WebCodecs ImageDecoder：图像格式处理与优化

            
async function decodeImage(imageData) {
  const decoder = new ImageDecoder({
    type: 'image/jpeg',
  });
  decoder.decode(imageData);
}

            

              
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async function decodeProgressive(imageData) {
  const decoder = new ImageDecoder({
    type: 'image/jpeg',
  });
  const frameStream = decoder.decode(imageData);
  for await (const frame of frameStream) {
    // Use the frame.bitmap for partial rendering
    console.log('Frame decoded');
  }
}

            

              
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async function decodeAnimatedGif(imageData) {
    const decoder = new ImageDecoder({ type: 'image/gif' });
    const frameStream = decoder.decode(imageData);
    for await (const frame of frameStream) {
        // Access frame.bitmap for each frame of the animation.
        console.log('Frame decoded from animated GIF');
    }
}

            

              
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async function getImageMetadata(imageData) {
  const decoder = new ImageDecoder({ type: 'image/jpeg' });
  const { imageInfo } = await decoder.decode(imageData).next();
  console.log('Width:', imageInfo.width);
  console.log('Height:', imageInfo.height);
  console.log('Color Space:', imageInfo.colorSpace);
}

            

              
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async function optimizeForMobile(imageData, maxWidth) {
  const decoder = new ImageDecoder({ type: 'image/jpeg' });
  const { imageInfo, frame } = await decoder.decode(imageData).next();
  let bitmap = frame.bitmap;

  if (imageInfo.width > maxWidth) {
    const ratio = maxWidth / imageInfo.width;
    const height = Math.floor(imageInfo.height * ratio);
    const canvas = document.createElement('canvas');
    canvas.width = maxWidth;
    canvas.height = height;
    const ctx = canvas.getContext('2d');
    ctx.drawImage(bitmap, 0, 0, maxWidth, height);
    bitmap = await createImageBitmap(canvas);
  }

  return bitmap;
}

            

              
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async function applyGrayscale(imageData) {
    const decoder = new ImageDecoder({ type: 'image/jpeg' });
    const frameStream = decoder.decode(imageData);
    for await (const frame of frameStream) {
        const bitmap = frame.bitmap;
        const canvas = document.createElement('canvas');
        canvas.width = bitmap.width;
        canvas.height = bitmap.height;
        const ctx = canvas.getContext('2d');
        ctx.drawImage(bitmap, 0, 0);
        const imageData = ctx.getImageData(0, 0, bitmap.width, bitmap.height);
        const data = imageData.data;

        for (let i = 0; i < data.length; i += 4) {
            const r = data[i];
            const g = data[i + 1];
            const b = data[i + 2];
            const gray = 0.299 * r + 0.587 * g + 0.114 * b;
            data[i] = gray;
            data[i + 1] = gray;
            data[i + 2] = gray;
        }
        ctx.putImageData(imageData, 0, 0);
        return await createImageBitmap(canvas);
    }
}

            

              
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// (Implementation of image loading, decoding, and rendering)

            

              
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try {
  // Decode image
} catch (error) {
  console.error('Image decoding error:', error);
  // Display fallback image or error message
}

            

              
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if ('ImageDecoder' in window) {
    // Use ImageDecoder
} else {
    // Use fallback method
}

            

              
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async function resizeImage(imageData, maxWidth) {
    const decoder = new ImageDecoder({ type: 'image/jpeg' });
    const { imageInfo, frame } = await decoder.decode(imageData).next();
    let bitmap = frame.bitmap;

    if (imageInfo.width > maxWidth) {
        const ratio = maxWidth / imageInfo.width;
        const height = Math.floor(imageInfo.height * ratio);
        const canvas = document.createElement('canvas');
        canvas.width = maxWidth;
        canvas.height = height;
        const ctx = canvas.getContext('2d');
        ctx.drawImage(bitmap, 0, 0, maxWidth, height);
        bitmap = await createImageBitmap(canvas);
    }

    return bitmap;
}

            

              
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// (Requires a Web Worker implementation for encoding.)

// In your main script:

async function convertToWebP(jpegImageData) {
  // Assume web worker is available to do the encoding in background.
  const worker = new Worker('webp-encoder-worker.js');

  return new Promise((resolve, reject) => {
    worker.onmessage = (event) => {
      if (event.data.error) {
        reject(new Error(event.data.error));
      } else {
        resolve(event.data.webpBlob);
      }
      worker.terminate();
    };
    worker.onerror = (error) => {
      reject(error);
      worker.terminate();
    };
    worker.postMessage({ jpegImageData });
  });
}

//In your web worker (webp-encoder-worker.js): 
// This example is incomplete. It would require a WebP encoding library.
// The following outlines a conceptual framework.

// import WebPEncoder from 'webp-encoder-library'; // hypothetical library

// self.onmessage = async (event) => {
//   try {
//     const jpegImageData = event.data.jpegImageData;
//     // Decode JPEG using ImageDecoder
//     const decoder = new ImageDecoder({ type: 'image/jpeg' });
//     const bitmap = (await decoder.decode(jpegImageData).next()).value.bitmap;

//     // Encode the bitmap to WebP (Requires a separate web worker library).
//     const webpBlob = await WebPEncoder.encode(bitmap, { quality: 75 });

//     self.postMessage({ webpBlob });
//   } catch (e) {
//     self.postMessage({ error: e.message });
//   }
// };

            

              
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async function createGifThumbnail(gifImageData, thumbnailWidth = 100) {
    const decoder = new ImageDecoder({ type: 'image/gif' });
    const frameStream = decoder.decode(gifImageData);
    let canvas = document.createElement('canvas');
    let ctx = canvas.getContext('2d');

    for await (const frame of frameStream) {
        const bitmap = frame.bitmap;
        canvas.width = thumbnailWidth;
        canvas.height = (thumbnailWidth / bitmap.width) * bitmap.height;
        ctx.drawImage(bitmap, 0, 0, canvas.width, canvas.height);
        // Only grab the first frame for the thumbnail
        break;
    }

    return canvas;
}

            

              
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// Main script (index.html)

const worker = new Worker('image-worker.js');

worker.onmessage = (event) => {
  // Handle results
};

worker.postMessage({ imageData: // your image data });

            

              
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// image-worker.js

self.onmessage = async (event) => {
  const imageData = event.data.imageData;
  // Decode and process the image using ImageDecoder here.
  // Send results back to the main thread with self.postMessage.
  // ...
};

            

              
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