C# 缩略图算法

using (var stream = new FileStream(physicalPath, FileMode.Open, FileAccess.Read))
{
Bitmap target = null;
int targetWidth, targetHeight;
try
{
using (var bitmap = new Bitmap(stream))
{
var size = bitmap.PhysicalDimension;
if (size.Width > size.Height)
{
if (size.Width > _maximumWidth)
{
targetWidth = _maximumWidth;
targetHeight = (int)size.Height * _maximumWidth / (int)size.Width;
}
else
{
targetWidth = (int)size.Width;
targetHeight = (int)size.Height;
}

if (targetHeight >= _maximumHeight)
{
targetWidth = targetWidth * _maximumHeight / targetHeight;
targetHeight = _maximumHeight;
}
}
else
{
if (size.Height > _maximumHeight)
{
targetHeight = _maximumHeight;
targetWidth = (int)size.Width * _maximumHeight / (int)size.Height;
}
else
{
targetHeight = (int)size.Height;
targetWidth = (int)size.Width;
}

if (targetWidth >= _maximumWidth)
{
targetHeight = targetHeight * _maximumWidth / targetWidth;
targetWidth = _maximumWidth;
}
}

SizeF targetSize = SizeF.Empty;

targetSize = new SizeF(targetWidth, targetHeight);

target = new Bitmap(targetWidth, targetHeight);
using (var graphics = Graphics.FromImage(target))
{
graphics.CompositingQuality = compositingQuality;
graphics.InterpolationMode = interpolationMode;
graphics.CompositingMode = CompositingMode.SourceCopy;

graphics.DrawImage(bitmap, 0, 0, targetSize.Width, targetSize.Height);
}
}

context.Response.ContentType = "image/jpg";
using (MemoryStream memoryStream = new MemoryStream())
{
target.Save(memoryStream, ImageFormat.Jpeg);
memoryStream.WriteTo(context.Response.OutputStream);
}
}
finally
{
if (target != null)
{
target.Dispose();
}
}
}