or arrow waypoints)
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3. Re-run the export script (Step 5)
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4. Re-validate
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### Step 6: Deliver the Result
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Read the PNG file to display it to the user. Provide the file path so the user can access it directly.
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## Troubleshooting
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**Setup fails**: Verify Node.js v18+ is installed (`node --version`). Ensure npm has network access for the initial Playwright/Chromium download.
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**Export times out**: The HTML page has a 30-second timeout. If it fails, check browser console output in the script's error messages. Common cause: esm.sh CDN is temporarily slow on first load.
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**Blank PNG**: Ensure elements include all required properties (see Step 4 defaults). Missing `seed`, `version`, or `fontFamily` on text elements can cause silent render failures.
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**"READY" never fires**: The `exportToBlob` call requires valid elements. Filter out `cameraUpdate` and other pseudo-elements before writing the `.excalidraw` file.
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