How to Create Image thumbnails

There are multiple ways to create a thumbnail. The simplest method is to use the. net method to create a thumbnail, or use a re-painting test. If compression is required, you can use JPEG compression.
Below I tested it in several ways and used it. the built-in method of net can ensure the quality of creating thumbnails, while the quality of creating thumbnails in the re-painting method is slightly poor, especially when processing small images, such as portraits, and local details become blurred. redraw is relative.. net thumbnails can be a little smaller, but the quality is really stubborn.

Code
/// <Summary>
/// Use the thumbnail extraction method provided by. net to create a thumbnail
/// </Summary>
/// <Param name = "imageStream"> image input stream </param>
/// <Param name = "thumbWi"> specify the thumbnail width </param>
/// <Param name = "thumbHi"> specify the thumbnail height </param>
/// <Returns> returns the thumbnail byte data </returns>
Private byte [] getImageByteData (Stream imageStream, int thumbWi, int thumbHi)
{
System. Drawing. Image image = System. Drawing. Image. FromStream (imageStream );
Int height = thumbHi;
Int width = thumbWi;
If (image. Height> height | image. Width> width)
{
// Change the thumbnail
System. Drawing. Image. GetThumbnailImageAbort callb = null;
// Extract the thumbnail with the specified size
System. Drawing. Image thumbnailImage = image. GetThumbnailImage (thumbWi, thumbHi, callb, new System. IntPtr ());
Image = thumbnailImage;
}

MemoryStream Stream = new MemoryStream ();
Image. Save (Stream, System. Drawing. Imaging. ImageFormat. Jpeg );
Byte [] byteData = Stream. ToArray ();
Return byteData;
}

/// <Summary>
/// Create a thumbnail using the redrawing Method
/// </Summary>
/// <Param name = "source"> source image </param>
/// <Param name = "thumbWi"> specify the thumbnail width </param>
/// <Param name = "thumbHi"> specify the thumbnail height </param>
/// <Param name = "maintainAspect"> whether to maintain the length/width ratio </param>
/// <Returns> </returns>
Private byte [] getImageByteData (Bitmap source, int thumbWi, int thumbHi, bool maintainAspect)
{
Byte [] ret = null;
// Call to create a thumbnail and create a thumbnail of the specified size based on the source Image
Bitmap thumbnail = CreateThumbnail (source, thumbWi, thumbHi, maintainAspect );

# Configure JPEG compression serving for region, which can be compressed by more than 90%, with the same quality as the source Image

// Configure the encoderParameters parameter required to compress the thumbnail
EncoderParameters encoderParams = new EncoderParameters ();

Long [] quality = new long [1];
Quality [0] = 75; // compression quality, range: 0 ~ 100, the compression quality is inversely proportional to the size

EncoderParameter encoderParam = new EncoderParameter (System. Drawing. Imaging. Encoder. Quality, quality );
EncoderParams. Param [0] = encoderParam;

// Obtain the array of image encoder objects
ImageCodecInfo [] arrayICI = ImageCodecInfo. GetImageEncoders ();

ImageCodecInfo policici = null;
// Obtain the JPEG encoding information in the array of encoding objects.
For (int x = 0; x <arrayICI. Length; x ++)
{
If (arrayICI [x]. FormatDescription. Equals ("JPEG "))
{
JpegICI = arrayICI [x];
Break;
}
}
# Endregion

MemoryStream MS = new MemoryStream ();
// Save the specified JPEG parameter to the memory stream
Thumbnail. Save (MS, javasici, encoderParams );
Ret = ms. ToArray ();
Thumbnail. Dispose ();

Return ret;
}

/// <Summary>
/// Create a thumbnail
/// </Summary>
/// <Param name = "source"> source image </param>
/// <Param name = "thumbWi"> thumbnail width </param>
/// <Param name = "thumbHi"> thumbnail height </param>
/// <Param name = "maintainAspect"> whether to maintain the length/width ratio </param>
/// <Returns> </returns>
Private Bitmap CreateThumbnail (Bitmap source, int thumbWi, int thumbHi, bool maintainAspect)
{
// If the source image height width is smaller than the scaled height width, the source image is directly returned.
If (source. Width <= thumbWi & source. Height <= thumbHi) return source;

System. Drawing. Bitmap ret = null;
Try
{
Int wi, hi;

Wi = thumbWi;
Hi = thumbHi;

If (maintainAspect)
{
// Maintain the aspect ratio regardless of the thumbnail size parameter
If (source. Width> source. Height)
{
Wi = thumbWi;
Hi = (int) (source. Height * (decimal) thumbWi/source. Width ));
}
Else
{
Hi = thumbHi;
Wi = (int) (source. Width * (decimal) thumbHi/source. Height ));
}
}

// Original code that creates lousy thumbnails
// System. Drawing. Image ret = source. GetThumbnailImage (wi, hi, null, IntPtr. Zero );
Ret = new Bitmap (wi, hi );
Using (Graphics g = Graphics. FromImage (ret ))
{
G. InterpolationMode = System. Drawing. Drawing2D. InterpolationMode. HighQualityBicubic;
G. FillRectangle (Brushes. White, 0, 0, wi, hi );
G. DrawImage (source, 0, 0, wi, hi );
}
}
Catch
{
Ret = null;
}

Return ret;
}