我想在文件位置的网格视图中显示缩略图图像。如何生成.jpeg文件？我在C#使用asp.net语言。
你必须在GetThumbnailImage类中使用Image方法：
https://msdn.microsoft.com/en-us/library/8t23aykb%28v=vs.110%29.aspx
这是一个粗略的例子，它采用图像文件并从中生成缩略图，然后将其保存回磁盘。
Image image = Image.FromFile(fileName);
Image thumb = image.GetThumbnailImage(120, 120, ()=>false, IntPtr.Zero);
thumb.Save(Path.ChangeExtension(fileName, "thumb"));
它位于System.Drawing命名空间中（在System.Drawing.dll中）。
行为：
如果图像包含嵌入的缩略图图像，则此方法将检索嵌入的缩略图并将其缩放到请求的大小。如果图像不包含嵌入的缩略图图像，则此方法通过缩放主图像来创建缩略图图像。
重要提示：上面Microsoft链接的备注部分警告某些潜在问题：
当请求的缩略图图像具有大约120×120像素的大小时，
GetThumbnailImage方法很有效。如果从具有嵌入式缩略图的图像请求大型缩略图图像（例如，300 x 300），则缩略图图像中可能会出现明显的质量损失。通过调用
DrawImage方法缩放主图像（而不是缩放嵌入的缩略图）可能更好。
以下代码将按照与响应成比例的方式编写图像，您可以为您的目的修改代码：
public void WriteImage(string path, int width, int height)
{
Bitmap srcBmp = new Bitmap(path);
float ratio = srcBmp.Width / srcBmp.Height;
SizeF newSize = new SizeF(width, height * ratio);
Bitmap target = new Bitmap((int) newSize.Width,(int) newSize.Height);
HttpContext.Response.Clear();
HttpContext.Response.ContentType = "image/jpeg";
using (Graphics graphics = Graphics.FromImage(target))
{
graphics.CompositingQuality = CompositingQuality.HighSpeed;
graphics.InterpolationMode = InterpolationMode.HighQualityBicubic;
graphics.CompositingMode = CompositingMode.SourceCopy;
graphics.DrawImage(srcBmp, 0, 0, newSize.Width, newSize.Height);
using (MemoryStream memoryStream = new MemoryStream())
{
target.Save(memoryStream, ImageFormat.Jpeg);
memoryStream.WriteTo(HttpContext.Response.OutputStream);
}
}
Response.End();
}
以下是如何创建较小图像（缩略图）的完整示例。此片段调整图像大小，在需要时旋转它（如果手机垂直放置），如果要创建方形拇指，则填充图像。此代码段会创建JPEG，但可以轻松修改其他文件类型。即使图像小于最大允许尺寸，图像仍将被压缩，并且其分辨率被改变以创建具有相同dpi和压缩级别的图像。
using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;
using System.IO;
//set the resolution, 72 is usually good enough for displaying images on monitors
float imageResolution = 72;
//set the compression level. higher compression = better quality = bigger images
long compressionLevel = 80L;
public Image resizeImage(Image image, int maxWidth, int maxHeight, bool padImage)
{
int newWidth;
int newHeight;
//first we check if the image needs rotating (eg phone held vertical when taking a picture for example)
foreach (var prop in image.PropertyItems)
{
if (prop.Id == 0x0112)
{
int orientationValue = image.GetPropertyItem(prop.Id).Value[0];
RotateFlipType rotateFlipType = getRotateFlipType(orientationValue);
image.RotateFlip(rotateFlipType);
break;
}
}
//apply the padding to make a square image
if (padImage == true)
{
image = applyPaddingToImage(image, Color.Red);
}
//check if the with or height of the image exceeds the maximum specified, if so calculate the new dimensions
if (image.Width > maxWidth || image.Height > maxHeight)
{
double ratioX = (double)maxWidth / image.Width;
double ratioY = (double)maxHeight / image.Height;
double ratio = Math.Min(ratioX, ratioY);
newWidth = (int)(image.Width * ratio);
newHeight = (int)(image.Height * ratio);
}
else
{
newWidth = image.Width;
newHeight = image.Height;
}
//start the resize with a new image
Bitmap newImage = new Bitmap(newWidth, newHeight);
//set the new resolution
newImage.SetResolution(imageResolution, imageResolution);
//start the resizing
using (var graphics = Graphics.FromImage(newImage))
{
//set some encoding specs
graphics.CompositingMode = CompositingMode.SourceCopy;
graphics.CompositingQuality = CompositingQuality.HighQuality;
graphics.SmoothingMode = SmoothingMode.HighQuality;
graphics.InterpolationMode = InterpolationMode.HighQualityBicubic;
graphics.PixelOffsetMode = PixelOffsetMode.HighQuality;
graphics.DrawImage(image, 0, 0, newWidth, newHeight);
}
//save the image to a memorystream to apply the compression level
using (MemoryStream ms = new MemoryStream())
{
EncoderParameters encoderParameters = new EncoderParameters(1);
encoderParameters.Param[0] = new EncoderParameter(Encoder.Quality, compressionLevel);
newImage.Save(ms, getEncoderInfo("image/jpeg"), encoderParameters);
//save the image as byte array here if you want the return type to be a Byte Array instead of Image
//byte[] imageAsByteArray = ms.ToArray();
}
//return the image
return newImage;
}
//=== image padding
public Image applyPaddingToImage(Image image, Color backColor)
{
//get the maximum size of the image dimensions
int maxSize = Math.Max(image.Height, image.Width);
Size squareSize = new Size(maxSize, maxSize);
//create a new square image
Bitmap squareImage = new Bitmap(squareSize.Width, squareSize.Height);
using (Graphics graphics = Graphics.FromImage(squareImage))
{
//fill the new square with a color
graphics.FillRectangle(new SolidBrush(backColor), 0, 0, squareSize.Width, squareSize.Height);
//put the original image on top of the new square
graphics.DrawImage(image, (squareSize.Width / 2) - (image.Width / 2), (squareSize.Height / 2) - (image.Height / 2), image.Width, image.Height);
}
//return the image
return squareImage;
}
//=== get encoder info
private ImageCodecInfo getEncoderInfo(string mimeType)
{
ImageCodecInfo[] encoders = ImageCodecInfo.GetImageEncoders();
for (int j = 0; j < encoders.Length; ++j)
{
if (encoders[j].MimeType.ToLower() == mimeType.ToLower())
{
return encoders[j];
}
}
return null;
}
//=== determine image rotation
private RotateFlipType getRotateFlipType(int rotateValue)
{
RotateFlipType flipType = RotateFlipType.RotateNoneFlipNone;
switch (rotateValue)
{
case 1:
flipType = RotateFlipType.RotateNoneFlipNone;
break;
case 2:
flipType = RotateFlipType.RotateNoneFlipX;
break;
case 3:
flipType = RotateFlipType.Rotate180FlipNone;
break;
case 4:
flipType = RotateFlipType.Rotate180FlipX;
break;
case 5:
flipType = RotateFlipType.Rotate90FlipX;
break;
case 6:
flipType = RotateFlipType.Rotate90FlipNone;
break;
case 7:
flipType = RotateFlipType.Rotate270FlipX;
break;
case 8:
flipType = RotateFlipType.Rotate270FlipNone;
break;
default:
flipType = RotateFlipType.RotateNoneFlipNone;
break;
}
return flipType;
}
//== convert image to base64
public string convertImageToBase64(Image image)
{
using (MemoryStream ms = new MemoryStream())
{
//convert the image to byte array
image.Save(ms, ImageFormat.Jpeg);
byte[] bin = ms.ToArray();
//convert byte array to base64 string
return Convert.ToBase64String(bin);
}
}
对于asp.net用户，可以举例说明如何上传文件，调整文件大小并在页面上显示结果。
//== the button click method
protected void Button1_Click(object sender, EventArgs e)
{
//check if there is an actual file being uploaded
if (FileUpload1.HasFile == false)
{
return;
}
using (Bitmap bitmap = new Bitmap(FileUpload1.PostedFile.InputStream))
{
try
{
//start the resize
Image image = resizeImage(bitmap, 256, 256, true);
//to visualize the result, display as base64 image
Label1.Text = "<img src=\"data:image/jpg;base64," + convertImageToBase64(image) + "\">";
//save your image to file sytem, database etc here
}
catch (Exception ex)
{
Label1.Text = "Oops! There was an error when resizing the Image.<br>Error: " + ex.Message;
}
}
}
以下是将高分辨率图像转换为缩略图大小的示例 -
protected void Button1_Click(object sender, EventArgs e)
{
//---------- Getting the Image File
System.Drawing.Image img = System.Drawing.Image.FromFile(Server.MapPath("~/profile/Avatar.jpg"));
//---------- Getting Size of Original Image
double imgHeight = img.Size.Height;
double imgWidth = img.Size.Width;
//---------- Getting Decreased Size
double x = imgWidth / 200;
int newWidth = Convert.ToInt32(imgWidth / x);
int newHeight = Convert.ToInt32(imgHeight / x);
//---------- Creating Small Image
System.Drawing.Image.GetThumbnailImageAbort myCallback = new System.Drawing.Image.GetThumbnailImageAbort(ThumbnailCallback);
System.Drawing.Image myThumbnail = img.GetThumbnailImage(newWidth, newHeight, myCallback, IntPtr.Zero);
//---------- Saving Image
myThumbnail.Save(Server.MapPath("~/profile/NewImage.jpg"));
}
public bool ThumbnailCallback()
{
return false;
}
来源 - http://iknowledgeboy.blogspot.in/2014/03/c-creating-thumbnail-of-large-image-by.html