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Если есть те кто работал в .NET и пытался сохранить файл в jpeg и после этого получал исковерканное сжатием изображение те поймут. Привожу класс расширитель взятый с просторов интернета так что пользуйтесь. В классе методы для изменения размера изображения без потери качества, сохранения изображения без потери качества и тд. Код ниже:

public static class ImageExtensionMethods
{

static private ImageCodecInfo GetEncoder(ImageFormat format)
{
return ImageCodecInfo.GetImageDecoders().SingleOrDefault(c => c.FormatID == format.Guid);
}

public static void SaveAsJpeg(this Image Img, string FileName, Int64 Quality)
{
ImageCodecInfo jgpEncoder = GetEncoder(ImageFormat.Jpeg);
Encoder QualityEncoder = Encoder.Quality;

using (EncoderParameters EP = new EncoderParameters(1))
{
using (EncoderParameter QualityEncoderParameter = new EncoderParameter(QualityEncoder, Quality))
{
EP.Param[0] = QualityEncoderParameter;
Img.Save(FileName, jgpEncoder, EP);
}
}
}

public static void SaveAsGif(this Image Img, string FileName, Int64 Quality)
{
ImageCodecInfo gifEncoder = GetEncoder(ImageFormat.Gif);
Encoder QualityEncoder = Encoder.Quality;

using (EncoderParameters EP = new EncoderParameters(1))
{
using (EncoderParameter QualityEncoderParameter = new EncoderParameter(QualityEncoder, Quality))
{
EP.Param[0] = QualityEncoderParameter;
Img.Save(FileName, gifEncoder, EP);
}
}
}

public static Image Resize(this Image Img, int Width, int Height, InterpolationMode InterpolationMode)
{
Image CropedImage = new Bitmap(Width, Height);
using (Graphics G = Graphics.FromImage(CropedImage))
{
G.SmoothingMode = SmoothingMode.HighQuality;
G.InterpolationMode = InterpolationMode;
G.PixelOffsetMode = PixelOffsetMode.HighQuality;
G.DrawImage(Img, 0, 0, Width, Height);
}

return CropedImage;
}

public static Image Resize(this Image Img, int Width, int Height)
{
return Img.Resize(Width, Height, InterpolationMode.HighQualityBicubic);
}

private static Rectangle EnsureAspectRatio(this Image Image, int Width, int Height)
{
float AspectRatio = Width / (float)Height;
float CalculatedWidth = Image.Width, CalculatedHeight = Image.Height;

if (Image.Width >= Image.Height)
{
if (Width > Height)
{
CalculatedHeight = Image.Width / AspectRatio;
if (CalculatedHeight > Image.Height)
{
CalculatedHeight = Image.Height;
CalculatedWidth = CalculatedHeight * AspectRatio;
}
}
else
{
CalculatedWidth = Image.Height * AspectRatio;
if (CalculatedWidth > Image.Width)
{
CalculatedWidth = Image.Width;
CalculatedHeight = CalculatedWidth / AspectRatio;
}
}
}
else
{
if (Width < Height)
{
CalculatedHeight = Image.Width / AspectRatio;
if (CalculatedHeight > Image.Height)
{
CalculatedHeight = Image.Height;
CalculatedWidth = CalculatedHeight * AspectRatio;
}
}
else
{
CalculatedWidth = Image.Height * AspectRatio;
if (CalculatedWidth > Image.Width)
{
CalculatedWidth = Image.Width;
CalculatedHeight = CalculatedWidth / AspectRatio;
}
}
}
return Rectangle.Ceiling(new RectangleF((Image.Width - CalculatedWidth) / 2, 0, CalculatedWidth, CalculatedHeight));
}

public static Image ResizeToCanvas(this Image Img, int Width, int Height, out Rectangle CropRectangle)
{
return Img.ResizeToCanvas(Width, Height, InterpolationMode.HighQualityBicubic, out CropRectangle);
}

public static Image ResizeToCanvas(this Image Img, int Width, int Height, InterpolationMode InterpolationMode, out Rectangle CropRectangle)
{
CropRectangle = EnsureAspectRatio(Img, Width, Height);
Image CropedImage = new Bitmap(Width, Height);

using (Graphics G = Graphics.FromImage(CropedImage))
{
G.SmoothingMode = SmoothingMode.HighQuality;
G.InterpolationMode = InterpolationMode;
G.PixelOffsetMode = PixelOffsetMode.HighQuality;
G.DrawImage(Img, new Rectangle(0, 0, Width, Height), CropRectangle, GraphicsUnit.Pixel);
}

return CropedImage;
}

public static Image ResizeToCanvas(this Image Img, int Width, int Height, RectangleF CR)
{
return Img.ResizeToCanvas(Width, Height, InterpolationMode.HighQualityBicubic, CR);
}

public static Image ResizeToCanvas(this Image Img, int Width, int Height, InterpolationMode InterpolationMode, RectangleF CR)
{
Image CropedImage = new Bitmap(Width, Height);
using (Graphics G = Graphics.FromImage(CropedImage))
{
G.SmoothingMode = SmoothingMode.HighQuality;
G.InterpolationMode = InterpolationMode;
G.PixelOffsetMode = PixelOffsetMode.HighQuality;
G.DrawImage(Img, new Rectangle(0, 0, Width, Height), CR, GraphicsUnit.Pixel);
}

return CropedImage;
}

}