## HTML5 キャンバスでピクセル完璧な画像のダウンスケーリングを実現するにはどうすればよいですか?

<code class="javascript">function downscaleImage(img, scale) {
    var imgCV = document.createElement('canvas');
    imgCV.width = img.width;
    imgCV.height = img.height;
    var imgCtx = imgCV.getContext('2d');
    imgCtx.drawImage(img, 0, 0);

    if (!(scale < 1) || !(scale > 0)) throw ('scale must be a positive number < 1');

    var sqScale = scale * scale;
    var sw = imgCV.width;
    var sh = imgCV.height;
    var tw = Math.floor(sw * scale);
    var th = Math.floor(sh * scale);

    var sBuffer = imgCV.getContext('2d').getImageData(0, 0, sw, sh).data;
    var tBuffer = new Float32Array(3 * tw * th);
    var sx, sy, sIndex, tx, ty, yIndex, tIndex;

    for (sy = 0; sy < sh; sy++) {
        ty = sy * scale;
        yIndex = 3 * ty * tw;

        for (sx = 0; sx < sw; sx++, sIndex += 4) {
            tx = sx * scale;
            tIndex = yIndex + tx * 3;

            var sR = sBuffer[sIndex];
            var sG = sBuffer[sIndex + 1];
            var sB = sBuffer[sIndex + 2];

            tBuffer[tIndex] += sR * sqScale;
            tBuffer[tIndex + 1] += sG * sqScale;
            tBuffer[tIndex + 2] += sB * sqScale;
        }
    }

    // Convert float array into canvas data and draw
    var resCV = document.createElement('canvas');
    resCV.width = tw;
    resCV.height = th;
    var resCtx = resCV.getContext('2d');
    var imgRes = resCtx.getImageData(0, 0, tw, th);
    var tByteBuffer = imgRes.data;

    var pxIndex;

    for (sIndex = 0, tIndex = 0; pxIndex < tw * th; sIndex += 3, tIndex += 4, pxIndex++) {
        tByteBuffer[tIndex] = Math.ceil(tBuffer[sIndex]);
        tByteBuffer[tIndex + 1] = Math.ceil(tBuffer[sIndex + 1]);
        tByteBuffer[tIndex + 2] = Math.ceil(tBuffer[sIndex + 2]);
        tByteBuffer[tIndex + 3] = 255;
    }

    resCtx.putImageData(imgRes, 0, 0);

    return resCV;
}</code>

<code class="javascript">function downscaleImageFast(img, scale) {
    var sw = img.width;
    var sh = img.height;
    var tw = Math.floor(sw * scale);
    var th = Math.floor(sh * scale);

    var resCV = document.createElement('canvas');
    resCV.width = tw;
    resCV.height = th;
    var resCtx = resCV.getContext('2d');
    resCtx.drawImage(img, 0, 0, sw, sh, 0, 0, tw, th);

    return resCV;
}</code>