创建缩略图图像

Question

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？我正在使用C#语言asp.net.

Answer 1

你必须GetThumbnailImage在Image类中使用方法:

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"));

Answer 2

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();
}

Answer 3

这是有关如何创建较小图像（缩略图）的完整示例。此代码段调整图像的大小，在需要时旋转图像（如果垂直握住电话），如果要创建方形拇指，则填充图像。该代码段创建了JPEG，但可以轻松地将其修改为其他文件类型。即使图像小于允许的最大尺寸，图像仍将被压缩，并且其分辨率也会更改，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，它是一个有关如何上载文件，调整文件大小并在页面上显示结果的小示例。

//== 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;
        }
    }
}