C#生成缩略图 等比例缩放

/// 获取缩略图
public static Image GetThumbnailImage(Image image, int width, int height)
{
if (image == null || width < 1 || height < 1)
return null;

// 新建一个bmp图片
Image bitmap = new Bitmap(width, height);

// 新建一个画板
using (var g = Graphics.FromImage(bitmap))
{
// 设置高质量插值法
g.InterpolationMode = InterpolationMode.HighQualityBicubic;
// 设置高质量,低速度呈现平滑程度
g.SmoothingMode = SmoothingMode.HighQuality;
// 高质量、低速度复合
g.CompositingQuality = CompositingQuality.HighQuality;
// 清空画布并以透明背景色填充
g.Clear(Color.Transparent);
// 在指定位置并且按指定大小绘制原图片的指定部分
g.DrawImage(image, new Rectangle(0, 0, width, height),
new Rectangle(0, 0, image.Width, image.Height),
GraphicsUnit.Pixel);
return bitmap;
}
}

/// <summary>
///     生成缩略图，并保持纵横比
/// </summary>
/// <returns>生成缩略图后对象</returns>
public static Image GetThumbnailImageKeepRatio(Image image, int width, int height)
{
var imageSize = GetImageSize(image, width, height);
return GetThumbnailImage(image, imageSize.Width, imageSize.Height);
}

/// <summary>
///     根据百分比获取图片的尺寸
/// </summary>
public static Size GetImageSize(Image picture, int percent)
{
if (picture == null || percent < 1)
return Size.Empty;
var width = picture.Width*percent/100;
var height = picture.Height*percent/100;
return GetImageSize(picture, width, height);
}

/// <summary>
///     根据设定的大小返回图片的大小，考虑图片长宽的比例问题
/// </summary>
public static Size GetImageSize(Image picture, int width, int height)
{
if (picture == null || width < 1 || height < 1)
return Size.Empty;

var imageSize = new Size(width, height);
var heightRatio = (double) picture.Height/picture.Width;
var widthRatio = (double) picture.Width/picture.Height;
var desiredHeight = imageSize.Height;
var desiredWidth = imageSize.Width;
imageSize.Height = desiredHeight;

if (widthRatio > 0)
imageSize.Width = Convert.ToInt32(imageSize.Height*widthRatio);

if (imageSize.Width > desiredWidth)
{
imageSize.Width = desiredWidth;
imageSize.Height = Convert.ToInt32(imageSize.Width*heightRatio);
}

return imageSize;
}

/// <summary>
///     获取图像编码解码器的所有相关信息
/// </summary>
/// <param name="mimeType">包含编码解码器的多用途网际邮件扩充协议 (MIME) 类型的字符串</param>
/// <returns>返回图像编码解码器的所有相关信息</returns>
public static ImageCodecInfo GetCodecInfo(string mimeType)
{
ImageCodecInfo[] codecInfo = ImageCodecInfo.GetImageEncoders();

return codecInfo.FirstOrDefault(ici => ici.MimeType == mimeType);
}

public static ImageCodecInfo GetImageCodecInfo(ImageFormat format)
{
ImageCodecInfo[] encoders = ImageCodecInfo.GetImageEncoders();
return encoders.FirstOrDefault(icf => icf.FormatID == format.Guid);
}

/// <summary>
/// 保存图片
/// </summary>
/// <param name="image"></param>
/// <param name="savePath"></param>
/// <param name="format"></param>
public static void SaveImage(Image image, string savePath, ImageFormat format)
{
SaveImage(image, savePath, GetImageCodecInfo(format));
}

/// <summary>
///     高质量保存图片
/// </summary>
private static void SaveImage(Image image, string savePath, ImageCodecInfo ici)
{
// 设置 原图片 对象的 EncoderParameters 对象
EncoderParameters parms = new EncoderParameters(1);
EncoderParameter parm = new EncoderParameter(Encoder.Quality, ((long) 95));
parms.Param[0] = parm;
image.Save(savePath, ici, parms);
parms.Dispose();
}

转自：http://www.codefans.net/articles/234.shtml