Sample Node.js to read metadata of PNG files created with Stable Diffusion web UI.

read-sdwebui-file-metadata.js

/**
 * Sample Node.js to read metadata of PNG files created with Stable Diffusion Web UI
 * 
 * Code from the following page is used as a reference
 * URL: https://qiita.com/javacommons/items/472e85be1b11098172b3
 */
const fs = require('fs').promises;

async function getPngInfo(fileName) {
    const signature = Buffer.from([137, 80, 78, 71, 13, 10, 26, 10]);
    const crcSize = 4;
    let result = "";

    const buffer = await fs.readFile(fileName);
    const realSig = buffer.slice(0, 8);
    if (!realSig.equals(signature)) {
        return "";
    }

    let position = 8;
    while (position < buffer.length) {
        const length = buffer.readUInt32BE(position);
        position += 4;

        const chunkType = buffer.slice(position, position + 4).toString();
        position += 4;

        if (chunkType === 'tEXt' || chunkType === 'iTXt') {
            const s = buffer.slice(position, position + length);
            position += length;
            if (s.slice(0, 10).toString() === 'parameters') {
                let sRest = s.slice(10);
                while (sRest.length > 0 && sRest[0] === 0) {
                    sRest = sRest.slice(1);
                }
                result = chunkType === 'tEXt' ? sRest.toString('latin1') : sRest.toString('utf8');
                break;
            }
        } else {
            position += length;
        }

        position += crcSize;
    }

    return result;
}

async function getPngInfoJson(fileName) {
    const infoString = await getPngInfo(fileName);
    const lines = infoString.split("\n");
    let phase = 0;
    let prompt = "";
    let negativePrompt = "";
    const data = {};

    for (const line of lines) {
        const re = /([A-Z][a-zA-Z0-9 ]*: [^,]+)(, )?/g;
        if (phase === 0) {
            if (line.startsWith("Negative prompt: ")) {
                negativePrompt = line.slice(17);
                phase = 1;
                continue;
            }
            const m = re.exec(line);
            if (m) {
                phase = 2;
            } else {
                prompt += "\n" + line;
                continue;
            }
        }
        if (phase === 1) {
            const m = re.exec(line);
            if (!m) {
                negativePrompt += "\n" + line;
                continue;
            }
            phase = 2;
        }
        if (phase === 2) {
            let match1;
            while ((match1 = re.exec(line)) !== null) {
                const [_, word] = match1;
                const rx1 = /([A-Z][a-zA-Z0-9 ]*): ([^,]+)(, )?/g;
                let match2;
                while ((match2 = rx1.exec(word)) !== null) {
                    const [_, name, value] = match2;
                    data[name] = value;
                }
            }
        }
    }

    data["Prompt"] = prompt.trim();
    data["Negative prompt"] = negativePrompt.trim();

    return JSON.stringify(data);
}

(async () => {
    const args = process.argv.slice(2);

    if (args.length !== 1) {
        console.error("Usage: node app.js <filename>");
        process.exit(1);
    }

    const fileName = args[0];

    try {
        const result = await getPngInfo(fileName);
        console.log('===PNG INFO===')
        console.log(result);

        const json = await getPngInfoJson(fileName);
        console.log('===PNG INFO (JSON)===')
        console.log(json);
    } catch (err) {
        console.error(`Error reading file ${fileName}: ${err.message}`);
        process.exit(1);
    }
})();