High quality image resize (C#)

You can use this class to resize images proportionally (maintaining aspect ratio), or to resize an image to an exact size and crop off excess (useful for making nice thumbnails):

 

public static class ImageExt {
  public static Image Resize(this Image img, int srcX, int srcY, int srcWidth, int srcHeight, int dstWidth, int dstHeight) {
    var bmp = new Bitmap(dstWidth, dstHeight);
    using(var graphics = Graphics.FromImage(bmp)) {
      graphics.InterpolationMode = InterpolationMode.HighQualityBicubic;
      graphics.CompositingMode = CompositingMode.SourceCopy;
      graphics.PixelOffsetMode = PixelOffsetMode.HighQuality;
      using(var wrapMode = new ImageAttributes()) {
        wrapMode.SetWrapMode(WrapMode.TileFlipXY);
        var destRect = new Rectangle(0, 0, dstWidth, dstHeight);
        graphics.DrawImage(img, destRect, srcX, srcY, srcWidth, srcHeight, GraphicsUnit.Pixel, wrapMode);
      }
    }
    return bmp;
  }

  public static Image ResizeProportional(this Image img, int width, int height, bool enlarge = false) {
    double ratio = Math.Max(img.Width / (double) width, img.Height / (double) height);
    if (ratio < 1 && !enlarge) return img;
    return img.Resize(0, 0, img.Width, img.Height, M2.Round(img.Width / ratio), M2.Round(img.Height / ratio));
  }
  public static Image ResizeCropExcess(this Image img, int dstWidth, int dstHeight) {
    double srcRatio = img.Width / (double) img.Height;
    double dstRatio = dstWidth / (double) dstHeight;
    int srcX,
    srcY,
    cropWidth,
    cropHeight;
    if (srcRatio < dstRatio) // trim top and bottom
    {
      cropHeight = dstHeight * img.Width / dstWidth;
      srcY = (img.Height - cropHeight) / 2;
      cropWidth = img.Width;
      srcX = 0;
    } else // trim left and right
    {
      cropWidth = dstWidth * img.Height / dstHeight;
      srcX = (img.Width - cropWidth) / 2;
      cropHeight = img.Height;
      srcY = 0;
    }
    return Resize(img, srcX, srcY, cropWidth, cropHeight, dstWidth, dstHeight);
  }
}
Posted in

3 thoughts on “High quality image resize (C#)

  1. Hi There,

    I have a bmp image that is 1500 x 1200 pixels I would like it to be 40% smaller, how would I do this with your code i.e. make the image 900 x 720 ?

    thank you

    Schuster

  2. Just need to do a little bit of math. Use

    ResizeProportional(img, img.Width*0.6, img.Height*0.6)

    I just noticed I used a function called

    M2.Round

    which I didn’t define. That’s simply

    (int)Math.Round

    .

  3. Hi Mark !, Thank you it helped me a lot and the result image has an great quality .

Leave a Reply

Your email address will not be published. Required fields are marked *