为什么调整png图像的大小会失去透明度？

Question

我正在尝试按如下方式调整图像大小.我将调整大小的图像返回到byte[]可以将其存储在数据库中.png图像的透明度丢失了.请帮助改善这一点.

private byte[] GetThumbNail(string imageFile, Stream imageStream, 
  int imageLen)
{
  try
  {
    Image.GetThumbnailImageAbort imageCallBack = 
      new Image.GetThumbnailImageAbort(ThumbnailCallback);
    Bitmap getBitmap = new Bitmap(imageFile);
    byte[] returnByte = new byte[imageLen];
    Image getThumbnail = getBitmap.GetThumbnailImage(160, 59, 
      imageCallBack, IntPtr.Zero);
    using (Graphics g = Graphics.FromImage(getThumbnail))
    {
      g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
      g.InterpolationMode = 
        System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
      g.DrawImage(getThumbnail, 0, 0, 160, 59);
    }
    using (MemoryStream ms = new MemoryStream())
    {
      getThumbnail.Save(ms, ImageFormat.Png);
      getThumbnail.Save("test.png", ImageFormat.Png);
      returnByte = ms.ToArray();
    }
    return returnByte;
  }
  catch (Exception)
  {
    throw;
  }
}

Answer 1

你的代码并不像你认为的那样完成......

您使用GetThumbnailImage来调整图像大小,然后将缩略图图像绘制到自身,这是毫无意义的.您可能在第一步中失去了透明度.

改为创建一个空白位图,并通过在空白位图上绘制来调整源图像的大小.

private byte[] GetThumbNail(string imageFile) {
  try {
    byte[] result;
    using (Image thumbnail = new Bitmap(160, 59)) {
      using (Bitmap source = new Bitmap(imageFile)) {
        using (Graphics g = Graphics.FromImage(thumbnail)) {
          g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
          g.InterpolationMode =  System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
          g.DrawImage(source, 0, 0, 160, 59);
        }
      }
      using (MemoryStream ms = new MemoryStream()) {
        thumbnail.Save(ms, ImageFormat.Png);
        thumbnail.Save("test.png", ImageFormat.Png);
        result = ms.ToArray();
      }
    }
    return result;
  } catch (Exception) {
    throw;
  }
}

(我删除了一些从未用于任何与结果有关的参数,比如imageLen参数,它只用于创建一个从未使用过的字节数组.)

Answer 2

尝试.MakeTransparent()在位图对象上使用调用.