trimCanvas

// functions to copy just the part with non-transparent pixels
// https://gist.github.com/timdown/021d9c8f2aabc7092df564996f5afbbf

function rowBlank(imageData, width2, y) {
      for (var x = 0; x < width2; ++x) {
          if (imageData[y * width2 * 4 + x * 4 + 3] !== 0) return false;
      }
      return true;
}

function columnBlank(imageData, width2, x, top, bottom) {
      for (var y = top; y < bottom; ++y) {
          if (imageData[y * width2 * 4 + x * 4 + 3] !== 0) return false;
      }
      return true;
}

function trimCanvas(c) {
      var ctx = c;
      var width2 = c.width;
      ctx.loadPixels();
      var top = 0, bottom = c.height, left = 0, right = width2;

      while (top < bottom && rowBlank(ctx.pixels, width2, top)) ++top;
      while (bottom - 1 > top && rowBlank(ctx.pixels, width2, bottom - 1)) --bottom;
      while (left < right && columnBlank(ctx.pixels, width2, left, top, bottom)) ++left;
      while (right - 1 > left && columnBlank(ctx.pixels, width2, right - 1, top, bottom)) --right;

      var trimmedWidth = right - left , trimmedHeight = bottom - top;
      var trimmed = ctx.get(left, top, trimmedWidth,trimmedHeight);

      image(trimmed,0,0, trimmed.width*.66, trimmed.height*.66);
}