C#无损压缩图片

话不多说，请看代码：

/// <summary>
    /// 根据指定尺寸得到按比例缩放的尺寸,返回true表示以更改尺寸
    /// </summary>
    /// <param name="picwidth">图片宽度</param>
    /// <param name="picheight">图片高度</param>
    /// <param name="specifiedwidth">指定宽度</param>
    /// /// <param name="specifiedheight">指定高度</param>
    /// <returns>返回true表示以更改尺寸</returns>
    private bool getpiczoomsize(ref int picwidth, ref int picheight, int specifiedwidth, int specifiedheight)
    {
      int sw = 0, sh = 0;
      boolean iszoomsize = false;
      //按比例缩放
      size tem_size = new size(picwidth, picheight);
      if (tem_size.width > specifiedwidth || tem_size.height > specifiedheight) //将**改成c#中的或者操作符号
      {
        if ((tem_size.width * specifiedheight) > (tem_size.height * specifiedwidth))
        {
          sw = specifiedwidth;
          sh = (specifiedwidth * tem_size.height) / tem_size.width;
        }
        else
        {
          sh = specifiedheight;
          sw = (tem_size.width * specifiedheight) / tem_size.height;
        }
        iszoomsize = true;
      }
      else
      {
        sw = tem_size.width;
        sh = tem_size.height;
      }
      picheight = sh;
      picwidth = sw;
      return iszoomsize;
    }
    /// <summary>
    /// 无损压缩图片
    /// </summary>
    /// <param name="sfile">原图片</param>
    /// <param name="dfile">压缩后保存位置</param>
    /// <param name="dheight">高度</param>
    /// <param name="dwidth">宽度</param>
    /// <param name="flag">压缩质量 1-100</param>
    /// <returns></returns>

    public bool getpicthumbnail(string sfile, string dfile, int dheight, int dwidth, int flag)
    {
      system.drawing.image isource = system.drawing.image.fromfile(sfile);
      imageformat tformat = isource.rawformat;
      int sw = isource.width, sh = isource.height;

      getpiczoomsize(ref sw, ref sh, dwidth, dheight);

      bitmap ob = new bitmap(dwidth, dheight);
      graphics g = graphics.fromimage(ob);
      g.clear(color.whitesmoke);
      g.compositingquality = compositingquality.highquality;
      g.smoothingmode = smoothingmode.highquality;
      g.interpolationmode = interpolationmode.highqualitybicubic;
      g.drawimage(isource, new rectangle((dwidth - sw) / 2, (dheight - sh) / 2, sw, sh), 0, 0, isource.width, isource.height, graphicsunit.pixel);
      g.dispose();
      //以下代码为保存图片时，设置压缩质量
      encoderparameters ep = new encoderparameters();
      long[] qy = new long[1];
      qy[0] = flag;//设置压缩的比例1-100
      encoderparameter eparam = new encoderparameter(system.drawing.imaging.encoder.quality, qy);
      ep.param[0] = eparam;
      try
      {
        imagecodecinfo[] arrayici = imagecodecinfo.getimageencoders();

        imagecodecinfo jpegiciinfo = null;

        for (int x = 0; x < arrayici.length; x++)
        {
          if (arrayici[x].formatdescription.equals("jpeg"))
          {
            jpegiciinfo = arrayici[x];
            break;
          }
        }
        if (jpegiciinfo != null)
        {
          ob.save(dfile, jpegiciinfo, ep);//dfile是压缩后的新路径
        }
        else
        {
          ob.save(dfile, tformat);
        }
        return true;
      }
      catch
      {
        return false;
      }
      finally
      {
        isource.dispose();
        ob.dispose();
      }
    }

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